Are Birds Dinosaurs? Why Your Pigeon Is a Living Theropod

Next time you see a pigeon pecking at crumbs, look closely: you are watching a dinosaur. That is not a figure of speech. In modern biology, birds are dinosaurs, the only branch of the dinosaur family that survived the asteroid impact 66 million years ago.

A family tree, not a ladder

Biologists group living things by ancestry. A group should include an ancestor and all of its descendants. Since birds descend from dinosaurs, leaving them out of “Dinosauria” would be like leaving bats out of mammals because they fly.

Birds sit within the theropods, the mostly two-legged, meat-eating dinosaurs that include Tyrannosaurus rex and Velociraptor. More precisely, they belong to a group of small, feathered theropods called paravians. That is why scientists sometimes call the other dinosaurs non-avian dinosaurs: dinosaurs that are not birds.

The first clue: Archaeopteryx

The idea is not new. In 1861, just two years after Darwin published On the Origin of Species, quarry workers in Germany found Archaeopteryx, a creature about 150 million years old. It had feathers and wings like a bird, but also teeth, clawed fingers and a long bony tail like a small dinosaur. In the 1860s, the biologist Thomas Henry Huxley pointed out the striking similarities between birds and small dinosaurs.

The idea fell out of favour for decades, and was revived in the 1960s and 70s when the palaeontologist John Ostrom described Deinonychus, an agile, bird-like predator.

The feathered dinosaur revolution

The decisive evidence came from China. In 1996, scientists described Sinosauropteryx, a small theropod covered in simple, hair-like feathers. Since then, fine-grained rocks in Liaoning Province and elsewhere have produced dozens of species of feathered dinosaurs, from fluffy ground-runners to Microraptor, which had long feathers on all four limbs.

Feathers, it turns out, evolved long before flight. They were probably first used for insulation, display and brooding eggs.

What birds share with dinosaurs

• Feathers: found on many theropods, not just birds.
• Hollow bones and air sacs: light, air-filled bones linked to a system of air sacs that makes breathing very efficient. Both birds and many theropods have them.
• A wishbone: the fused collarbone, or furcula, that you find in a roast chicken also occurs in T. rex and Velociraptor.
• Nesting behaviour: fossils of Citipati, found in Mongolia, show adults sitting on their nests with their arms spread over the eggs, exactly like a brooding bird.
• Similar eggs and growth: some dinosaurs laid eggs with bird-like shells and grew quickly, like birds.

A 2015 review by Stephen Brusatte and colleagues concluded that the features of birds were built up gradually over tens of millions of years in their dinosaur ancestors, rather than appearing suddenly.

How birds survived

When the asteroid struck, all large dinosaurs died. Some small birds survived. Scientists think it helped that they were small, could fly to find food and shelter, and that some had toothless beaks suited to eating seeds, which could last in the soil through the long, dark impact winter.

Today there are about 11,000 species of birds, more than twice as many as there are mammals. In a sense, the age of dinosaurs never ended.

In short

• Birds are dinosaurs: the only dinosaur branch that survived the extinction 66 million years ago.
• They evolved from small, feathered theropods, relatives of T. rex and Velociraptor.
• Archaeopteryx (1861) and many feathered dinosaurs from China support the link.
• Birds and theropods share feathers, hollow bones, air sacs and a wishbone.
• Fossils show dinosaurs brooding their eggs like birds.
• Small size, flight and seed-eating beaks may have helped birds survive.