I feed birds to carpets and short tails. Most have difficulty with birds the first few times. They seem unsure how to find the head. They get the hang of it soon enough. About that shape difference; judge the bird at it's widest which is the butt end. With rodents, if the snake can get past the shoulders, it's a done deal. Not so with birds, gotta get past that butt. Go by girth not weight.