I know this is an old thread, but I'm researching quail raising and this came up so it's still active on search engines. So I want to point out for others that comments made regarding the size of the animal and the feed conversion ratio should be taken with a grain of salt. A major consideration has to be that even if a cow would process the same food more efficiently than a chicken, for example, cows and chickens don't eat the same food. The FCR, or food conversion rate for chickens is roughly 2:1 for chickens compared to roughly 8:1 for cows. At least part of that is because chickens naturally eat a more energy rich diet, including grubs and insects as well as plant material while the cows are eating nothing but grass. Theoretically of physiologically this could be construed as an unfair advantage for the chicken that needs to be balanced out for a true comparison. From a permaculture perspective, though, it makes little sense to penalize them for being more effective foragers. Put a chicken and a cow out in the same pasture and the chicken should put on meat more efficiently than the cow.
More generally, while I've been out of the loop for a while, what I remember from agricultural economics is that actually smaller animals have better food conversion rates and a little internet research seems to bear this out. In the commercial feedlot setting, you have cows at 6:1, swine at 3.5:1, poultry at 2:1, and fish at 1.5:1.
It's worth noting here that FCR is an imprecise measure for meat yield because it actually measures not only just the mass of the food intake (rather than the energy content) but also the body mass of the animal (rather than the meat yield). Different animals will have a different ratio of edible meat to total body mass. For example, a dressed chicken ready for the pot will weigh about 75% of its live weight, while a beef carcass hanging in a meat locker (with bones, like the chicken) is typically around 65% of its live weight. This proportion actually favors chickens even more, and will get even more chicken favored once you take the bones out because of their lower bone density.
