I have a small pressure cooker for the tough birds but found slow cooking in a crock pot gets them just as tender, and easier to moniter. I over did one in the pressure cooker once, little bones everywhere, it was a mess.
I'm going to try something new with some cockerels I have in the freezer. I'm going to debone them and give them a few day soak in Salamida state fair chicken sauce. That should tenderize them some. Wrap them up with butcher string and slow grill them. Just waiting for a nicer day.
One thing I never thought of is I heard the birds should be refrigerated for a couple days so the muscles relax before freezing. I have always just tossed them in the freezer.
I always hang my deer to 'age' for a week if the weather is right, if not I quarter them up put in garbage bags and put them in our extra fridge.