This past May, when it finally sank in that I was going to be stuck at home for a very long time because of the pandemic, I took up a hobby that had never especially appealed to me before: birding. I cleaned my neglected bird feeder and filled it with seed, retrieved my binoculars from a gear bag in the basement, and started having my morning coffee outside, slowly learning to identify species based on body size, feather colors, beak shape and song. At last count I had logged 39 species from the confines of my suburban backyard. These hours spent observing birds—the goldfinches congregating at the feeder, the pileated woodpeckers drumming in the trees, the turkeys strutting across the lawn, the ruby-throated hummingbirds hovering above their favorite blooms, the red-shouldered hawks circling overhead—have given me a newfound appreciation for their diversity. And I am seeing only a sliver of the actual richness of avian forms. With more than 10,000 species alive today, birds constitute the most diverse group of land vertebrates (backboned animals) on Earth. How did they come to be so spectacularly varied?
Birds are dinosaurs, the only lineage to survive to the present day. They arose in the Jurassic period, between 200 million and 150 million years ago, from the theropods, a group of two-legged carnivorous dinosaurs whose members include both the behemoth Tyrannosaurus rex and the daintier Velociraptor. For tens of millions of years birds evolved alongside other dinosaurs, diversifying into a number of small-bodied, fast-growing, feathered fliers, along with a few large-bodied, flightless forms. One group, the so-called neornithines, or new birds—distinguished by their fused foot and anklebones and by certain traits in the bones that support the wings—would eventually give rise to modern avian-kind.
Scientists have tended to view modern bird diversity as the result of a burst of evolutionary activity that occurred after the fateful day 66 million years ago when a six-mile-wide asteroid struck Earth, dooming 75 percent of plant and animal species, including the nonbird dinosaurs and most bird groups. Exactly why the neornithine lineage alone survived this apocalypse is uncertain, although the recent discovery of a 66.7-million-year-old neornithine bird fossil from Belgium called Asteriornis, a relative of today's ducks and chickens, suggests that being small and living in a shoreline environment may have helped. In any case, the idea was that after the mass extinction, the neornithine birds had the place largely to themselves. Free of competition from other dinosaurs (not to mention a whole bunch of other vertebrates that also perished, including the pterosaurs, those flying reptiles that had long ruled the skies), birds abruptly exploded into a multitude of forms to fill the many newly vacant ecological niches.
Now a new analysis has turned up intriguing evidence that their extraordinary diversity might not have originated that way. In a study of hundreds of bird and dinosaur skulls, Ryan Felice of University College London, Anjali Goswami of the Natural History Museum in London and their colleagues found that in the aftermath of the mass-extinction event, the pace of birds’ evolution actually slowed way down compared with that of their dinosaur predecessors, rather than accelerating as expected. The paper, published in PLOS Biology, reveals the rate of evolution during the radiation of a major vertebrate group and hints at factors that may have played a key role in determining its course.
Fossils that preserve the entire skeleton of an animal are extremely rare, so comparative studies of fossil material tend to focus on a particular region of the body. The team looked at skulls because they serve many functions, from supporting sense organs to enabling feeding to attracting mates to defending themselves. “Birds have incredible diversity in the shape of their skulls,” Felice observes. Consider hawks versus hummingbirds, he says, or pigeons versus pelicans. “Did birds evolve their highly variable skulls by evolving more rapidly than their nonavian dinosaur ancestors?” Felice asks. That might seem like a narrow question, but “it gets toward an understanding of how diversity evolves,” he explains. “If a group of organisms is really diverse, do they achieve their diversity quickly in an explosive burst? Or is it slow and steady?”
To investigate, the team carried out a detailed shape analysis of 391 well-preserved skulls from modern birds and extinct dinosaurs using high-resolution 3-D scans of the specimens. The scientists used the results to reconstruct the animals’ evolution. Typically skull-shape comparisons rest on the use of established landmarks—such as sutures and bumps—that all the various species under evaluation share. But the larger the study group, the fewer the points of correspondence. As a result, investigations that focus on traditional landmarks lose much of the information about skull shape. “Our approach takes those landmarks and uses them as anchors for curves that connect up those landmarks and, in doing so, outline and delimit the individual bones of the skull,” Goswami says. “Our automated approach then takes a generic template of points and fits the exact same template to all the specimens in our data set by using the landmarks and curves to identify the regions of interest. So you can get points distributed across the surface of a bone in a consistent way, regardless of whether the bones you are looking at look like the flat, bony structure under the beak of a duck or the tall, biting [snout] of a T. rex.”
What the researchers found was that dinosaurs evolved 1.5 to three times faster than birds in all regions of the skull. After the mass-extinction event brought the Mesozoic era to a close and ushered in the Cenozoic era, birds branched into most of the major modern groups, from hummingbirds and penguins to birds of prey and songbirds. But they evolved this diversity far more slowly than their Mesozoic dinosaur forerunners. “Their rate of morphological change declines just as they are taking off as a radiation,” Goswami says.
Why the sudden deceleration? Goswami thinks it reflects a shift in priorities for skull function. Whereas dinosaur skulls have elaborate display and fighting structures, as well as complex feeding mechanisms that require large areas for jaw-muscle attachment, bird skulls are mostly dedicated to housing and protecting the animals’ comparatively large brain, she explains.
Bird-evolution experts who were not involved in the new research praised the team's methodology and the vast number of species they included in their study.
The finding that dinosaurs had a much faster rate of skull evolution than modern birds might seem strange considering the variety of bills in birds such as spoonbills, flamingos and pelicans, says Daniel Ksepka of the Bruce Museum in Greenwich, Conn. Their sundry shapes suggest a high rate of evolution in the beak, which is a major component of the skull. But a closer look reveals that these distinctive bills are the exception rather than the rule, he says. “There are plenty of groups where dozens of related species share a pretty similar skull shape, like warblers or parrots, suggesting relatively little skull evolution,” Ksepka observes.
In contrast, some groups of dinosaurs clearly had sky-high rates of skull evolution. Among the ceratopsians (Triceratops and its kin), for instance, “each species had a unique arrangement of horns and crests. And these seem to have evolved rapidly because of their value for attracting mates,” Ksepka says. “So many dinosaurs had these elaborate skull ornaments, but they are very rare in birds—the cassowary is one awesome exception,” he adds. The large, flightless cassowary, a relative of the emu found in the tropical forests of Papua New Guinea and northeastern Australia, has a prominent bony crest atop its head. “It's likely that feathers took over the display role, as we have plenty of modern birds with plain-shaped skulls but beautiful feathered head crests. Just look at your friendly backyard cardinals and blue jays.”
The discovery that bird skulls resulted from relatively low evolutionary rates “is essentially opposite from what we know of the rest of the skeleton,” says Stephen Brusatte of the University of Edinburgh, another outside expert. Previous studies by Brusatte and others have focused on parts of the body other than the skull and found that these regions evolved faster in birds than in other dinosaurs. “What this means, I think, is that the origin of birds was driven by rapid and remarkable changes to the skeleton, particularly turning the arms into wings for flight. The heads were less important in this transition, and they probably lagged behind the rest of the skeleton.” Early on in their evolution, birds seem to have hit on a head design that worked for them, with such features as a beak, big eyes and a large brain, he says: “Birds didn't need to radically change any of these things in order to adapt to different niches.” Instead, Brusatte suggests, “after birds split off from other dinosaurs and went into the skies, they adapted to new niches by changing their body sizes, wing shapes and flying styles more than their heads.”
Such mosaic evolution, in which different parts of the body evolve at different rates, is known to have occurred in many organisms, including humans. Ksepka notes that the ceratopsians’ high rate of skull evolution contrasts starkly with barely discernible changes in their limb bones. Meanwhile modern warblers, he says, exhibit very little change in skull shape but have evolved “a kaleidoscope of color patterns.”
But Goswami has a hunch that other parts of the bird skeleton may have also evolved on a relatively leisurely timetable. Nonbird dinosaurs transitioned between bipedal and quadrupedal body plans several times over the course of their evolution and did a lot of different things with their forelimbs, she points out—think of T. rex’s puny arms compared with a titanosaur's tree trunks. In contrast, once birds became specialized for flight as their forelimbs morphed into wings, among other changes, they never really evolved completely new body plans—presumably because of the developmental or functional constraints of being a bird. “I expect that future studies with sampling as broad as ours will also start to find that birds are, quite frankly, not keeping up with the pace of evolution observed in the other dinosaurs,” Goswami says.
Of course, the birds are no less spectacular for that downturn. They survived fire and brimstone, conquered the skies and diversified into the dazzling array of feathered wonders that share the planet with us today. Slow and steady won the race.