Review: welfare perspectives on hens kept for egg production.

The wild ancestors of chickens, along with those of most other farm animals, were preadapted to domestication because their lack of specialized requirements allowed them to adapt to a wide variety of environments provided by humans. Currently most commercial chickens kept for table-egg production are incubated, reared, and maintained as productive adults in high-density, artificial environments. Nevertheless, there are limits to adaptability as indicated by behavioral, physiological, immunological, and individual productivity indicators when environmental conditions become extreme. However, with the exception of obvious injury, no single criterion is likely to be adequate. Multiple indicators are required to obtain reliable evaluation of whether husbandry practices and environmental conditions reduce hens' welfare significantly. Concern for the well-being of hens has led to the phasing out of cages in two European countries. Although cages are known to be associated with some problems of well-being, it is known also that they have some welfare advantages for hens over alternative systems of production and they have definite economic advantages for producers. Therefore, it is doubtful whether the use of cages should be denied without exploring further the possibilities of cage modification or genetic selection aimed at improving the well-being of hens in such environments. Ethical perspectives relative to animals have been evolving since the time of Aristotle more than 300 yr B.C. Recent developments include divergence of welfare concerns between utilitarian and animal rights based philosophies. The utilitarians generally agree that animals may be used for human benefit if unnecessary pain and suffering are avoided and humane care and management criteria are met. Fundamentally, rights-oriented groups reject such exploitation. The general public exhibits a continuum of attitudes towards animals. However, there are indications that they are moving towards a protectionist attitude toward farm animals. Many requirements for good husbandry are known. However, uncertainty prevails in some areas, particularly with the necessity of routine procedures such as beak trimming and the amount of space needed, optimal group size, and whether nests, roosts, litter, and other quality of environment features are necessities or luxuries in terms of hens' welfare. Floor and feeder space and group size seem to be of paramount importance, and space that is adequate for well-being seems to be greater than that which yields the greatest net income. Genetic solutions to several behavior-related problems that compromise hens' well-being seem to be feasible and worthy of greater emphasis by commercial poultry breeders.