The bold contention proclaiming that birds are dinosaurs – specifically, that birds are theropod dinosaurs (meaning they have hollow bones and three-toed limbs) and part of the group that includes Velociraptor and its kin – is, I hope, familiar.
It’s undeniably invigorating, from the point of view of palaeontologists and dinosaur aficionados, to proselytize and promote the dinosaurian nature of birds. “Ooh, look at the cute little dinosaurs,” proclaim the self-assured when looking at parakeets or sunbirds. In a similar vein, a few scientists have argued that the dinosaurian nature of birds is something that everyone interested in birds should care about: that knowing about the deep origins of our favorite feathered animals is important for a list of reasons.
Maybe this is true. After all, it can’t be a bad thing to know more about a subject. But it can also be argued that matters of avian origins just aren’t relevant or important to such issues as conservation, birdwatching, and aviculture.
And – are birds dinosaurs anyway? Anyone who’s followed the discussion on bird origins will know that the “birds are dinosaurs” position has been contested, and charged, not just with being wrong, but of being part of a paleontological conspiracy, and of mis-framing the context in which birds evolved. A few scientists have been or remain committed to the toppling of the “birds are dinosaurs” idea, the best-known being Professor Emeritus Dr. Alan Feduccia of the University of North Carolina. Feduccia has staked his career on birds not being dinosaurs.
But here’s the deal. For far too long, birders and ornithologists have been led astray by the “birds are not dinosaurs” (or ‘BAND’) team. Because at least some BAND promotors are themselves ornithologists, the ornithological literature has tended to frame the bird origins debate as if we should lean in a BAND direction, and even that those who promote a dinosaurian model for bird origins just don’t know, or care about, birds. As someone with expertise in the study of dinosaur fossils and who knows, watches, and loves birds, I need to tell you that this view is categorically wrong and worryingly off-base.
The evidence showing that birds are theropod dinosaurs is outstandingly good. Since the mid-1990s, hundreds of fossil skeletons of near-bird dinosaurs and archaic birds have been discovered in Chinese rocks that date to between 165 and 120 million years ago. They demonstrate that feathers were widespread among these animals, and in fact that essentially all Velociraptor-like theropods were fully feathered. Species within related groups (like tyrannosaurs) were feathered, too. The fossils reveal a diversity in feather form consistent with models of feather evolution. The least bird-like of these animals had a pelt of hair-like filaments, while the more bird-like of them had complex vaned feathers, complete with microscopic barbs and barbules.
The many species that have been named from these sediments are, frankly, baffling in their diversity and in how they might be related to one another. One thing they show is that there were numerous different “near-bird” and “archaic bird” groups living at the same time, each diversifying and specializing in its own way but all sharing a list of anatomical details with the animals that ultimately gave rise to the birds of today. The famous Archaeopteryx from the Jurassic of Germany – long celebrated as the “first bird,” as the one animal bridging the gap between reptiles and birds – is no longer alone, but one of many equally old feathered theropods. Birds evolved a list of anatomical peculiarities later in history (like a toothless bill, and a short tail with a fan-like arrangement of feathers), but a time traveller looking at the small, feathered theropods of the Jurassic would simply not have been able to work out which one was ancestral to modern birds.
None of these animals were quite like living birds, and consequently it’s difficult to describe their appearance or lifestyles succinctly. The species belonging to predatory groups (like dromaeosaurs, the group that includes Velociraptor) might have looked something like long-snouted, long-tailed caracaras or ground-dwelling hawks. Meanwhile, the omnivores and herbivores (like troodontids and oviraptors) perhaps vaguely recalled pheasants or curassows, though with longer faces and bulkier tails.
Something else these fossils show is that the earliest members of the bird group began their history as adaptable generalists. They could climb, glide, and even forage at the water’s edge, but they were mostly woodland-dwelling animals of the forest floor, foraging predominantly for arthropods and small vertebrates. Only much later did birds undergo the great diversification in anatomy and ecology that gave rise to swifts and hummingbirds, waders, seabirds, dedicated climbers, and wood-probers like woodpeckers, aerial predators like falcons, and so many others.
Rare fossils also provide glimpses into the specific behavioral traits of near-bird theropods. We know that they sat on top of their nests and brooded their eggs, that predatory species ejected pellets containing the bones and scales of their prey, that herbivores swallowed stones and used a gizzard, and even that these animals huddled together to sleep, and tucked their faces into their plumage at the same time. These are all traits we think of as avian, yet here they are, in theropods that are not birds nor necessarily ancestral to them. The picture, again, is that birds are just one of many related theropod groups, all of which were similar during the earliest phases of their evolution.
If this is true, why did birds and birds alone survive the extinction event that ended the Age of Dinosaurs, 66 million years ago? The answer seems to be that birds alone evolved sophisticated flight, tiny size, and an ability to rely on seeds and other small items. Near-bird groups did not, and it is likely this that explains their extinction.
Birds are, indeed, dinosaurs. But it is their uniqueness that seemingly explains their persistence, and ultimately their success and prevalence in a world where other dinosaur groups – for so long their contemporaries – are no more.
This essay is derived from the book DINOPEDIA: A Brief Compendium of Dinosaur Lore by Darren Naish. Copyright © 2021 by Darren Naish. Reprinted by permission of Princeton University Press.