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Xantus de Vesey 1860
3, see text
Xantus de Vesey 1860
3, see text
The spotted owl (Strix occidentalis) is a species of true owl. It is a resident species of old-growth forests in western North America, where it nests in tree holes, old bird of prey nests, or rock crevices. Nests can be between 12 to 60 metres (39 to 197 ft) high and usually contain two eggs (though some will contain as many as four). It is a nocturnal owl, which feeds on small mammals and birds.
This owl has a length of 43 cm (17 in), a wingspan of 114 cm (45 in), and a weight of around 600 g (21 oz). Its eggs are a little over 50 mm (2.0 in) long, and are white and smooth with a slightly grainy texture. The female sits on the eggs and cares for the young, while the male provides food for them. Juvenile spotted owls have an average survival rate of 11%, with an average birth rate of 0.58 owls per pair.
The spotted owl is similar in appearance to the barred owl but has cross-shaped markings on the underparts, whereas the barred owl is alternately barred on the breast and streaked on the belly. Barred owls are larger and grayer than spotted owls. In recent years the California and northern subspecies of spotted owl have been displaced by barred owls, which are more aggressive, have a broader diet and occur in more varied habitats. Though the two species may hybridize in areas where displacement is occurring, they are quite genetically distinct, for example, differing 13.9% in certain gene sequences.
The nearly contiguous range of the northern spotted owl extends from southwestern British Columbia south through western Washington and Oregon to Marin County on the north-central coast of California. The California spotted owl's range overlaps that of the northern spotted owl in the southern Cascade Range, and extends south through the western Sierra Nevada to Tulare County. They also occur in discrete populations in mountainous areas of coastal and southern California from Monterey County to northern Baja California. In the United States the Mexican spotted owl occurs in disjunct populations in mountain ranges and canyons of Utah, Colorado, Arizona, New Mexico, and extreme western Texas. In Mexico it ranges from Sonora, Chihuahua, Nuevo León, and eastern Coahuila through the Sierra Madre Occidental and Sierra Madre Oriental as far south as Michoacán.
Northern spotted owls occur in ponderosa pine/Douglas-fir (Pinus ponderosa/Pseudotsuga menzeizii) forests in the eastern Cascade Ranges of Washington and in Douglas-fir/evergreen hardwood forests in northwestern California. Throughout much of their range they use stands of the following coniferous species: Douglas-fir, western hemlock (Tsuga heterophylla), western redcedar (Thuja plicata), grand fir (Abies grandis), Pacific silver fir (A. amabilis), Sitka spruce (Picea sitchensis) and redwood (Sequoia sempervirens).
California spotted owls occur in hardwood, coniferous, and coniferous-hardwood forests. Occupied coniferous habitats include mixed coniferous forests, California red fir (A. magnifica), and eastside pine forests which are composed of ponderosa pine and/or Jeffrey pine (Pinus jeffreyi). Redwood/California bay (Umbellularia californica), ponderosa pine/hardwood, and live oak-bigcone Douglas-fir (Quercus chrysolepis or Q. agrifolia-Pseudotsuga macrocarpa) are hardwood-mixed coniferous forests used by California spotted owls. They also occur in hardwood habitats including riparian and oak (Quercus spp.) woodlands. For example, in the Tehachapi Mountains of southern California they occurred in stands dominated by canyon live oak (Q. chrysolepis).
Mexican spotted owls occur in varied habitats. Ponderosa pine-Gambel oak (Q. gambelii) and mixed-conifer forests, usually dominated by Douglas-fir and/or white fir (Abies concolor), are often used. In Arizona, ponderosa pine-Gambel oak vegetation was selected, and roosting Mexican spotted owls in New Mexico generally preferred mixed-conifer and mixed-conifer/oak forests. The majority of roosting sites in southern Arizona were in mixed-conifer or pine-oak habitats, but some occurred in Madrean evergreen woodland and interior chaparral. Pinyon-juniper (Pinus spp.-Juniperus spp.) woodlands provide nonbreeding habitat and may be used to some extent during the breeding season. However, in New Mexico, pinyon-juniper and open ponderosa pine woodlands were avoided. Other woodlands used by Mexican spotted owls include riparian woodland, encinal oak (Q. emoryi, Q. arizonica, Q. oblongifolia, Q. grisea) woodland, pine (Pinus leiophylla, P. engelmannii, P. ponderosa, P. strobiformis) woodland with evergreen oak (e.g. Q. chrysolepis) understories, and Arizona cypress (Cupressus arizonica) woodland. Montane meadows are used to some extent for foraging. Habitats such as mountain shrub and desert scrub are used during the winter by dispersing juveniles and possibly migratory adults.
Although spotted owls are not generally migratory, some individuals, especially California and Mexican spotted owls, migrate short distances (less than 31 miles (50 km) between winter and breeding ranges. Migratory California spotted owls leave their breeding grounds from October to December and return from February to mid-April. Some Mexican spotted owls in Arizona, New Mexico, Colorado, and Utah leave their breeding grounds in November and December and return from January to April. According to a literature review of all three spotted owl subspecies, migratory spotted owls move through vegetation types not typically considered suitable spotted owl habitat.
The spotted owl's breeding season occurs from early spring to late summer or fall. Breeding spotted owls begin prelaying behaviors, such as preening and roosting together, in February or March. In western Oregon, laying occurred from 9 March to 19 April, with an average date of 2 April. Northern spotted owl eggs in western Oregon were incubated for about 30 days and hatched from 8 April to 20 May. Hatchlings fledged after 34 to 36 days and reached independence in August and September. In coniferous forests of north-central Arizona, average time from fledgling to independence for Mexican spotted owls was 87 days in one year and 101 days in another. In western Oregon, dispersal of juvenile (young of the year) northern spotted owls was documented in October, and from early September to early November. Information on California spotted owl dispersal is sparse. A review summarizes studies of northern spotted owl dispersal and suggests that dispersal of both subspecies is likely to occur in September and October. Mexican spotted owls usually disperse in September, but have been observed dispersing from August to October.
Spotted owl pairs are monogamous and rarely renest after failed breeding attempts. The clutch size is two eggs, but can be one, three, or very rarely four eggs. The average clutch size of northern spotted owls in western Oregon was two eggs.
Spotted owl reproductive output is variable. In Douglas-fir/hardwoods, mixed-conifer, and Oregon white oak (Quercus garryana) forests of northwestern California, average annual northern spotted owl reproductive output ranged from 0.150 to 0.810 fledged young/pair. The percentage of years (n=5) in which pairs in coniferous forests and Douglas-fir/hardwood woodlands produced at least one fledging ranged from zero to 80%. In a study of both northern and California spotted owls in California, fledging rate varied from zero to 100% over eight years and two study sites. In oak woodland and coniferous forests of the southern Sierra Nevada, the average annual reproductive output of California spotted owl varied from 0.07 to 1.67 young/pair over a nine-year period.
Several factors contribute to the spotted owl's variable reproductive success. For instance, spotted owl pairs do not breed every year. Over a five-year period in western Oregon, the percentage of nesting northern spotted owls averaged 62% and ranged from 16% to 89%.
Another source of the variation in reproductive output is spotted owl age. Spotted owls are generally reproductive by three years of age. They may, albeit rarely, breed as young as one year old. In Washington and Oregon, the majority of radio-marked northern spotted owls were paired by two years of age, and recruitment of banded owls into the territorial population occurred at an average age of 2.4 years. The average number of fledglings produced by one-year-old northern spotted owls in 11 study areas across their range was 0.074 fledglings/territorial female. Two-year-olds fledged an average of 0.208 young/territorial female, and individuals three years and older fledged an average of 0.372 young/territorial female. Increased reproductive output of northern spotted owls three years or older compared to juveniles and subadults (one- and two-year-olds) has been observed in several study areas. In Arizona and New Mexico, Mexican spotted owls two years and older had greater fecundity than one-year-olds.
Site characteristics can also affect the reproductive output of spotted owls. Increased elevation was negatively associated with reproductive success of northern spotted owls in coniferous forests of the eastern Cascade Range in Washington, and of California spotted owls in coniferous forests in northeastern California. In areas throughout their range with less than 20% suitable habitat, northern spotted owls averaged only 0.33 fledglings/pair, while in areas with more than 60% suitable habitat they averaged 0.93 fledglings/pair. Northern spotted owls in nonglaciated montane slopes of the eastern Cascade Range of Washington averaged 0.57 fledglings/year, significantly (P<0.01) more than those in glacially scoured regions, which averaged 0.38 fledgling/year. In the Sacramento Mountains of New Mexico, Mexican spotted owls in mixed-conifer forests had higher average annual reproductive output (0.38 female fledged/territory) than those in ponderosa pine and Colorado pinyon-alligator juniper (Pinus edulis-Juniperus deppeana) woodlands (0.13 female fledged/territory).
Weather and food have a strong influence on spotted owl reproductive success. Good weather, such as lower precipitation and higher minimum temperatures during the nesting season, is associated with higher reproduction.
Juvenile spotted owls disperse in late summer to fall. Dispersing juvenile northern spotted owls in Oregon and Washington often settle in a wintering range from October or November until February to April, and from there dispersed to a breeding-season home range. Some individuals remained in this home range, while others occupied a series of temporary home ranges until settling sometime from 2 to 5 years of age. Although variable, most spotted owls disperse less than 19 miles (31 km). Only 8.7% of northern spotted owls in Washington and Oregon dispersed over 31 miles (50 km).
Occasionally adult (>two years old) spotted owls disperse from their territories. An annual average dispersal rate of adult northern spotted owl was 6.6% in Oregon and Washington. Females, three- to four-year-old adults, and individuals with no mate in the current or previous season were most likely to disperse from their territory. The average adult dispersal distance was 3.8 miles (6.1 km).
Spotted owls are long lived, and 16- to 17-year-old northern spotted owls have been documented in Oregon. Average annual survival rate estimates are usually more than 0.8 for adults of all three subspecies, although average annual survival rates as low as 0.75 have been reported for northern spotted owls.
Survival rates of juvenile and subadult spotted owls are low and generally more variable than adult survival rates. A meta-analysis of northern spotted owl data estimated annual juvenile survival rates at 21–29%. Annual survival rates of 1-year-old northern spotted owls from 11 study areas varied from 42% to 86%, while annual survival rates of 2-year-olds ranged from 63% to 89%. In New Mexico, Mexican spotted owls from one to two years old had average annual survival rates of 64.4%. Mexican spotted owls less than a year old had average annual survival rates of 11% in New Mexico and 17.9% in Arizona.
The most common causes of spotted owl mortality are predation and starvation. Most juvenile northern spotted owls in Washington and Oregon died from predation (68%) or starvation (26.2%). Both adult and juvenile Mexican spotted owls in New Mexico died from starvation and predation. Juveniles also die from exposure. Accidents are an additional source of mortality. Given that 67% of northern spotted owls in Washington and Oregon that died from other causes were diseased, illness may increase the risk of mortality.
|This section may contain an excessive amount of intricate detail that may only interest a specific audience. (June 2014)|
Spotted owls occur in closed-canopy, uneven-aged, late-successional and old-growth forests; Mexican spotted owls also occur in deep, steep-walled canyons with little canopy cover. Many habitat measurements were taken in plots between 0.1 and 2 acres (0.040 and 0.809 ha). In this section, these will be referred to as "small plots."
Spotted owls occur at a range of elevations, with higher elevations occupied at lower latitudes. Northern spotted owls occur at elevations from 70 to 6,600 feet (21 to 2,012 m), with the majority in the lower portions of this range. In coniferous forests of northwestern California, nest sites ranged from 118 to 4,944 feet (36 to 1,507 m), with 94% occurring below 4,000 feet (1,200 m). In mixed evergreen and mixed-conifer forests of northwestern California, roosting northern spotted owls avoided areas above 2,950 feet (900 m). In coniferous forests of the Klamath, Coast and Cascade regions in Oregon and the Olympic peninsula of Washington, nest locations were significantly lower (P<0.001) in elevation than random sites within northern spotted owl's home ranges. In coniferous forests of southwestern Washington, important owl locations (e.g., nest sites, multiple detection sites) averaged 3,170 feet (970 m), which was significantly (P<0.001) lower than the 3,510 foot (1,070 m) average elevation at random sites. In coniferous forests of the eastern Cascade Range of Washington, elevation of northern spotted owl nest sites was negatively associated with latitude (P<0.001), and site occupancy and reproductive rates were inversely associated with elevation.
California spotted owls occur on sites from about 1,000 to 8,500 feet (300 to 2,590 m), with individuals in southern California generally occurring at higher elevations. A detailed summary of California spotted owl habitat associations reports nest sites occurring at an average elevation of 5,300 feet (1,600 m) in the northern Sierra Nevada and 6,000 feet (1,800 m) in southern California. In white fir-mixed-conifer stands of the Lassen National Forest in northeastern California, elevation at California spotted owl nest areas was inversely associated with site occupancy and reproductive output.
Most of Mexican spotted owl nests in Arizona and New Mexico (95%) were in trees at elevations from 6,000 to 8,500 feet (1,800 to 2,600 m), with 72% occurring from 6,500 to 7,500 feet (2,000 to 2,300 m). In Colorado, Mexican spotted owls occurred from 5,820 to 9,100 feet (1,770 to 2,770 m), with 17 of 20 from 6,500 to 7,800 feet (2,000 to 2,400 m). In Saguaro National Park, Mexican spotted owl territories did not occur below 7,000 feet (2,100 m). In another southern Arizona study site, Mexican spotted owl nest/roost sites occurred from 5,820 to 7,620 feet (1,770 to 2,320 m).
Aspects at nesting and roost sites in mixed evergreen and mixed-conifer forests of northwestern California did not differ from availability. In the western Cascade Range of southwestern Washington, aspects of spotted owl sites did not differ significantly (P>0.05) from random sites. Similarly, data from low to mid-elevation forests of northwestern California also indicated that aspects at nest sites did not differ from random sites. In southwestern Oregon, north to east aspects were used more frequently and south to southwest aspects were used less frequently than expected in summer. In contrast, spring and fall roost sites occurred more frequently on south and southwest aspects (P<0.05). The aspect at nest sites in the eastern Cascade Range of Washington averaged 35° northeast, which was significantly (P=0.015) different from the average aspect of 48° northeast on random sites in the nest stand. Over 50% of summer roosting and foraging observations occurred on north-facing slopes in predominantly mixed-evergreen forests of northwestern California.
California and Mexican spotted owls may select northern slopes and/or avoid southern slopes. In the central Sierra Nevada, roost sites, but not nest sites, faced north (x=16° north) significantly (P<0.05) more often than a uniform distribution. One of the many factors associated with higher reproductive rates in oak woodland of the southern Sierra Nevada was nesting on north-facing slopes. Although likely confounded with vegetation characteristics, California spotted owl nests did not occur on sites with southern aspects in foothill riparian and oak woodlands in the southern Sierra Nevada. However, the average aspects of nesting and roosting sites in the San Bernardino Mountains were not significantly different from average aspects of random sites. In southern Arizona, 7 out of 10 of Mexican spotted owl nest/roost sites were on northwest facing slopes. In Saguaro National Park most roost sites occurred on northwest facing slopes. Nearly 50% of Mexican spotted owl nests occurred on north or northeast aspects in Arizona and New Mexico study sites. Spotted owls often occur on steep slopes, and sometimes steep slopes are selected more than would be expected based on their availability. For example, in coniferous forests of the western Cascade Range in Washington, slopes at northern spotted owl sites averaged 54.1%, significantly (P<0.001) steeper than the 46.2% average slope on random sites. In mixed evergreen and mixed-conifer forests of northwestern California, gentle (15–30%) slopes were avoided (P<0.03) for roosting. Slope averaged 49% at roost sites in southwest Oregon and 58% at nest sites in low- to mid-elevation coniferous forests of northwestern California, but these slopes were common for the area. In mixed-evergreen and mixed-conifer forests of northwestern California, steep slopes were used by nesting northern spotted owls in proportion with availability. California spotted owl nesting and roosting sites were significantly (P<0.001) steeper than random sites in the San Bernardino Mountains. Slopes of nest sites in foothill riparian and oak woodlands of the southern Sierra Nevada ranged from 0% to 105%. Mexican spotted owl nest areas in Arizona were significantly (P<0.001) steeper (x=38.5%) than random sites (x=20.6%). In steep, rocky, canyons of southern Arizona, the average slope of nest and roost sites was 34%.
Spotted owls seem to select the lower portions of slopes, at least in summer. Although most of the data are from studies on the northern spotted owl, there is evidence that California and Mexican spotted owls also select slope bases. In mixed-evergreen and mixed-conifer forests of northwestern California, northern spotted owls nested and roosted on the lower third of slopes significantly more, the middle third in proportion with, and the upper third of slopes significantly less than expected based on availability during the breeding season (α=0.05). On sites in coniferous forests in Oregon and Washington, 95% of nest sites were on the bottom or middle third of slopes, although this was only significantly (P<0.025) more than random sites within home ranges in the Klamath Mountains. In southwestern Oregon, lower slopes were used significantly more than expected for roosting during summer. However, in the spring, fall, and winter northern spotted owls used upper and mid-slopes significantly more than expected (P<0.01). In foothill riparian and oak woodlands in the southern Sierra Nevada, California spotted owl nest sites were typically on the lower third of slopes.
In coniferous forests of the eastern Cascade Range of Washington, topography may influence northern spotted owl reproductive success. In glacially scoured, topographically varied landscapes, average reproductive output was 0.38 fledglings/year, while in more gently rolling, montane slopes it averaged 0.57 fledglings/year (P<0.01).
In some regions, northern spotted owls use areas near water. In mixed-evergreen forests of northwestern California, the summer roost sites of 10 northern spotted owls averaged 466 feet (142.1 m) from water, which was significantly (P<0.01) shorter than the average 743 feet (226.6 m) from random locations to water. In managed timberlands in the coastal redwood vegetation zone of northwestern California, northern spotted owl nest areas were closer to water than randomly selected plots (P=0.032). Nest sites in low- to mid-elevation conifer forests of northwestern California averaged 385 feet (117.3 m) from water. On two sites in the Coast and Cascade Ranges in western Oregon, 84% of nests were within 820 feet (250 m) of a stream or spring. In southwestern Oregon, roost sites were significantly (P<0.01) closer to water in summer (x = 240 feet (74 m)) than in winter (x = 325 feet (99 m)). A literature review states that Mexican spotted owls occur in canyons with perennial water sources.
Reproductive rates of northern and California spotted owls are strongly influenced by weather. Low northern spotted owl reproductive output in Douglas-fir/hardwood and mixed-conifer forests of northwestern California was associated with cold, wet springs. Average productivity in the eastern Cascade Range of Washington also declined with increasing precipitation, from 0.10 young/year on sites receiving more than 118 inches (300 cm) of precipitation a year to 0.96 young/year on sites receiving less than 20 inches (51 cm) of precipitation a year. High California spotted owl reproductive rates were associated with less precipitation and higher minimum temperatures during the breeding season (March–May). For example, in mixed-conifer forests, reproductive output averaged 1.585 fledglings/pair in breeding seasons with less than 8 inches (20.7 cm) of rain and 0.307 fledglings/pair in breeding seasons with more than 8 inches (200 mm) of rain. In breeding seasons with more than 8 inches (200 mm) of rain, California spotted owl reproductive output was greater when the minimum April temperature was above 28 °F (−2 °C) (0.473 fledglings/pair) than when it was below 28 °F (0.183 fledglings/pair). The number of pairs breeding in each of these situations was small, ranging from 2 to 7.
Survival and occupancy rates of Northern spotted owls may also be affected by weather. Northern spotted owl survival in Douglas-fir/hardwood and mixed-conifer forests of northwestern California was detrimentally affected by cold, wet springs. In coniferous forests of the eastern Cascade Range of Washington, precipitation was inversely related to site occupancy. There is no information on the effect of weather on California or Mexican spotted owl survival or site occupancy.
Good quality habitat likely buffers the effects of weather. For example, data from Douglas-fir/hardwood and mixed-conifer forests of northwestern California suggest that decreases in northern spotted owl survival associated with cold, wet weather were more gradual in landscapes with features considered high quality compared to landscapes with low-quality features. The habitat features associated with nest sites and the negative impact of precipitation on California spotted owl reproduction led researchers to speculate that high canopy cover and foliage volumes could reduce through fall precipitation and wind penetration at nest sites.
Weather may also influence spotted owl's habitat selection. In Saguaro National Park, the average daytime temperature at Mexican spotted owl roosts was significantly cooler than the surrounding ambient temperature (P<0.05), with an average difference between roost and ambient temperatures of 5.1 °F (2.9 °C). In northern Arizona, nesting sites had significantly (P<0.001) lower temperatures and were above 95 °F (>35.2 °C) less often than randomly selected areas. Cooler sites were associated with increased canopy cover (P=0.001). Canyons occupied by Mexican spotted owls in Zion National Park had higher humidity than canyons where owls were not detected, and roost sites in Utah occurred in canyons with lower temperatures than randomly selected canyons. North and others suggest that the characteristically high foliage volume at California spotted owl nest sites in oak woodland and mixed-conifer forests of the southern Sierra Nevada may reflect selection for microhabitats that provide the most cover in inclement weather.
Northern spotted owls in western Oregon roosted higher in the canopy in cold wet weather than in warm or hot weather. However, relationships between roost sites and climatic variables were weak in mixed-conifer and Douglas-fir forests in southwestern Oregon. Summaries of the role of northern spotted owl thermoregulation in the selection of old-growth habitat are included in.
Northern spotted owls are strongly associated with mature and old-growth forests. In Douglas-fir/hardwood, mixed-conifer, and Oregon white oak forests of California, 500 acre (200 ha) plots centered on nesting and roosting sites contained significantly (P=0.003) more mature and old-growth habitat than random plots. In mixed-evergreen forests of northwestern California, northern spotted owls preferentially selected (P≤0.005) foraging and roosting sites in mature or old-growth stands within home ranges. In coniferous forests of the western Cascade Range of southwestern Washington, there was significantly (P<0.02) more forest 130 years old or older in 500 acre (200 ha) areas around nest sites compared to random sites. In a western Oregon study, over 90% of roosting and nesting locations were in old-growth coniferous forests, and northern spotted owls foraged in these forests significantly (P<0.05) more than would be expected based on availability within their home ranges. In coniferous forests in southwestern Oregon, 83% of northern spotted owls selected old-growth forests and used an average of 1.5 times the amount of old growth than would be expected based on availability in the landscape. However, there was no difference in stand age between small nest and random plots in the eastern Cascade Range of Washington, and only 12% of nest stands on the eastern slope of the Cascade Range in Washington and Oregon were classified as old growth.
The use of younger stands by northern spotted owls is mixed. For instance, in coniferous forests of the western Cascade Range in southwestern Washington, northern spotted owls selected (P<0.02) stands less than 49 years old more than expected based their availability. In the coastal redwood zone, forests more than 60 years old and those less than seven years old were used in proportion to their availability, while stands from 31 to 60 years of age were used for nesting more (P≤0.039) than expected based on availability. However, in mixed-evergreen and Klamath montane forests of northwestern California, pole timber and seedling-sapling stands less than about 35 years old were not used for nesting. Roosting individuals used the pole timber stands, but less (P<0.001) than would be expected based on availability. In mixed-evergreen forests of northwestern California, summer foraging and roosting sites occurred significantly (P≤0.05) less often in young- to intermediate-aged stands than expected based on availability.
Greater amounts of mature and old-growth forests have been associated with improved northern spotted owl persistence and reproductive output in some areas. For instance, persistence on territories in southwestern Oregon was significantly (P<0.03) higher with increased amounts of forests more than 120 years old in the 8,870 acre (3,590 ha) area around the nest site. The average number of fledglings per site was significantly (P<0.05) higher in 990 acre (400 ha) areas of western Washington, western Oregon, and northwestern California with more than 60% mature (>80 years) forest than in areas with 20% or less mature forest.
In other locations the relationship between forest age and northern spotted owl reproductive success has been ambiguous. In forests of the northern California Coast Ranges, greater reproductive success was negatively associated with clearcuts less than six years old at two scales, positively associated with 21- to 40-year-old forest at three scales, and negatively associated with 61- to 80-year-old forest at all five scales investigated. In the eastern Cascade Range of Washington, the amount of late-successional forest (dominated by trees with >25-inch (64 cm) diameter at breast height (DBH)) within a 500 acre (200 ha) area of the nest was negatively associated with reproductive rate. Previously occupied territories that were not used in the last three years of the study had significantly less forest in seedling and sapling stages and significantly more forest dominated by pole-sized trees than territories occupied during this period (P<0.05).
Northern spotted owl home range size may be influenced by the amount of old-growth habitat available. In mixed-conifer and Douglas-fir forests of southwestern Oregon, home range size was inversely correlated (r=-0.83) with the percentage of old-growth habitat. The amount of old growth used ranged from 1,330 to 2,360 acres (538–955 ha), which was less variable than the 1,920- to 8,980 acre (777-3,635 ha) home range sizes. The northern spotted owl recovery plan also includes citations for an inverse relationship between the amount of old-growth and home range size.
Data suggest that California and Mexican spotted owls select old growth and/or avoid young stands. Of oak-pine and riparian forests within California spotted owl home ranges in the Sierra National Forest, 91% were old growth. In Douglas-fir, ponderosa pine, and white fir dominated forests of the Sierra Nevada, early-successional stands including clearcuts, shrublands, and plantations were avoided by foraging individuals. Only 2% of telemetry locations occurred in these cover types, while they made up 30% of the available habitat. Home ranges were also contained significantly (P<0.001) less of these cover types than would be expected based on availability. Increased old-growth and mature tree basal areas were also characteristic of stands occupied by California spotted owls. In mixed-conifer forest of the Sierra Nevada only 13% of the vegetation within California spotted owl home ranges was classed in the greater than 21-inch (530 mm) DBH category. The low reproductive rate in this area suggests that it was not providing high quality habitat. In northern Arizona some foraging Mexican spotted owls selected old-growth mixed-conifer and ponderosa pine forests (generally >200 years old) more than expected based on availability within the home range, while managed forests in this area were avoided. Continuous forests used by Mexican spotted owls are typically old-growth forests and territories in Arizona usually contain mature trees, as well as other features associated with mature and old-growth habitats.
Spotted owls usually select areas with large trees associated with mature and old-growth stands. In coniferous forests of Tahoe National Forest, foraging California spotted owls used large (≥21 inch (53.2 cm) DBH) tree stands significantly (P<0.005) more than expected based on availability. A summary of a California spotted owl habitat study in the Tahoe National Forest reports significantly more foraging sites in stands of large (20- to 35-inch (51–89 cm) DBH) trees than expected based on availability and significantly more foraging sites in these stands than in stands of medium-sized (11- to 20-inch (28–51 cm)) trees (P<0.01). An analysis of data from the national forests of the Sierra Nevada showed that California spotted owls nested in stands of medium to large (≥24-inch (61-cm DBH)) trees more than expected based on availability. Several sources note Mexican spotted owls' use of large, mature trees, including roosting in areas with high densities of relatively large Douglas-fir and southwestern white pine (Pinus strobiformis) in coniferous forests of New Mexico. Northern spotted owl high-use sites in coniferous forests of northwestern Washington had higher densities of trees greater than 31.5 inches (80 cm) DBH and higher foliage volumes than rarely used sites (P<0.1). In low to mid-elevation conifer forests of northwestern California, nest stands had significantly more (P<0.005) conifers greater than 35 inches (90 cm) DBH than would be expected based on availability. In the eastern Cascade Range of Washington, the average height of the dominant canopy in small nest plots was significantly (P≤0.02) taller than canopy height on random plots within nest stands. The average size of trees greater than 39 inches (100 cm) DBH was significantly (P<0.001) larger at nest sites than random sites within home ranges in coniferous forests in Oregon and Washington. However, trees greater than 21 inches (>53.4 cm) DBH did not occur at significantly greater densities or basal areas on nest sites than on random sites. Trees from 25 to 126 feet (7.6–38.3 m) in height were significantly more dense on nesting sites than random sites (P<0.1), while density of trees in the tallest (>176 feet (53.7 m)) categories were similar on nesting and random sites.
Northern and California spotted owls may select habitats dominated by intermediate-sized trees in some areas. In the eastern Cascade Range of Washington, the average density of intermediate-sized (14 to 24 inches (35–60 cm) DBH) Douglas-fir trees in small northern spotted owl nest plots was significantly (P=0.03) greater than that in random plots within nest stands. Basal area of trees 21 inches (53.3 cm) DBH or smaller was significantly (P<0.001) greater on small nest plots than on random sites in coniferous forests in Oregon and Washington. California spotted owl home ranges contained significantly (P<0.001) more forests comprised predominantly of 11- to 21-inch (27.0-53.1 cm) DBH trees than expected based on availability in coniferous habitats within Tahoe National Forest. Although the amount of variation in tree DBH was similar on northern spotted owl nest sites and random sites in coniferous forests of Oregon and Washington, tree size variability may be important to spotted owls in some portions of their range.
Areas with more large trees may provide higher quality habitat. In previously logged forests in the northern Coast Ranges of California, northern spotted owls with greatest reproductive success had territories with a greater density of remnant large trees than less successful individuals. These differences were significant at the 120 acre (50 ha, P=0.042) and 208 acre (114 ha, P=0.052) scales. In contrast, areas with dense small trees may be associated with lower site fidelity. In the eastern Cascade Range in Washington, areas with few 5- to 7-inch (13–19 cm) DBH trees were used more often than those with more of these pole-sized trees. Abandoned territories in this study area contained significantly (P=0.049) more pole-sized stands than occupied territories. In conifer forests of southern Sierra Nevada, high foliage volume above the nest was related to California spotted owl nest success. In the Lassen National Forest, stands dominated by large (>24 inches (61 cm) DBH) trees were associated with greater occupancy and apparent survival, and those areas with more large remnant trees were associated with increased nest success. Nest areas dominated by small trees had lower site occupancy and reproductive output. However, pairs in stands dominated by medium-sized (12–24 inches (30–61 cm) DBH) trees had higher nest success than those in stands dominated by large trees.
Although a common feature of northern spotted owl nest plots, mistletoe infestation in nest plots was similar to infestation in unoccupied stands and nest stands in the Cascade Range. For instance, there was no significant difference (P>0.3) in mistletoe infestation rating between small northern spotted owl nest plots and nest stands, although 84% of nest plots were infested. Although the mistletoe infestation rate was not investigated, medium to large (>11 inches (27.5 cm) DBH) trees at California spotted owl foraging locations had significantly (P<0.001) lower vigor than those at random locations in coniferous habitats of Tahoe National Forest.
Areas used by spotted owls generally have greater than 40% canopy cover and often have more than 70% canopy cover. Despite variation in tree height and DBH, canopy cover varied little, ranging from 88% to 95% on small nest plots in grand fir forests of eastern Washington. Areas containing 90% of telemetry locations had greater than 40% canopy cover in this region. Review of several studies found that northern spotted owl nesting and roosting sites in the California Klamath and Coast regions had more than 80% cover. Canopy cover within 82 feet (25 m) of California spotted owl nest sites in foothill riparian and oak woodlands in the southern Sierra Nevada averaged 86%; and in white fir-mixed-conifer forests in northeastern California, canopy cover was often greater than 80% in a similar-sized area around nest sites. Cover of vegetation above 7 feet (2.1 m) is typically more than 70% at California spotted owl nest sites, although canopy cover as low as 30% to 40% has been observed at higher elevations. Although a review notes the occurrence of Mexican spotted owls in sparsely vegetated habitats, other reviews state that they occupy stands with more than 40% or 60% canopy cover. In ponderosa pine-Gambel oak forests of Arizona, canopy cover was greater than 40% with only two roost stands having canopy cover from 25% to 40%. In addition, 75% of roost stands had more than 60% canopy cover.
Spotted owls may select areas with high canopy cover. In coniferous forests in Oregon and Washington, canopy cover in small plots near northern spotted owl nests was significantly (P<0.095) greater than at random plots within spotted owl home ranges. However, when data were not pooled across study areas, only nest sites in the Olympic and Cascade regions had significantly (P<0.004) greater canopy cover than random plots. Six California spotted owls in coniferous habitats of the Tahoe National Forest used stands with more than 40% canopy cover significantly more, and stands with less than 40% canopy cover significantly less than expected based on availability (P<0.005). In addition, California spotted owl owl home ranges contained significantly (P<0.02) more forest with more than 70% canopy cover than expected. California spotted owls in coniferous forests in the Sierra Nevada consistently selected areas with high canopy cover. In northern Arizona, Mexican spotted owl nest stands averaged 75.2% canopy cover, which was significantly (P<0.001) higher than the 53.8% average canopy cover in random stands. In a ponderosa pine-Gambel oak forest in Arizona, stands with more than 60% canopy cover were used for roosting and foraging in both the breeding and nonbreeding seasons more than expected based on availability. Stands used for roosting had significantly (P≤0.03) greater canopy cover than stands that were not used for roosting. Other studies that have found greater canopy cover in areas used by Mexican spotted owls than random areas are summarized in.
Spotted owls may also select habitats with high live tree basal areas. The average live tree basal area on small plots at and around northern spotted owl nests in the eastern Cascade Range of Washington was significantly (P=0.09) greater than that of random plots within nest stands. In forests of the northern Coast Ranges in California, there was a significantly greater proportion of the >69 m²/ha basal area category within 17 acres (7 ha), and significantly lower proportions of the less than 23 m²/ha basal area class within 17 acres (7 ha) and 124 acres (50 ha) of owl sites compared to random sites (P<0.05). California spotted owls in the Sierra Nevada and in southern California nested and roosted in areas with greater average conifer and total live basal areas than random locations. In northern Arizona, basal area in Mexican spotted owl nest stands averaged 37.9 m²/ha, which was significantly (P<0.001) greater than the average 25.4 m²/ha in random stands. The basal area at Mexican spotted owl nests in the Basin and Range East region was significantly (P=0.0121) greater than in random forest stands, and in the upper Gila region the basal area at nests was significantly (P<0.0001) greater than within nest and random forest stands. In Douglas-fir/hardwood forests of northwestern California, the smaller northern spotted owl males foraged in stands with higher tree density than stands used by the larger females.
Investigations of the impact of canopy cover and basal area on spotted owl habitat have found different relationships. In forests in the northern Coast Ranges of California, activity centers of northern spotted owl pairs in the upper 50th percentile of reproductive success had higher proportions of the 23 to 69 m²/ha basal area classes and lower proportions of the >69 m²/ha basal area category. Many of these relationships were significant (P≤0.053) at scales from 124 to 983 acres (50-398 ha). In young forests of mainly western hemlock and Sitka spruce on the Olympic peninsula, sites used multiple times had an average canopy cover of 85.6%. Although high, this was significantly (P=0.03) lower than canopy cover on unused sites or sites within northern spotted owls' home ranges that had been used once. In coniferous forests in the Sierra Nevada, there were weak but significant correlations (0.29<r<0.37, P≤0.04) between canopy cover and California spotted owl reproduction. The amount of forests with 0 to 39% canopy cover was negatively correlated, and the amount of forest with more than 40% cover was positively correlated with reproductive output at the 3 scales investigated (178-1,063 acres (72 ha-430 ha)). There was about 10% more habitat with canopy cover >50% on sites that consistently produced young compared to unproductive sites. This difference was due to increased frequency of nesting pairs with increasing canopy cover, not higher reproduction by nesting pairs.
Although some Northern and California spotted owls select and avoid certain cover types, trends related to cover type or species composition are not consistent. Mexican spotted owls apparently respond more consistently to cover type. Roost sites in southern New Mexico occurred primarily in mixed-conifer forest, and owls selected mixed-conifer stands for roosting in all but 1 season/site combination. Selection of mixed-conifer habitats was also observed in the Black Range and San Mateo Mountains of New Mexico and in the upper Gila Mountains region of Arizona. In Arizona, the majority of foraging and roosting occurred in ponderosa pine-Gambel oak forests. Foraging and roosting stands in pine-oak areas also had generally higher Gambel oak density and basal areas than unused stands. Similar trends have been observed in the Upper Gila Recovery Unit and in the Black Range and San Mateo Mountains. Gambel oak's contribution to canopy cover, canopy layering, and prey habitat in mixed-conifer-hardwood and pine-oak communities likely influences its selection by Mexican spotted owls. A review notes selection of white fir in the Basin and Range East region and several firs (Abies spp.) in Utah. There was significantly more Douglas-fir, Gambel oak, and limber pine (Pinus flexilis) on roost sites than random sites in the upper Gila Mountains region. Roosting Mexican spotted owls may have selected areas with greater southwestern white pine densities in southwestern New Mexico. Ponderosa pine and pinyon-juniper woodland were avoided by Mexican spotted owl in southern New Mexico. Ponderosa pine was also more abundant in random stands than in nest stands in the Upper Gila Mountains and Basin and Range East recovery units. In southern New Mexico, Mexican spotted owls had greater survival, increased fecundity, and smaller home ranges in a mesic area dominated by mixed-conifer forest than in a xeric area dominated by ponderosa pine forests and Colorado pinyon-alligator juniper woodlands.
Northern spotted owls are strongly associated with multilayered forests. An uneven-aged, multiple-layered canopy is consistently included in descriptions of northern spotted owl habitat. Increased canopy layering was a significant predictor of northern spotted owl presence in Olympic National Park. In coniferous forests of northwestern Washington, the height class diversity, a measure of canopy layering, was significantly (P<0.1) greater in stands with more than 10% of telemetry locations compared to stands that were used less often. In coniferous forests on the eastern slope of the Cascade Range in Oregon and Washington, nest sites had more sapling, pole, and large trees, while unoccupied stands within 2,953 feet (900 m) of the nest site had more medium-sized trees. This indicates a more layered canopy in occupied stands. However, the average number of canopy layers in small northern spotted owl nest plots was not significantly different from that on random sites within nest stands in the eastern Cascade Range of Washington.
California spotted owls also used multilayered forests. Foraging sites of six California spotted owls had significantly (P<0.001) more vegetation layers than random locations in coniferous stands in the Tahoe National Forest. In foothill riparian and oak woodlands in the southern Sierra Nevada, California spotted owl nest sites occurred in areas with multilayered canopies. The subcanopy was a dense (743 stems/acre) layer of trees less than 5 inches (13 cm) DBH that averaged 13 feet (4 m) tall. The major canopy layer was moderately dense (129 stems/acre) and was composed of trees 5 to 30 inches (13–76 cm) DBH and 31 to 60 feet (9–18 m) tall. The upper canopy was sparse (1 stem/acre) and composed of trees 31 to 60 inch (79–152 cm) DBH and 68 to 102 feet (21–31 m) tall.
Areas occupied by spotted owls have varied understories. Tall shrub cover was significantly (P<0.098) greater on small plots around nest sites than on random sites within home ranges in coniferous forests in Oregon and Washington. In forests in the western Cascade Range, stands frequently used by northern spotted owls for foraging had lower herb and shrub cover than random stands in the same age class. In foothill riparian and oak woodlands in the southern Sierra Nevada, California spotted owl nest sites had little vegetative ground cover, with an average of 20% small shrub cover and 21% grass cover. An average of 60% of the forest floor was covered by small litter. Higher litter cover was found on roost sites than on random sites in southern Utah.
In some areas, northern spotted owls select habitats with more snags. For instance, in coniferous forests Oregon and Washington, basal area of relatively sturdy snags was significantly (P<0.001) greater in small plots near nests than in random sites within home ranges. In young coniferous forests on the Olympic Peninsula in Washington, sites used repeatedly by northern spotted owls had significantly (P=0.0007) more snags greater than 20 inches (51 cm) DBH than random or single-use sites. Similarly, in coniferous forest of Oregon, snags larger than 16 inches (40 cm) DBH were more abundant in stands used frequently for foraging than in random sites in forests of the same age class. Snag volume was also greater (P<0.1) on northern spotted owl high use sites than on less often used sites in coniferous forests of northwestern Washington. Stands with snag basal areas less than 142.1 m³/ha were used less frequently for foraging. Basal area of highly decayed snags (P<0.001) and density of small snags (P=0.08) were greater on small nest plots in the eastern Cascade Range of Washington than on random plots within nest stands. However, basal area of hard snags was significantly lower (P<0.01) on nest plots. Densities of larger snag size classes, and basal areas of snags in other decay classes were similar on nest sites and random sites within nest stands. In coniferous forests of Washington and Oregon, the abundance of snags was similar on nest sites and unoccupied stands within a 0.6-mile (0.9 km) radius. In old-growth coniferous forests of Olympic National Park, snag diameter was a significant predictor of owl presence. However, the relationship changed with study area, with larger snags associated with owl presence on the eastern side of the Bailey Range, and smaller snags associated with owl presence on the western side.
California and Mexican spotted owls may select habitats with abundant snags. Snag basal area at California spotted owl nest and roost sites was significantly (P<0.05) greater than at random sites. Foraging locations in coniferous forests of the north-central Sierra Nevada also had significantly (P<0.001) greater snag basal area than random locations. In foothill riparian and oak woodlands in the southern Sierra Nevada, California spotted owl nest sites averaged 14 snags/acre. A review and analysis of habitat data tentatively recommends snag basal areas in Sierra mixed-conifer forests of 15 to 30 ft²/acre in foraging habitat and 30 to 55 ft²/acre in roosting and nesting habitat. It also suggests that snags greater than 15 inches (38 cm) DBH comprise 7 to 17 ft²/acre of foraging stands and 20 to 30 ft²/acre of nesting and roosting stands. Mexican spotted owl roosting and foraging sites in coniferous forests of Arizona had significantly (P<0.001) greater snag densities and basal areas than random sites. A review reports significantly (P<0.0001) greater average snag densities in areas close to Mexican spotted owl nest sites in the Upper Gila Mountains Recovery Unit in Arizona and New Mexico. In small nest plots, snag density averaged 63.9/ha. Nest stands averaged 44.0 snags/ha and random stands averaged 17.6 snags/ha. Snag basal area was also significantly (P=0.0003) higher in nest stands than in random stands. Occupied canyons in Zion National Park also had higher snag basal areas than canyons where Mexican spotted owls were not detected.
Northern spotted owl may select habitats with more coarse woody debris in some areas, possibly due to the apparent dependence of northern flying squirrels (Glaucomys sabrinus) and other prey species on coarse woody debris for cover and truffle production. In coniferous forests in Oregon and Washington, the volume of highly decayed logs was significantly (P≤0.025) greater in small plots near northern spotted owl nests than in random sites within home ranges. The volume of downed logs with diameters greater than 20 inches (50 cm) was significantly (P=0.0002) greater on nesting and frequently used foraging sites than on random sites in coniferous forest in the western Cascade Range in Oregon. However, in the eastern Cascade Range of Washington, the average volume of coarse woody debris in small nest plots was similar to that in random plots within nest stands. Log abundance was also similar in nest stands and unoccupied stands within a 0.6-mile (0.9 km) radius of nest sites in coniferous forests on the eastern slope of the Cascade Range in Washington and Oregon.
Coarse woody debris is likely an important feature of Mexican and California spotted owl habitats. In coniferous forests of Arizona there were significantly (P<0.001) more logs with greater than 12-inch (30.5 cm) diameters and lengths of 10 or more feet (3 m) on sites used by eight Mexican spotted owls for roosting and foraging than on random sites. The average amount of coarse woody debris was 97.8 m³/ha at nest sites and 94.7 m³/ha in nest stands in the Upper Gila Mountains Recovery Unit of Arizona and New Mexico. This was significantly greater (P=0.0006) than the average of 54.6 m³/ha in random stands. Nest sites in the Basin and Range East Recovery Unit in New Mexico also had higher (P=0.0061) log volumes than random stands. Six California spotted owls selected 0.1 acre (0.04 ha) foraging plots with significantly (P<0.001) more cover of coarse woody debris than on random sites in the Tahoe National Forest. Winter roost sites in the Eldorado National Forest also had significantly (P<0.05) more coarse woody debris than random sites. Another review states that coarse woody debris is more influential in coniferous forests than in riparian/hardwood habitats, due to the role of hypogeous fungi in supporting spotted owl prey in coniferous forests. In foothill riparian and oak woodlands in the southern Sierra Nevada, the amount of coarse woody debris greater than 10 inches (25 cm) in diameter ranged from 156 to 331 ft³/acre.
Although similar to nesting and roosting habitat requirements, foraging habitat requirements are likely less strict. Like nesting and roosting sites, foraging areas are generally older, have higher canopy covers, greater tree densities, and more snags and coarse woody debris than random sites. In the Coast Ranges of southern Oregon, northern spotted owls foraged in forests older than expected. In northwestern California, foraging sites were older than sites that were infrequently used. However, California and northern Spotted owls may use younger forests for foraging than for nesting or roosting. For instance, stands as young as 27 years old were used by foraging northern spotted owls in coniferous forests of the western Cascade Range in Oregon. Although Mexican spotted owls foraged in old stands and stands with more than 60% canopy cover in ponderosa pine-Gambel oak forests in Arizona, canopy cover on foraging sites was lower than on roosting sites. California spotted owls did not select areas with greater canopy cover at the patch scale (≥5 acres, 2 ha) as consistently for foraging as they did for roosting in the Sierra Nevada. Log volume and snag basal area and density were similar on foraging and roosting sites on sites in Arizona and New Mexico, but stands with less than 60 ft²/acre basal area, less than 25% canopy cover, and very high Gambel oak densities were used for foraging and not for breeding-season roosts. Mexican spotted owl foraging sites also differed from roosting sites in study areas near Flagstaff and Alpine, Arizona, with large coarse woody debris, density of snags, tree density, and canopy cover significantly (P<0.001) higher on roosting sites. Some sources suggest that foraging habitat has more open area under the canopy to allow for spotted owl flight. Increased habitat heterogeneity may also be important to foraging spotted owls by providing more varied prey. Predominant prey is likely to contribute to differences in spotted owl habitat selection across their range, since spotted owl prey species occur in a variety of habitats. For instance, in coniferous forests of southwestern Oregon and the Olympic Peninsula in Washington, flying squirrel densities in old-growth forests were about twice those in younger stands, while dusky-footed woodrats (Neotoma fuscipes) in northwestern California tend to occur in early-successional shrublands. Young forest stands did not positively affect California spotted owl reproduction or site occupancy in a white fir-mixed-conifer study area on the Lassen National Forest. Northern flying squirrel was the major prey item in this area. In contrast, in areas where dusky-footed woodrats are a substantial component of the diet, such as in some areas of northwestern California, some interspersion of younger or more open stands with mature and old-growth forests is likely beneficial. Northern spotted owl home range size in Douglas-fir and mixed-conifer study areas in southwestern Oregon and northwestern California was inversely correlated with proportion of woodrats (Neotoma spp.) in the diet (r=–0.8, P<0.005). It is suggested that the larger mass and tendency of woodrats to occur at higher densities than flying squirrels makes for more efficient foraging and allows for smaller home ranges. The 1,700-ha area of old growth used by northern spotted owls in western hemlock forests of Washington was much greater than the 500-ha area of old growth used in mixed-conifer and Douglas-fir forests of Oregon, possibly due to the greater prey biomass density in the Oregon study area (388 g/ha) than the Washington study area (61 g/ha).
The habitat of Northern and California spotted owls during the nonbreeding season is generally similar to breeding-season habitat, with some minor differences. Mexican spotted owls winter in lower elevation habitats that are more open and shrubby than breeding season habitats. Based on a review of radio-telemetry data from studies in Arizona, New Mexico, Colorado, and Utah, roost stands used by Mexican spotted owls in the nonbreeding season had less basal area of live trees and hardwoods and lower canopy cover than breeding season roost stands. In addition, there were more forests with more than 60% canopy cover in breeding season ranges than in winter ranges. In ponderosa pine-Gambel oak forests of Arizona, roosting sites occurred in more variable habitats in the nonbreeding season than in the breeding season. Nonbreeding season roost sites occurred in areas with basal areas less than 60 ft²/acres, less than 25% canopy cover, and very high oak densities, while these habitats were not used for roosting during the breeding season.
Spotted owls may disperse through vegetation that is more open than typical habitat. Several articles note northern spotted owl dispersal through cover types generally considered unsuitable habitat, including relatively open areas. Mexican spotted owls dispersed through pinyon-juniper woodlands, mountain shrubland, desert scrub, and subalpine and mixed-conifer forests in southern Utah.
Northern spotted owls apparently select large habitat patches. On Washington's Olympic Peninsula, the average area of habitat patches in 8,038 acre (3,253-ha ) circles centered on pair locations was significantly (P<0.01) larger than in circles centered on random sites. In Douglas-fir forests of northern California, frequency of occurrence increased as stand size increased (P<0.1) from <25 acres (10 ha) to 52 to 247 acres (21-100 ha).
Less is known about California and Mexican spotted owls' relationships to patch size. In mixed-conifer forests of the central Sierra Nevada, 1,129 acre (457 ha) areas used by California spotted owls were not significantly (P=0.547) larger habitat patches than random areas. However, in a model of California spotted owl dynamics and reserve design, persistence was greater when reserved areas were arranged in fewer larger patches than when they occurred in more smaller patches. Nest stands in study areas throughout California averaged 100 acres (40.5 ha). A review suggests that during winter, smaller forest patches may be used than in the breeding season.
Although the results are mixed, spotted owls may be negatively impacted by fragmentation and isolation. This has most often been observed at scales from 500 (200 ha) to several thousand acres. In Douglas-fir/hardwood and mixed-conifer forests of northwestern California, northern spotted owl nest sites were significantly (P<0.01) less fragmented than random plots at approximately 500 acre (200 ha) and 1,100 acre (450 ha) scales. At approximately 2,000- to 10,100 acre (800-4,100 ha) scales, fragmentation on northern spotted owl nest sites and random sites were not significantly (P>0.05) different. A similar pattern was observed in Douglas-fir forests of northern California, with no significant associations between northern spotted owl frequency and percent clearcut or total edge at large (2,500 acre (1,000 ha)) scales. Northern spotted owl frequency also had no significant association with distance to a clearcut or total length of edge within the plot at small scales (25 acre (10 ha)). However, at the stand scale, northern spotted owl frequency was negatively associated with a measure of the percentage of the stand's perimeter that bordered clearcuts (P<0.1). Although several measures of fragmentation did not differ (P≥0.14) between northern spotted owl and random areas at a large scale in the Olympic Peninsula in Washington, a measure of northern spotted owl habitat isolation was significantly (P<0.01) lower in 8,038 acre (3,253-ha) circles centered on pair locations than in random circles. Effects at large scales were observed in mixed-conifer and Douglas-fir landscapes in southwestern Oregon. Northern spotted owls in fragmented areas had lower densities, less home range overlap between members of the same pair, higher home range overlap between members of neighboring pairs, and greater incidence of mate changes than owls in study areas with clumped habitat.
Spotted owls may be more selective of habitat in areas closer to their nest sites. In mixed-conifer and ponderosa pine-Gambel oak forests of north-central Arizona, Mexican spotted owls were most selective within about 500 acres (200 ha) surrounding the nest site. In California's Klamath physiographic region, differences in habitat selected by northern spotted owls and those available at random were greatest at the 200-ha scale compared to the 500- and 900-ha scales. However, sample sizes were larger at the 200-ha scale.
Spotted owl home ranges are generally large, but sizes are variable. The average home range size of northern spotted owl pairs varies from 1,030 acres (417 ha) in coniferous forests of Oregon to 14,169 acres (5,734 ha) on Washington's Olympic Peninsula. In riparian hardwood forests of the Sierra National Forest, California spotted owl had comparatively small home ranges, varying from 661 to 985 acres (267–399 ha), while those in mixed pine, white fir, and California red fir forests of the Lassen National Forest had home ranges varying from 7,061 to 12,473 acres (2,857–5,048 ha). Median California spotted owl pair home range sized up to 18,706 acres (7,570 ha). A Mexican spotted owl review includes individual home range estimates from 645 acres (261 ha) in the upper Gila Mountains to 3,672 acres (1,487 ha) on the Colorado Plateau. Pair home range estimates ranged from 2,548 acres (1,031 ha) in Arizona to 2,780 (1,125 ha) in New Mexico. In some cases, Mexican spotted owls can spend a substantial portion of their time in a small portion of their home range. For example, in riparian areas, pinyon-juniper, and mixed-conifer woodlands of southern Utah, 70% of radio locations occurred within an area averaging 689 acres (279 ha), which is less than one-third of the 2,179 acre (882 ha) area that was occupied by 95% of radio locations.
Spotted owls have smaller home ranges in the breeding season than in the nonbreeding season. For example, in the western hemlock zone of the southern Oregon Coast Ranges, average northern spotted owl breeding season home range was 1,497 acres (606 ha), compared to an average nonbreeding season home range size of 3,509 acres (1,420 ha). Radio-telemetry data on Mexican spotted owls from studies in Arizona, New Mexico, Colorado, and Utah resulted in estimates of breeding season home ranges that varied from 563 to 1,250 acres (228–506 ha), while nonbreeding season home range estimates varied from 902 to 2,540 acres (365-1,028 ha). In a ponderosa pine-Gambel oak forest of Arizona, nonbreeding home range size were significantly (P=0.008) larger than breeding home ranges. However, in the Sierra National Forest there was an interaction between cover type and season on California spotted owl territory size, such that nonbreeding territories were larger than breeding territories in mixed-conifer forest but smaller in oak/pine woodland.
Other sources of variability in home range size include habitat requirements and prey availability. Spotted owls likely require a certain amount of old-growth forests. A literature review states that a pair of northern spotted owls uses an average of 2,000 to 2,500 acres (10 km2) of old-growth forests. In areas where this resource is clumped and abundant, spotted owl home ranges are generally smaller. For example, in coniferous forests in southwestern Oregon, northern spotted owl home range size varied from 1,320 acres (533 ha) in clumped mixed-conifer forest to 7,190 acres (2,908 ha) in fragmented Douglas-fir forest. Several articles note the inverse relationship between home range sizes and the amount of old growth within them. Spotted owls also require less area in certain cover types. For instance, Mexican spotted owl home ranges in the Sacramento Mountains of New Mexico dominated by mixed-conifer forests were significantly (P<0.04) smaller than those dominated by ponderosa pine forests and pinyon-juniper woodland. In addition, home range size was inversely related (P≤0.003) to the amount of mixed-conifer forest in the home range. The variation between different habitat types may be related to the availability of spotted owl prey. For instance, the low numbers and inconsistent availability of prey on Washington's Olympic peninsula was suggested as a cause for the large northern spotted owl home ranges in this region.
Female spotted owls may have larger home ranges than males. In mixed-evergreen forests of northwestern California, female northern spotted owl summer home ranges averaged 1,329 acres (538 ha), which was larger than the average male home range of 835 acres (338 ha). In pine-oak forest in Arizona, female Mexican spotted owls had considerably larger (P=0.073) home ranges than males. However, there was no difference in the size of male and female California spotted owl home ranges in mixed-conifer and oak-pine woodlands of the Sierra National Forest and mixed pine, red and white fir in the northern Sierra Nevada.
The amount of overlap between members of pairs and between adjacent owls varies with season. In the western hemlock zone of the southern Oregon Coast Ranges, home range overlap of northern spotted owl pairs was 74% to 97% during the breeding season and 64% to 91% during the nonbreeding season. In the Coast and Cascade ranges in western Oregon home range overlap averaged 68% between members of a pair and 12% for individuals in neighboring territories. Home range overlap between members of California spotted owl pairs in coniferous forests of the Tahoe National Forest was 47% to 63%.
Spotted owl densities also vary with habitat and location. In mixed-conifer and Douglas-fir forests in southwestern Oregon, density of resident northern spotted owl pairs ranged from 0.046 pairs/km² in fragmented Douglas-fir forest to 0.190 pairs/km² in clumped mixed-conifer habitat. In coastal redwood forests, Douglas-fir forests, and oak woodlands of northwestern California, average density of northern spotted owls was 0.209 owls/km², but varied from 0.092 to 0.351 owls/km² across subregions. Ecological densities, defined as owls per area of suitable habitat, differed significantly (P<0.001) across the subregions and varied from 0.373 to 1.049 owls/km². Mexican spotted owl densities averaged 0.275 owls/km² in mixed-conifer forests, 0.08 owls/km² in pine forests, and 0.022 owls/km² in pinyon-juniper woodlands of the Sacramento Mountains New Mexico.
|This section may contain an excessive amount of intricate detail that may only interest a specific audience. (June 2014)|
Spotted owls do not build their own nests. They rely on sites such as trees and snags with cavities or broken tops, and platforms associated with abandoned raptor or squirrel nests, witches' brooms (caused by mistletoe infection) and debris accumulations. Large, old trees are most often used by spotted owls for nesting. Species used as nest trees vary with region and subspecies.
Several studies indicate that tree cavities are most commonly used for nesting by spotted owls, while the extent of platform use varies. In coniferous forests in Oregon, 60% to 93% of nests were in trees with broken tops. Additionally, broken-topped trees (>21 inches (53.3 cm) DBH with 1 or more secondary crowns) had significantly (P<0.001) higher basal area and density in small plots on and around nest sites than in random plots within spotted owls' home ranges. Platform use may be more common in areas that lack large, old trees and snags and have a greater abundance of witches' brooms. Compared to other habitats within their range, northern spotted owls use platforms more often in mixed-evergreen and mixed-conifer forests. California spotted owls in southern California use platforms more frequently than those in the Sierra Nevada. Platform use also occurred more frequently in oaks than in conifers in the southern Sierra Nevada. The average DBH of California spotted owl platform nest trees was significantly (P<0.01) smaller than that of cavity nest trees in foothill riparian and oak woodlands in the southern Sierra Nevada. In grand fir-dominated stands in eastern Washington, northern spotted owls nested in witches' brooms on trees as small as 12 inches (30 cm) DBH. Mexican spotted owls use cliffs and comparatively open areas as nest sites more frequently than the other subspecies. Fletcher and Hollis (1994, as cited by) found 9.7% of 248 Mexican spotted owl nests in cliffs, while Steger and others noted only 1 out of 41 California spotted owl nests in a rock cliff in the southern Sierra Nevada.
Spotted owls often nest in old trees in mature and old-growth forests. Sixty-five percent of northern spotted owl nests sites in coniferous forests of Oregon were in trees greater than 120 years old. On two sites in the Coast and Cascade Ranges in western Oregon, 90% of nest sites were in unmanaged old-growth forests, 4% were in mature forests, and 6% were in late-successional forests (70–80 years) with 5 or fewer residual old-growth trees per hectare. In low- to mid-elevation coniferous forests of northwestern California, the minimum nest tree age averaged 288 years, with a range of 57 to 688 years. In coniferous forests in the Cascade Range of southwestern Washington, northern spotted owl site centers, such as the nest tree or locations of fledged young, did not occur in stands less than 49 years old, and 31% were in stands greater than 180 years old. Most species of nest trees used by nesting California spotted owls in oak woodland and coniferous forests of the southern Sierra Nevada averaged more than 227 years of age.
Selection of nest tree species varies with spotted owl subspecies and location. In Oregon, California, and most of Washington, more than 80% of northern spotted owl nests occurred in Douglas-fir. However, on the Olympic Peninsula in Washington, where Douglas-fir was not present in all stands, northern spotted owl nests occurred with about equal frequency in Douglas-fir, western hemlock, and western redcedar. A review states that California spotted owl's use of nest trees seemed related to availability, with Douglas-fir used most often in northern Sierra Nevada coniferous forests, and bigcone Douglas-fir, Jeffrey pine, and live oak (Quercus chrysolepis and/or Q. agrifolia) used primarily in southern California coniferous forests. California spotted owls nested most frequently in pines and firs in white fir-mixed-conifer stands in the Lassen National Forest. In foothill riparian and oak woodlands in the southern Sierra Nevada, nests occurred in California sycamore (Platanus racemosa, 5%), ponderosa pine (5%), and several oaks (88%) including interior live oak (Q. wislizenii, 34%), California black oak (Q. kelloggii, 22%), and canyon live oak (20%). In oak woodland and coniferous forests of the southern Sierra Nevada, California spotted owls selected giant sequoia (Sequoiadendron giganteum) and California black oak trees, with nests occurring in these species significantly (P<0.05) more often than their relative abundance. Habitat associations in Arizona and New Mexico, 50% of nests were in Douglas-fir, 20% in Gambel oak, and 19% in white fir. A study in ponderosa pine-Gambel oak communities found 6 of 11 nests in Gambel oak and the rest in ponderosa pine, while in the Tularosa Mountains of New Mexico, 78% of nests were in Douglas-fir, 11% were in white fir, 7% were in ponderosa pine, and 4% were in southwestern white pine.
Many of the habitat characteristics discussed in the Preferred Habitat section have been observed in the area immediately around nest sites. For instance, overhead cover at Northern and California spotted owl nest sites is high. In oak woodlands above-nest foliage volumes of >900 m³ in a 0.12 acre (0.05-ha) plot around nest sites were associated with greater reproductive output. Tree density was also higher near California spotted owl nest sites in the Sierra Nevada. Snags and coarse woody debris are also more abundant near some spotted owl nest locations.
Some spotted owl nest sites are used repeatedly. California spotted owls may use the same nest annually, rotate the nests used, or use new nests each year. Nests in oak woodland and coniferous forests of the southern Sierra Nevada used in 3 or more seasons had reproductive outputs more than twice those of nests used only once. Although nests can be used multiple times, the attrition rate may be high. Over the course of an eight-year study in the Coast and Cascade Ranges in western Oregon, 17% of nest sites deteriorated to the extent that they were no longer usable.
Characteristics of spotted owl roosting cover are similar to those of nesting cover. For example, in Douglas-fir/hardwood forests of northwestern California, northern spotted owl roost plots had significantly (P<0.05) greater canopy cover than infrequently used plots. In coniferous forests of Washington and Oregon, 86% to 88% of roost sites were in old forests, which were dominated by trees greater than 39 inches (100 cm) DBH and included patches of younger forests less than 10 acres (4 ha) in size. In the central Sierra Nevada, California spotted owl nesting and roosting plots had similar canopy cover, snag basal area, ground cover of vegetation and coarse woody debris, and basal area, size, and condition of trees with DBH 11 inches (27.5 cm) or greater. California spotted owl selection for high canopy cover was more consistent for roosting sites than nesting sites, according to data from the central Sierra Nevada and southern California. In the central Sierra Nevada, California spotted owl roost plots occurred in mixed-conifer forest dominated by trees larger than 12-inches (30 cm) DBH significantly (P<0.001) more than random sites. In coniferous forests of Arizona, Mexican spotted owl roost plots had greater (P≤0.05) canopy cover, coarse woody debris, and tree and snag densities than random plots. In Saguaro National Park, roost plots had greater canopy cover, more trees, more vegetation layers, and greater basal areas than random plots. In addition, trees in the roost plots were taller and had larger DBH than those on random sites (P≤0.05). However, Mexican spotted owl roost trees are generally smaller than nest trees. Spotted owl roosting and nesting habitat are also similar at larger scales.
In mixed-conifer and Douglas-fir forests of southwestern Oregon, northern spotted owl roost sites averaged 45 feet (13.7 m) above ground and ranged from ground level to 256 feet (78 m). Roost trees averaged 24 inches (61 cm) DBH and 83 feet (26 m) tall. These roost sites averaged 69% overhead cover.
Spotted owls often roost in Douglas-fir, but several species are used. In mixed-conifer and Douglas-fir forests of southwestern Oregon, 46% of northern spotted owl roost sites were in Douglas-fir. Thirteen percent of roosts occurred in western hemlock and 9% were in western redcedar. Northern spotted owls also roosted in bigleaf maple (Acer macrophyllum), incense-cedar (Calocedrus decurrens), grand fir, giant chinkapin (Chrysolepis chrysophylla), Pacific madrone (Arbutus menziesii), and canyon live oak. In Saguaro National Park about 60% of Mexican spotted owl roosts were in Douglas-fir. Gambel oak, southwestern white pine, ponderosa pine, New Mexico locust (Robinia neomexicana), and a white pine snag were also used for roosting. In the upper Gila Mountains, 54% of roosts were in Douglas-fir and 21% were in Gambel oak.
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Spotted owls are nocturnal, sit-and-wait predators. They often hunt from a perch and swoop or pounce on prey. They also grab arboreal prey from tree boles and limbs. They do occasionally hunt during the day and will hawk prey such as insects or bats.
Diet may affect spotted owl's reproductive output. In California, the average prey size was significantly (P<0.01) larger during years that northern and California spotted owls bred (x=115 g) compared to nonbreeding years (x=75 g). Although mean prey biomass of nesting northern spotted owls was generally higher than that of non-nesting owls in coniferous habitats of the Olympic peninsula and eastern Cascade Range in Washington, the difference was only significant (P<0.05) in 2 of 21 territories. Nest success in coniferous forest, oak woodland, and riparian deciduous habitats of the Sierra National Forest was not related to the northern flying squirrels, proportion of biomass.
Although spotted owls' diet varies with location, the majority consists of a few mammalian species. Species taken most often are northern flying squirrels and woodrats, including dusky-footed, bushy-tailed, (N. cinerea), and Mexican woodrats (N. mexicana). Northern flying squirrels commonly comprise greater than 30% of the diets (all percentages are by mass) of northern spotted owls and California spotted owls in conifer forests of the Sierra Nevada. Woodrats comprise the majority of the diet of all three subspecies, in at least some portion of their ranges. In mixed-conifer habitats of northwestern California and the Klamath Mountains of Oregon, dusky-footed woodrats comprised more than 50% of northern spotted owl diets. Woodrats comprise the predominant portion of California spotted owl and Mexican spotted owl diets.
In some portions of its range, much of the spotted owl's diet is composed of several other mammal species. White-footed mice (Peromyscus spp.), such as deer mice (Peromyscus maniculatus), comprised 2% to 30% of the total prey by northern spotted owl and 16.6% to 30.7% by California spotted owl. This genus represents from 0.7% to 6% of northern and California spotted owl diets. White-footed mice generally occur more frequently and comprise a larger portion of the diet of Mexican spotted owls than that of the other two subspecies, with percent biomass as high as 17.3% for Mexican spotted owls in pine-oak forests of northern Arizona. Pocket gophers (Thomomys spp.) comprised up to 16.2% of northern spotted owl diets, 18.5% of California spotted owl diets, and 15.5% of Mexican spotted owl diets. Although voles (Microtus, Clethrionomys, and Phenocomys spp.) comprised up to 31.0% of the total prey by northern spotted owl and 37.6% by Mexican spotted owl, mass percentage of voles in spotted owl diets is generally less than 5%. However, at two sites in Arizona, voles made up 15% and 16.1% of the Mexican spotted owl diets (Microtus spp.). In Washington, snowshoe hares (Lepus americanus) comprised up to 16.3% of the northern spotted owl diets. On one study area in Oregon 24.9% of prey biomass was snowshoe hares and brush rabbits (Sylvilagus bachmani), and 6.2% in the Klamath Mountains was brush rabbits. Rabbits (Sylvilagus spp.) comprised up to 29.6% of Mexican spotted owl diets, although values in the 7% to 8% range are more common. Red tree voles (Arborimus longicaudus) represented as much as 10.3% of northern spotted owl diets in Douglas-fir and western hemlock forests of Oregon and averaged 4% in the Coast Ranges of northwestern California. For more detail on variation in the use of red tree voles across the northern spotted owl's range in Oregon see. Red tree voles do not occur within the range of California or Mexican spotted owls. Although squirrels such as Tamiasciurus and Tamias do not usually comprise substantial portions of the diet, these and other diurnal species making up fairly high percentages (5–20%) of the California spotted owl's diet in some areas. Northern and California spotted owls occasionally consume moles and shrews, and Mexican spotted owls infrequently eat bats.
Nonmammalian prey species include birds, amphibians, reptiles, and insects. Birds, such as smaller owls (Strigidae), jays (Corvidae), songbirds (Passeriformes), and woodpeckers (Picinae) comprise less than 10% of the spotted owl's diet. Insects can occur at fairly high frequencies in owl pellets but generally make up a very small percentage of spotted owl diets. Amphibians and reptiles are rarely preyed upon.
The extent to which various taxa are eaten by spotted owls varies temporally. Both annual and seasonal variation in the composition of spotted owl diets has been observed. For instance, pocket gophers, voles, insects, and rabbits are hunted more by spotted owls in the summer than in the winter. In coniferous forests of the Sierra National Forest, birds comprised 12.9% of the California spotted owl's diet during the breeding season but only 4.6% during the nonbreeding season.
Birds and mammals, such as the fisher (Martes pennanti), are likely predators of eggs and young spotted owls. Northern goshawks (Accipiter gentilis) and crows may prey on juvenile spotted owls, while great horned owls (Bubo virginianus), red-tailed hawks (Buteo jamaicensis), and golden eagles (Aquila chrysaetos) are likely predators of both juvenile and adults. Great horned owls and barred owls likely compete with spotted owls for food and space in some areas. Barred owls may have a negative effect on northern spotted owl survival and fecundity in some areas.
The northern spotted owl and Mexican spotted owl subspecies are listed as threatened in the United States under the Endangered Species Act (ESA), which is administered by the United States Fish and Wildlife Service (USFWS). The California spotted owl is not considered to be threatened nor endangered by the USFWS. However, it is a species of special concern by the state of California and the United States Forest Service (USFS). The northern spotted owl was one of a few cases where the "God committee", a provision of the Endangered Species Act, has been invoked to decide whether or not to open up more federal forest for commercial logging. In a battle between two federal agencies, the Bureau of Land Management and the Fish and Wildlife Service, the committee ruled for the exemption of 1700 acres to the ESA, potentially allowing the extinction of the species. Legal battle went on at different levels, creating a complex case study in environmental law.
As of 2006[update] the northern spotted owl is in rapid decline with about a 50% annual population loss along the northern edge of its range (northern Washington state and south-western British Columbia). Fewer than 30 breeding pairs remained in British Columbia, and the species was expected to be extirpated from Canada within a few years.
All subspecies of the spotted owl are often the subject of disagreement between conservationists and loggers, cattle grazers, developers, and other organizations whose activities can affect forest conservation. In February 2008, a federal judge reinforced a U.S. Fish and Wildlife Service decision to designate 8,600,000 acres (35,000 km2) in Arizona, Utah, Colorado and New Mexico as critical habitat for the owl. The decision had been challenged by the Arizona Cattle Growers' Association, but Judge Susan Bolton upheld the designation. According to the Center for Biological Diversity, "having critical habitat will ensure that U.S. Forest Service logging does not limit the bird's recovery or drive it into extinction."
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