In this rich agricultural region of Northern California, ranchers have been turning chickens too old to lay eggs into compost at a rate of a half-million hens a year.

But some chickens not properly euthanized have been seen crawling out of the compost piles, earning them the name "zombie chickens" — and hatching a debate over what else might be done with them and other "spent hens."

A food bank proposed making sausage to feed the poor. A reptile enthusiast suggested using them as food for large exotic pets like pythons and alligators. And an industry group said in the future they could be used as fuel for power plants.

But for now, according to egg farmers in Sonoma County, composting is the only affordable option. The last California rendering plant stopped taking the hens in May.

"If there was something that could be done, it would be done," said Petaluma egg farmer Arnie Reibli.

The egg-laying birds have only a pound of usable meat, compared to the 5-pound chickens typically raised for eating. Slaughtering the chickens, even to transport them unprocessed and frozen whole, would likely cost more than composting them, Reibli said.

"Unfortunately, it's less expensive to go out and buy the birds than process them," said David Goodman, executive director of the Redwood Empire Food Bank in Santa Rosa, which had considered the sausage-making plan.

To kill the chickens, farmers suffocate them in sealed boxes filled with carbon dioxide, a practice that has drawn the ire of animal rights groups. Afterward, the hens are layered in mounds of sawdust.

A new European technology that turns dead cows into fuel to generate electricity — and that could be the fate of spent hens someday, said Rich Matteis, head of the Pacific Egg and Poultry Association.

But "that's not something that's going to be available anytime soon," he said.