One typically learns in school that a bird is a bipedal, warm-blooded animal, a vertebrate, covered in feathers and lays eggs. Some can fly, but not all of them. And, until we started to think about extinct animals, this was enough. It was pretty clear what was a bird, or a mammal, or a reptile, when you only looked at things that were alive today.
But when you start to consider extinct organisms, the picture becomes significantly less clear. In school, typically the answer for what the first bird was, is Archaeopteryx. Archaeopteryx was a small, bipedal dinosaur, covered in feathers, with clear wing feathers, and was probably warm blooded. Problem solved!
Except as we learn more and more about dinosaurs and how they evolved, it becomes increasingly clear that Archaeopteryx was not unique, and it was not the first dinosaur to show these traits. Wing feathers are actually present in such dinosaurs as Velociraptor and Oviraptor, animals we don’t typically call “birds.” These animals were also bipedal and warm-blooded. They couldn’t fly, though, so perhaps that’s what sets Archaeopteryx apart – it’s ability to fly.
But Archaeopteryx couldn’t even fly. It did not have the musculature necessary to power its own flight – the most it could do was glide! And given that flight is an optional characteristic for birds anyway, it seems rather unfair to Ostriches and Emus as well as Archaeopteryx to make flight a necessary characteristic of being 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.
So the group Avemetatarsalia is given the name Pan-Birds, indicating its inclusion of birds and all things closer to birds than to anything else. And all modern birds are in the group Neornithes, and are called Crown Birds. But what is the first bird?
Well let’s look at our characteristics. If we’re going off of feathers, the classic thing that one thinks of when they hear the word “feather” – a vane running down the middle with barbs coming off the sides – were definitely first seen in animals such as Oviraptor, a large, chicken-parrot like animal that you probably would call a bird if you saw it today, but is not considered a traditional “bird.” It is possible, however, that such feathers were present in things even earlier than that – animals such as Ornithomimus, which, again, one probably would call a bird today, but wasn’t traditionally.
Still, it seems strange to draw the line at this point. After all, we don’t just call these vaned structures – known scientifically as pennaceous feathers – feathers? We also call most fluffy features on bird feathers, such as the downy feathers. And these were present in a lot of animals, not just Ornithomimus and later relatives. When did the first things that we would consider feathers even evolve?
