They kill them, of course.
San Jose Mercury News - 38 roosters saved in raid on cockfighting operation:
But the birds -- groomed, trained and pumped full of chemical supplements to make them fighters -- are not eventually bound for some kind of retirement refuge for rehabilitation.
"They can't be put up for adoption. They can't be around other birds or they'd fight," said Judi Adams, the SPCA's humane investigations supervisor. "They may need to be euthanized."..."There are no groups that take in seized cockfighting birds or serene refuges where they could live out their natural lives. 'If there was a way to rehabilitate them, we'd definitely pursue that,' Adams said. Donating the birds to a soup kitchen is not feasible either. Their bodies probably contain a chemical stew of vitamins and growth hormones that were pumped into the fighting cocks. 'No one is inspecting the meat for quality or safety,' Adams said. 'In some ways, those killed in fights are the lucky one. Their suffering is over,' she said."
Note the phrasing: "They may need to be euthanized."