Feeding birds to an anaconda that is already feeding well on rodents might be asking for trouble. Most anacondas really like birds and can easily get stuck on them, refusing to eat anything else. Sometimes chicks or ducklings are the only way to get a newborn started feeding. These are usually offered as a last resort and once feeding, owners work long and hard to get their snakes off of birds and onto rodents since they are much easier to aquire for most people. Since your snake has already accepted mice, and even seemed to have a little digestive issue with the bird, I would definitely stick to a rodent diet. Also I would try to switch to appropriately sized rats as soon as possible to avoid it getting hung up on mice.