All young anacondas, regardless of species, are essentially programmed to initially take small birds as their first prey. Some young anacondas will take rodents from the beginning, but that is usually the exception, and not the rule. I've had a few neonate greens hold out for as long as 4 months before feeding, and that is not overly unusual, but I feel that you should always have them feeding by no longer than 6 months or you could run into problems. Day-old chicks are usually the most available and tend to work very well to get most young to start feeding. The absolute best are young ducklings, but those are not as readily available. Young nestling pigeons will also work well if you can find them. Once they are started well on birds, I've never had much trouble getting them switched over to rats after a few months on birds. Let us know how it goes.
Kelly