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Cannibalism is a behavioural problem that occurs most frequently amongst domestic hens reared for egg production, although it can also occur amongst domestic turkeys, pheasants reared as gamebirds, and other poultry species. Cannibalism occurs when one bird pecks at the skin of another and devours its flesh. Cannibalism can follow on from severe feather pecking which has caused denuded areas on the bird. Cannibalism can result in significant mortality within the flock and also causes a decrease in production as the pecked birds become stressed. Vent pecking, sometimes called 'cloacal cannibalism', is considered to be a separate form of cannibalistic pecking as this can be observed in well-feathered birds and the vent or cloaca is targeted exclusively.
Poultry species which exhibit cannibalism are omnivores. For example, hens in the wild often scratch at the soil to search for seeds, insects and even larger animals such as lizards or young mice, although they are mainly herbivorous in adulthood. Feather pecking is often the initial cause of an injury which then attracts the cannibalistic pecking of other birds – perhaps as re-directed foraging or feeding behaviour. In the close confines of modern farming systems, the increased pecking attention is easily observed by multiple birds which join in the attack, and often the escape attempts of the cannibalised bird attract more pecking attention.
Chicks brooded with a hen had lower mortality levels due to feather pecking and cannibalism compared to non-brooded chicks. This may indicate the hen guides the chicks to peck at more rewarding substrates, such as food or litter.
Cannibalism among layer hen flocks is highly variable and when it is not problematic, mortalities among production systems are generally similar. Published data on the prevalence of cannibalism could be misleading due to the inclusion of vent-pecking by some researchers but not others. Mortalities, due mainly to cannibalism, can be up to 15% in egg laying flocks housed in aviaries, straw yards, and free-range systems. Because egg laying strains of chickens can be kept in smaller group sizes in cage systems, cannibalism is reduced leading to a lowered trend in mortality as compared to non-cage systems. In a study which examined ‘skin damage’ (most of which would have been cause by pecking) on hens at the end of their productive life, damage was worst in hens from free range systems, followed by barns, furnished cages and best in conventional or battery cages.
Beak-trimming is the most common method of preventing or reducing injuries by cannibalism. In a three-year study of floor-housed laying hens, death by cannibalism was reported as 7% in beak-trimmed birds but was increased to 18% in non-trimmed birds.
Increased group sizes in larger cages or floor systems can elevate the risk of cannibalism and feather pecking, probably due to the spread of the behaviour through social learning.
Lights are sometimes provided in nest-boxes to attract hens to use the nests, but this practice has been correlated with an increased risk of cannibalism.
Rearing chicks with access to perches by four weeks of age has been associated with increased use of perches, and reduced cannibalism, in adulthood.
A sib-selection programme has genetically selected a low mortality line which shows decreased mortality from cannibalism compared to a control line.
Cannibalism may be reduced by fitting hens with a range of eyewear. Rose-tinted spectacles or contact lenses have been used. Opaque spectacles, or blinders, have also been used. For both spectacles and blinders, there are versions that are held in place by circlips into the nares of the bird, or others in which a pin pierces through the nasal septum: this latter method is illegal in the UK. It is theorized that like placing red filters over windows or keeping the birds in red light, the coloured lenses prevent the birds from recognising the blood or raw flesh of other hens and thus diminish cannibalistic behaviour.