When one looks at birds like this puffin, it can be hard to reconcile its cute appearance with its place in the animal kingdom. The thing is, this adorable puffin has something in common with a rattlesnake, in that it’s a reptile (Image credit: Ray Hennessy, Unsplash licence, Image Cropped).

You read that correctly, birds are reptiles. Now, I can hear you saying “but we learned that they are a different group of organisms, and that reptiles are just those scaly animals that have cold blood?” While reptiles don’t have cold blood per se, some of them DO have feathers. And can fly. In this post I hope to convince you of the fact that the puffin pictured above, and all of its avian relatives, belong with the snakes, lizards, crocodiles, and turtles in the reptile group.

What ARE birds, anyway?

One of my favorite web comics is that of the Tyrannosaurus rex transitioning slowly into a chicken. In the first panel, it towers over a group of humans, scaring them and basking in its monstrous, scaly glory. But then it starts to change, the massive predator begins to shrink and grow feathers, and it goes from being the “terrible king lizard” (actual meaning of Tyrannosaurus rex, despite it not being a lizard) to being too small and cute to be a threat. While this is humorous and the T. rex likely already had feathers, it calls attention to one of the scientific facts that I would wager a fair amount of the non-scientific public knows: birds are the modern-day descendants of dinosaurs.

The cool thing about this is that birds are not only the descendants of dinosaurs, they ARE dinosaurs. Molecular data tells us that during the Triassic period (251-199 million years ago) the major groups of what are today considered reptiles evolved, and these are the relatives of a group that were the ancestors of crocodiles and dinosaurs. About 65 million years ago a massive extinction event wiped out all but one group of small, feathered dinosaurs (most dinosaurs were likely feathered, we know now). These dinosaurs eventually developed over time into what we now call birds. So, despite their shared evolutionary history and close relation to other reptiles like crocodiles (I challenge you to find someone who would say crocodiles aren’t reptiles), why are birds usually neglected when it comes to reptiles?

Fossilized skeleton of an Archeopteryx, the classic example of the transition between the huge, non-flying dinosaurs and today’s modern birds. It even LOOKS like a chicken (Image credit: James L. Amos, National Geographic Society, CC0 1.0).

The General Public

Part of the problem may be the tendency that we as humans have to differentiate things based on how similar they are. Sure, birds lay eggs like a lot of other reptiles do, but they are also covered in feathers and most of them fly. How could these feathered, flying animals be the same kind of animals as the scaly, legless snakes? Part of the problem may lie with how dissimilar birds are, as a group, to their closest relatives, the crocodilians (alligators, crocodiles, caimans, etc.). Most birds are smaller than a human, have feathers covering most of their bodies, and plenty of them fly. Crocodilians, on the other hand, can’t fly, don’t have feathers, and lots of them are much larger than humans. To bring this point home using another group of animals, I will use humans and their closest primate relatives.

Most people (religious fundamentalists aside) know that chimpanzees are the closest relatives to humans in the animal kingdom, but broadly speaking they look almost nothing like us. They walk on their hands and feet, have bristly hair covering the majority of their bodies, and move in the trees as easily as they do on the ground. We humans, however, tend to be relatively hairless (at least compared to the chimps), walk exclusively on our feet, and we have come to dominate the world in a way that no other species before us has. As a matter of fact, plenty of people wouldn’t name humans when talking about primates, despite the fact that we ARE primates. So, why the distinction? It may be a cultural hangover from the days where people considered themselves “special” or “chosen”, and thus above the “primitive” animals, but I think it has more to do with how different we are from them in both appearance and behavior. Despite these differences, we are primates just like our chimp cousins, and birds are reptiles.

In the Literature

This bias isn’t unique to the general public, we as scientists are also guilty of separating birds from their fellow reptile relatives. When researching this topic, I looked up papers to get a sense of how the scientific community discusses and refers to birds. I used the search terms “bird + reptile + phylogeny” to find papers showing the evolutionary history of the reptile group, and almost all of the relevant papers used terms like “birds and reptiles” or “birds and non-avian reptiles”. Why the distinction and separation?

Part of the issue may lie with the classical system of classification, called the Linnaean system after Carolus Linnaeus, where animals are divided into groups based off of their physical similarity to one another. In this system, reptiles were organisms that could not regulate their own body temperature (ectothermic) and had scales, so birds did not fit into this group. It wasn’t until the 1940’s that the science of phylogeny, using studies of ancestral states and grouping organisms based off of how similar they are GENETICALLY, was able to show that birds, lizards, turtles, snakes, and crocodilians were all descended from the original reptile ancestor.

Related: Whales Are Fish: Weird Perspectives on Classification

A second possibility is that it is simply easier to separate birds from the “non-avian reptiles”. In my experience with the literature, most studies using birds are concerned with some aspect of a bird-exclusive trait (like flight or roosting in large colonies), and those using other reptiles like turtles are more concerned with some aspect of their non-avian biology, like how to overwinter as an ectothermic animal. So, unless a researcher is writing a paper on the evolutionary history of these organisms, why would you put them together?

Where Does This Leave the Birds?

So, why does any of this matter? Sure, in the end these arguments are just humans trying to put things into neat little boxes. We want to classify them and marvel at our ability to “solve” the evolutionary history of the tree of life. The birds don’t care what we call them (unless it’s an emu, they don’t take insults well). It is important, however, to use the proper terms when discussing science. We can’t fall back onto our preconceived notions of how things are when the evidence for the contrary is staring us right in the face.

Also, when it comes down to it, birds being reptiles is pretty cool.

Adam Hasik an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.