Caged hens experience no more stress than their free-range counterparts, according to research at the University of Sydney’s Poultry Research Unit. Lead researcher Jeff Downing told the London Telegraph that free-range hens are more prone to manure-borne diseases and parasites, that extreme temperatures are more stressful to hens than the method of their housing, and that caged hens have greater protection from both the elements and predators. If hens have no cover, they are constantly in fear of attack by predators, Downing says.
The study measured corticosterone in the white of eggs laid by caged, barn-reared, and free-range birds. Downing said. He added that the hormone is only one stress measure, but a good one.This is not the first study for find real-world problems with letting chickens out of their cages, says the Center for Consumer Freedom. In October 2003, researchers at Veterinary College in Hanover, Germany, found that cage-free hens produced fewer eggs, are more susceptible to disease, and die prematurely twice as often.