What, if anything, is a bird?
Frankly, scientists have trouble deciding at what point an animal is considered a bird. They know this point is somewhere in the group of animals that are more closely related to birds than they are to any other modern animal – this group is called Avemetatarsalia, and includes all dinosaurs (including modern birds,) pterosaurs, and close relatives. We know all these animals are more closely related to living birds than to any other group of animals through the process of cladistics. Paleontologists and biologists look at the different characteristics of animals, and test what is the simplest explanation for how those characteristics evolved, and thus how the animals themselves evolved. Crocodiles, the closest living relatives to birds, are used as an external marker – if anything is as closely related to crocodiles as it is to birds, then they aren’t in Avemetatarsalia.