Systematics = evolutionary relationships.
Although I've said birds are remarkably similar, there is an astonishing range in size. A Bee Hummingbird has a wing span 4 in and weighs 0.5 oz. A Wandering Albatross' wing span is 11.5 ft and it weighs 25 lb. Ostrich weigh 350 lb.
Bird shapes (i.e., bills, wings, and feet) reflect adaptations of birds to habitat and feeding methods. By looking at bird parts, you ought to be able to predict a bit about how they feed and behave. This is called predictive morphology
We name birds according to a system of binomial nomenclature.