You have to protect your birds. Whether that means trapping and bringing the cats to the pound, eradicating them, or something else. After all, if no one is going to be around to feed them, they could starve to death. It would be kinder for them to be put down quickly and humanely, rather than let them suffer that way. Or, who knows? If you don't have cats of your own, having a couple of them around could be beneficial as far as keeping the rodent population down. (If they're not threatening your chickens, that is) You have to make the choice as to what to do with the feral cats that come your way.