It really depends on the kind of bird that you're butchering. Egg layers don't have a lot of muscle to begin with, so the majority of their weight is going to be in the bones and innards. (Feathers don't account for much of the weight.) We've dispatched extra layer roos, only to be disappointed by the meat yield, plus they don't taste as good (to us they have a different flavor). They tend to have really thin breasts and scrawny everything else. They pelvic opening is relatively small, and it's hard to clean them out.
Dual purpose birds have more muscle, but still not like what you get with meat birds. They yield more breast meat, but not much. I would say that you are probably getting closer to half the body weight in muscle. They seemed to be easier to clean, but not much. They're okay for stew, soup, and frying.
Meat birds have a very high yield of meat. You'll be surprised at how much muscle they have compared to their insides, and their pelvic area is very wide, making it very easy to remove the insides. (I thought they looked kind of empty for their size!
) Most of the work goes into pulling the feathers with them. They have a huge amount of white meat, and it's very tender compared to the other two. They grow pretty fast, so they can be butchered relatively young.
I hope this helps some!
Stacey