Well, certainly no higher form of life will pass through the digestive tract of a bird. However, birds are most likely the primary source of "spontaneous pond stocking". I have observed egrets eating gambusia, and seen live babies (from a the ruptured belly of a pregnant female)stuck to the bird's beak. I think the babies would live at least 20 minutes. I beleive fertilized eggs can occasionally stick to a bird's feet. Also, birds will fly across water carrying a fish to the nest. Other birds will often harrass the bird, and it will instinctively drop the fish to get away. Nature finds a way!
