What do farmers do with old chickens?

So, I'm curious here, when it comes to farmers and their old chickens, what's the typical practice? Do they have a specific process for dealing with these birds once they've reached a certain age or stage of life? Is there a humane way that farmers often go about it, or is it more of a practical approach focused on resource management? And if there are any legal or ethical considerations that farmers have to keep in mind when dealing with old chickens, what are those? I'd love to hear more about how farmers navigate this aspect of their operations.