Flights of Fancy in Avian Evolution
From mousebirds to terror birds, the class Aves has encompassed a remarkable diversity of species over the past 150 million years.
People would never imagine the astonishing diversity of birds that once existed on Earth if they think only of the tiny songsters most commonly encountered around their homes and workplaces, such as cardinals, sparrows, and finches. These birds are a lovely part of our backyard life, but they represent a tiny sliver of the ecological roles that birds have explored over their 150-million-year history.
Over the course of avian evolution, specialized lineages of birds have played the parts of landscape-altering browsers, marine divers, nocturnal hunters, and even terrestrial apex predators. Many of the most unusual species are now extinct. Others diversified into the forms we see today. As prolific rock deposits in places like China, Wyoming, and Peru yield new discoveries, fossils are providing an increasingly rich source of insight into this evolutionary journey. More than 300 new fossil species of birds have been named since 2000, filling in the scientific picture of bird evolution.
The earliest known bird, Archaeopteryx lithographica, lived 150 million years ago during the Jurassic Period. Dinosaurs such as the armored Stegosaurus armatus and gigantic long-necked Brachiosaurus altithorax roamed the landscape, and plants like conifers, ginkos, and cycads dominated the flora. Archaeopteryx did not have the skies to itself, but instead lived alongside flying reptiles called pterosaurs.
During the Jurassic, the continents were in the process of breaking apart. Africa, South America, Australia, and Antarctica remained united into the supercontinent Gondwana. Much of Europe was inundated with shallow seas at this time, and the eleven known Archaeopteryx skeletons found their final resting places on the bottom of a lagoon in an area that is now part of present-day Germany.
Over the following 84 million years, a large number of primitive bird species, equipped with the evolutionary baggage of teeth, hand claws, and long bony tails, appeared and lived alongside their dinosaurian relatives. Then, 66 million years ago, an asteroid impact triggered a mass extinction that wiped out the nonavian dinosaurs, the flying pterosaurs, marine reptiles like plesiosaurs, and many other groups. Some birds survived, but many species went extinct, including all the toothed forms.
In the aftermath of this mass extinction, “modern” birds—those belonging to the radiation that includes over 10,000 living species—took the stage. Out of the chaos of extinction emerged survivors that diverged and adapted to fill the ample niches opened up by the enormous loss of species. This period of diversification following the Cretaceous-Paleogene extinction gave rise to groups both familiar and strange, including the tree-loving parrots and mousebirds, aquatic penguins, giant soaring pseudotoothed birds, and land-dwelling terror birds.
The Eocene Explosion
According to the fossil record, bird diversity expanded during the Eocene Epoch, which began 56 million years ago. The Eocene was characterized by a warm and mostly ice-free climate worldwide, with temperate and subtropical forests extending across much of the globe. Fossil-rich rock units, such as the Green River Formation of the western United States and the Messel Formation of Germany, formed as sediments accumulated at the bottom of ancient lakes, often burying animal and plant remains that later fossilized. Many important groups of birds make their first appearance in these ancient lakebed treasure troves.
Eocene fossil skeletons reveal the new roles that birds began to play as they diversified into a plethora of new lineages. Over these Eocene lakes the earliest known frigate birds glided in search of fish to catch (or steal from other birds). Early members of the swift lineage sped across the sky, taking insects on the wing. In the trees, primitive members of the parrot lineage maneuvered through the branches using specialized grasping feet. Along muddy shorelines, some of the first curve-billed ibises probed for invertebrate prey. A radiation of nocturnal birds was also under way, with the first appearances of fossil relatives of the fruit-loving oilbird, the camouflaged insect-ambushing potoos, and the wide-beaked predatory frogmouths, whose modern descendants indeed consume frogs.
Heyday of the Mousebirds
Most people have never seen a mousebird. These small, social birds are found creeping through vegetation in their namesake mouse-like manner, sometimes clinging upside down using their dexterous grasping toes. Mousebirds are unknown to most because the six living species only occur in sub-Saharan Africa. We might thus assume mousebirds were always minor players in the story of avian evolution. Not so!
Fossils show that during the Eocene and Oligocene Epochs (56 million to 23 million years ago), mousebirds underwent a fascinating diversification that gave rise to a kaleidoscope of different forms. Among the species that flourished in North America and Europe during that golden age of mousebirds were the sharp-billed Chascacocolius cacicirostris, which specialized in prying open bark or hard fruits, the long-winged Celericolius acriala, which most likely chased down insects, and the parrot-skulled Oligocolius psittacocephalon, known from a skeleton packed with seeds. These forms had all vanished by 25 million years ago, possibly losing out to the wildly successful songbirds. They are only one example of many evolutionary radiations that were followed by diversity collapse.
Returning to the Sea
Since the first terrestrial vertebrates evolved nearly 400 million years ago, multiple lineages returned to the oceans, giving rise to secondarily aquatic groups such as whales, seals, and sea turtles. Penguins are the best-known avian lineage to make this transition, but they are by no means the only one.
Penguins made the plunge just a few million years after the Cretaceous mass extinction, during the Paleocene. Some species like Kairuku grebneffi subsequently reached standing heights of about 1.3 meters—30 percent taller than the largest penguin living today, the Emperor penguin. Plotopterids, a poorly understood, extinct group of diving birds, followed penguins into the sea during the Eocene. Whereas penguins are almost exclusively from the Southern Hemisphere (with the exception of the Galápagos penguin), plotopterids lived along the coasts of Japan and the Pacific Northwest from 35 to 15 million years ago. Despite their penguin-like appearance, plotopterids are thought to have been more closely related to gannets and cormorants.
A third group, the Alcidae, also adapted to marine diving in the Neogene. Like penguins and plotopterids before them, two lineages of stout, web-footed alcid birds lost flight and evolved flipper-like wings during the Miocene and Pliocene. The Great Auk represented a less advanced step on this journey back to the sea, because these birds never evolved the more stiffened wing and miniaturized scale-like wing feathers seen in penguins. Tragically, this species was wiped out in the mid-19th century because of overhunting. Farther back in time, from the Miocene to the Pleistocene, a different lineage of alcids, the Lucas Auks, took the same adaptive route further, evolving shorter, more flipper-like wing bones. The Great Auk once ranged along the northern Atlantic Ocean; in contrast, Lucas Auks were exclusively Pacific Ocean inhabitants.
Together, these four lineages present a classic study in convergence, the evolution of similar features in independent lineages. Each of these groups solved the problem of moving underwater in the same way: by adopting a style of locomotion called wing-propelled diving. Convergence occurred readily because bird wings are adapted for flying through a fluid—air. But in these cases of convergence, the birds “fly” through seawater, a fluid that is almost 800 times as dense as air.
Reign of the Terror Birds
Today, carnivorous mammals such as lions and wolves fill the apex predator role in most terrestrial ecosystems. However, over much of the Cenozoic Era, predatory birds struck fear into the mammals of South America.
Phorusrhacids, commonly known as terror birds, were a lineage of giant, flightless terrestrial birds that appeared shortly after the extinction of the nonavian dinosaurs. In a way, phorusrhacids represented a repeat of evolutionary history. Like their dinosaurian predecessors from the Cretaceous Period, they were huge-headed, bipedal, egg-laying killing machines. Add teeth, replace the tiny wings with tiny arms, and throw in a bony tail and you would essentially have a smaller version of Tyrannosaurus rex.
The largest bird skull ever discovered measures a whopping 70 centimeters long (shown above). This huge head belonged to the titanic Kelenken guillermoi, a species that lived during the middle of the Miocene (about 15 million years ago). Today, the only surviving relatives of terror birds are two species of seriema. These carnivorous birds reach heights of just 90 centimeters or so. Nevertheless, they terrorize rodents and small reptiles with their sharp claws and beaks.
Flight of the Sea Monsters
Among the last marvels of prehistoric times were the gigantic, “pseudotoothed” Pelagornithidae. These birds survived all the way up to the Pliocene Epoch, vanishing about 3 million years ago. Humans (genus Homo) appeared in Africa about 2.4 million years ago, and so our earliest direct ancestors just missed out on the sight of these enormous birds soaring through the skies.
With a wingspan approaching 20 feet, the magnificent Pelagornis chilensis dwarfed albatrosses and condors. Bones from Pelagornis chilensis and close relatives have been found on all seven continents, indicating these birds were exceptional long-range travelers, crossing whole oceans with efficient, gliding flight. Among the bizarre features of pelagornithids were their jaws full of tooth-like bony spikes. Unlike true teeth, which are made of enamel and sit inside sockets, these pseudoteeth were specialized outgrowths of the skull bones.
All of the extinct bird species illustrated so far died out due to natural factors such as sea level change, competition with other groups, or shifting forest environments. However, some astounding birds would still be extant today if not for human callousness. The list of casualties includes birds that range in size from minuscule to gigantic and in ecology from creeping insect hunters to crushers of large fruits. These birds have one factor in common: They all inhabited islands.
Throughout human history, wherever new people alighted as they migrated throughout the world, the situation turned grim for endemic birds. More than 500 extinctions of avian species on islands appear to have occurred in association with human arrivals. The timing of these extinctions ranges from roughly 50,000 years ago to just a few years in the recent past. In some cases, such as the moa of New Zealand, humans hunted a particular species for food, but in many others, indirect effects such as the introduction of nonnative predators or burning of woodlands were the drivers of extinction.
The list of vanished island species includes the largest terrestrial birds ever to have lived. In Madagascar, the elephant bird Aepyornis maximus reached heights of approximately 10 feet. In New Zealand, a radiation of a dozen or so moa species (many huge in size) browsed the land, filling the role that deer do in other ecosystems. These titans are well known to scientists and museum-goers, but many less imposing taxa disappeared as well. These oddities included the flightless, nocturnal, scent-foraging Kaua’i mole duck of Hawaii, which appears to have died out about 1,500 years ago, the iconic dodo and its cousin the solitaire (relatives of pigeons and doves wiped out in the 17th and 18th centuries), and several ground-dwelling New Zealand wrens including the tiny bush wren, which died out in 1972 after rats breached its last remaining stronghold and relocation efforts failed.
Although extinction is a natural part of evolution, the recent extinctions that have afflicted island birds are not natural. Whereas the Cretaceous mass extinction wiped out some avian lineages and opened new doors for others, today we are seeing avian biodiversity lost and replaced with homogenous sets of invasive species, reducing the resilience of these island ecosystems.
A dangerous misconception is that flightless birds like the dodo were destined for extinction because they were slow, foolish, or otherwise poorly adapted. However, these species fell victim not due to some flaw but because the environment to which they were well suited shifted too quickly. For example, species that nest on the ground on islands lacking land mammals benefit by staying hidden from aerial predators like hawks. This advantageous behavior suddenly becomes perilous when nonnative terrestrial predators such as cats are released.
Perhaps it is easier to turn away from this history of human-caused extinctions, but doing so will exacerbate the problem: The same phenomenon is occurring today in places like Guam, where introduced snakes are wiping out endemic birds. There are still opportunities to prevent the addition of more species to the island extinction toll. Whether we will act in time, though, remains a pressing question.