A study suggests free range chickens are not necessarily 'happier' than their caged cousins.
Last week, Woolworths announced it will phase out eggs laid by caged hens by 2018 in response to animal welfare concerns.
Dr Jeff Downing, from the Faculty of Veterinary Science at the University of Sydney, says caged hens may not be any more stressed than free range hens.
His research, funded by the Australian Egg Corporation, has tested the stress levels of chickens in a range of egg production systems.
Dr Dowling says there is no distinct difference between the stress levels encountered by caged, barn or free range chickens.
"What we found in any of these production systems is that there can be big variations between the stress levels that we measure from flocks on different farms.
"What's happening on the farm itself seems to be more important than actually the production system and the levels of stress the hens are experiencing," he said.
Dr Downing identifies stress levels in chickens by measuring the quantity of stress hormones, corticosterone, in the albumen of their eggs.
"The albumen develops over a four-hour period the day before the egg is laid.
"My work showed that there is a direct relationship between the level of these stress hormones in blood plasma and the egg, so we're able to use eggs as a non-invasive measure."
Dr Downing found that environmental factors, such as heat, and social factors are the main causes of stress in chickens.
"In evolutionary terms, hens lived in small group sizes. Once you get into very large group sizes, there is so much social interaction that this can be quite stressful for some hens.
"There is far more potential in these big group sizes for social stress."
Dr Downing says Woolworths' move to phase out eggs laid by caged hens will offer no guarantees to consumers.
"If you went into a supermarket and you just looked at the production system and you chose eggs based on that, there is no guarantee that those eggs came from a farm where hens were less stressed than another production system or another farm."
Federal Agriculture Minister Barnaby Joyce says he is concerned by the deal that Woolworths has announced and says he is not sure that farms can supply enough eggs if caged hen production methods are phased out.
"I don't think many consumers realise what free range can mean.
"Caged hen production systems are often very clean and tightly monitored and are often very high tech with happy and healthy hens.
"They can control temperature, parasites, to provide a clean delivery of food. These are vastly superior to what you would get in many free range operations.
"With large-scale egg production, you can get chooks walking over other chooks, eating other chooks' faeces. I am not sure if that is what you have in mind with free range.
"Also, a lot of producers have just invested to change the cage sizes to fulfil new requirements, and and they have done that in goodwill, thinking that that will fulfil all of the requirements. You can't keep putting costs on top of costs.
"I have set up a meeting between Woolworths and the egg industry to try and talk through some of these concerns."
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