Interesting question, one that I'm sure could provide many different answers. I have fed the snakes that I've had over the years a mixture of chickens and rabbits. I've had some snakes that would only eat rabbits, some that would only eat birds, and some that would eat both indiscriminately. I had a small retic once that only ate quail and chickens. Wouldn't touch anything with fur. Same thing for a green anaconda I had about 10 years ago -- from the get go she would only eat fowl. She started with chicks and went up from there to large chickens. I never had a problem feeding my snakes quail/chickens. They grew well and showed no side effects from the birds. However, I do believe that feeding mammals is better and should be done when possible. If I have a snake and it will eat mammals, that's what it gets. If it won't, then I try fowl. I've heard that mammals have more nutritional value than birds, though you could supplement a pre-killed chicken with vitamins and minerals. One thing to keep in mind with chickens raised commercially -- they are often given hormones to speed their growth, as well as other medications, and this may not be good for the snake over the long haul.
I would stick to rabbits if the snake was happy with them.
RP