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"The birds and the bees" is an English-language idiomatic expression and euphemism that refers to courtship and sexual intercourse, and is usually used in reference to teaching someone, often a child, about human (and/or animal) reproduction and the reproductive system. The "birds and the bees talk" is generally the event in most children's lives in which they learn the truth about "where babies come from." The phrase is evocative of the metaphors and euphemisms often used to avoid speaking openly and technically about the subject.
According to tradition, the birds and the bees is a metaphorical story sometimes told to children in an attempt to explain the mechanics and good consequences of sexual intercourse through reference to easily observed natural events. For instance, bees carry and deposit pollen into flowers, a visible and easy-to-explain example of male fertilisation. Another example, birds lay eggs, a similarly visible and easy-to-explain example of female ovulation.
Word sleuths William and Mary Morris hint that it may have been inspired by words like these from the poet Samuel Coleridge (1825): 'All nature seems at work ... The bees are stirring—birds are on the wing ... and I the while, the sole unbusy thing, not honey make, nor pair, nor build, nor sing.'"
Even earlier instances of this idiomatic expression appear in the Cavalier poet, Thomas Carew's (pronounced Carey) work, "The Spring" (c.1640), in which, Carew uses earth and its change of seasons as a metaphorical depiction of women and their sensuality (The Norton Anthology of English Literature 1696). To abet his ends, Carew alludes to the "birds in the bees" in lines 7-8 with the use of "swallow", "cuckoo", and "humble-bee" as seen here (lines included are 5-8): "But the warm sun thaws the benumbed earth/And makes it tender; gives a sacred birth/To the dead swallow; wakes in hollow tree/The drowsy cuckoo and the humble-bee/Now do a choir of chirping minstrels bring" (emphasis added; lines 5-9 from "The Spring").
Dr. Emma Frances Angell Drake (b. 1849) wrote a section of a publication called The Story of Life which was published in 1909. This piece was later picked up and included in Safe Counsel, a product of the Eugenics movement in the late 19th and early 20th century. The author tells her daughters "when you discovered the tiny blue eggs in the robin's nest and I told you that wrapped in each shell was a baby robin that was growing there, kept warm by the mamma bird..." the narrative continues on in vague terms without actually describing sexual intercourse. Later she describes the father's role in reproduction like this; "Sometimes it is the wind which blows the pollen dust from one plant to the other, and sometimes it is the bees gathering honey from the flowers. As they suck the honey from the blossoms some of the plant dust sticks to their legs and bodies, and as they go to another plant in search of sweets this is rubbed off and so the parts of the father and mother plant get together and the seed is made fertile." Safe counsel was reprinted at least 40 times from 1893 through 1930 and may have been widely enough repeated to have contributed to the euphemism, "the birds and the bees." 
Several sources give credit to Cole Porter for coining the phrase. One of the musician's more famous songs was "Let's Do It, Let's Fall in Love." In Porter's publication from 1928, the opening line for the chorus carried derogatory racial references like "Chinks" and "Japs", later changed following CBS recommendation and NBC adaptation:
And that's why birds do it, bees do it
Even educated fleas do it
Let's do it, let's fall in love