In the poultry industry, "spent" hens that are too old to lay eggs are typically killed and composted. They are not used for meat, as these chickens typically yield one pound of edible meat, compared to five pounds from a bird that has been raised to be eaten. The industry practice is to place the birds in a container filled with carbon monoxide until asphyxiated, and then covered with sawdust to facilitate composting. The compost is typically sold to farmers.
However, sometimes hens that have not been properly euthanized will survive to climb out of the pile. Such birds are nicknamed Zombie Chickens by some workers. Animal rights advocates view the appearance of zombie chickens as evidence that elderly hens are not being killed humanely. One unidentified source has stated that one out of every 20,000 hens survive the process. 
Zombie chickens are celebrated in the Troma production of Poultrygeist: Night of the Chicken Dead as well as the comedic/horror novel Cluck: Murder Most Fowl by Eric D. Knapp.
References^ Young, Tobias (2006-11-22). "Recycling chickens: Farmers turn to composting amid collapsed spent-hen market". The Press Democrat. http://www1.pressdemocrat.com/apps/pbcs.dll/article?AID=/20061122/NEWS/611220399/1033/NEWS01.
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