When Jim Stauffer of Petaluma saw a chicken crawling out of a mound of compost like the living dead, he knew something had changed at the egg farm next door.

"We called them zombie chickens," Stauffer said. "Some of them crawled right up out of the ground. They'd get out and stagger around."

What changed was the method used to get rid of "spent hens," which are chickens that no longer produce eggs. And the change isn't just in Petaluma; it's throughout the country.

The market for spent-hen meat has collapsed. Since May, there isn't a California facility willing to take them.

That means finding a way to dispose of more than a half-million spent hens a year - and that's just in the Sonoma County area, mostly around Petaluma, where chickens and eggs have been an agricultural staple for a century.

As a last resort, many farmers have turned to killing the chickens and using them to make piles of compost.

Hens are placed in a sealed box which is filled with carbon monoxide. Within seconds the chickens are unconscious. Less than two minutes later, they die from lack of oxygen.

Farmers say the method for euthanizing and composting the chickens is humane and health officials say they have heard no complaints.

The dead chickens are layered into a mound of sawdust. In about a month, it turns into compost, farmers said.

They said the incident described by Stauffer, in which about two dozen chickens crawled out of compost piles, was an anomaly probably caused by inexperience.

One farmer said no chickens survive the process, which he personally oversees. The biggest chicken farmer in the region said usually two survive out of 40,000 gassed.