You'd think it would be an easy matter to tell the story of bird evolution--after all, it was the striking adaptations of finches on the Galapagos Islands that, in the 19th century, led Charles Darwin to formulate the theory of evolution. The fact is, though, that gaps in the geological record, differing interpretations of fossil remains, and even the exact definition of the word "bird" have prevented experts from coming to a consensus about the distant ancestry of our feathered friends. Still, most paleontologists agree on the broad outlines of the story, which goes as follows. (See a complete list of prehistoric birds and a gallery of prehistoric bird pictures.)
Archaeopteryx & Friends - The Birds of the Mesozoic Era
Although its reputation as the "First Bird" has been overblown, there are good reasons to consider Archaeopteryx the first animal to inhabit a place more on the bird than on the dinosaur end of the evolutionary spectrum. Dating from the late Jurassic period, about 150 million years ago, Archaeopteryx sported such avian features as feathers, wings and a prominent beak, though it had some distinctly reptilian traits as well (including a long, bony tail, a flat breastbone, and three claws jutting out of each wing). It's not even certain that Archaeopteryx could fly for extended periods of time, though it would easily have fluttered from tree to tree. (Recently, researchers announced the discovery of another "basal avilian," Aurornis, that predated Archaeopteryx by 10 million years; it's unclear, though, if this was any more a true "bird" than Archaeopteryx was.)
From whence did Archaeopteryx evolve? Here's where matters become a bit ambiguous. While it's reasonable to assume that Archaeopteryx derived from small, bipedal dinosaurs (Compsognathus is often cited as a likely candidate, and then there are all those other "basal avilians" of the late Jurassic period), that doesn't necessarily mean that it lay at the root of the entire modern bird family. The fact is that evolution tends to repeat itself, and what we define as "birds" may have evolved multiple times during the Mesozoic Era--for example, it's possible that two famous birds of the Cretaceous period, Ichthyornis and Confuciusornis, as well as the tiny, finch-like Iberomesornis, evolved independently from raptor or dino-bird forebears.
But wait, things get even more confusing. Because of gaps in the fossil record, not only could birds have evolved multiple times during the Jurassic and Cretaceous periods, but they could also have "de-evolved"--that is, become secondarily flightless like modern ostriches, which we know descended from flying ancestors. Some paleontologists believe certain birds of the late Cretaceous, like Hesperornis and Gargantuavis, may have been secondarily flightless. And here's an even more dizzying idea: what if the small, feathered raptors and dino-birds of the age of dinosaurs were descended from birds, and not the other way around? A lot can happen in the space of tens of millions of years! (For example, modern birds have warm-blooded metabolisms; it's entirely likely that small, feathered dinosaurs were warm-blooded as well.)
After the Mesozoic - Thunder Birds, Terror Birds, and the Demon Duck of Doom
A few million years before the dinosaurs went extinct, they had pretty much disappeared from South America (which is a bit ironic, considering that's where the very first dinosaurs probably evolved, back in the late Triassic period). The evolutionary niches that had once been occupied by raptors and tyrannosaurs were quickly filled by large, flightless, carnivorous birds that preyed on smaller mammals and reptiles (not to mention other birds). These "terror birds," as they're called, were typified by genera like Phorusrhacos and the big-headed Andalgalornis and Kelenken, and prospered until a few million years ago (when a land bridge opened between North and South America and mammalian predators decimated the giant bird population). One genus of terror bird, Titanis, managed to prosper in the southernmost reaches of North America; if it sound familiar, that's because it's the star of the horror novel The Flock.)
South America wasn't the only continent to spawn a race of giant, predatory birds. The same thing happened about 30 million years later in similarly isolated Australia, as evidenced by Dromornis (Greek for "running bird," even though it doesn't seem to have been particularly fast), some individuals of which attained heights of 10 feet and weights of 600 or 700 pounds. You might assume that Dromornis was a distant but direct relative of the modern Australian ostrich, but it seems to have been more closely related to ducks and geese.
Dromornis appears to have gone extinct millions of years ago, but other, smaller "thunder birds" like Genyornis lasted well into early historical times, until they were hunted to death by aboriginal human settlers. The most notorious of these flightless birds may be Bullockornis, not because it was particularly bigger or deadlier than Dromornis but because it has been given a particularly apt nickname: the Demon Duck of Doom.
Rounding out the roster of giant, predatory birds was Aepyornis, which (wouldn't you know it) dominated another isolated ecosystem, the Indian Ocean island of Madagascar. Also known as the Elephant Bird, Aepyornis may have been the biggest bird of all time, weighing close to half a ton. Despite the legend that a full-grown Aepyornis could drag off a baby elephant, the fact is that this imposing bird was probably a vegetarian. A relatively late newcomer on the giant bird scene, Aepyornis evolved during the Pleistocene epoch and lasted well into historical times, until human settlers figured out that a single dead Aepyornis could feed a family of 12 for weeks!
Victims of Civilization: Moas, Dodos and Passenger Pigeons
Although giant birds like Genyornis and Aepyornis were done in by early humans, most of the attention in this regard centers on three famous birds: the moas of New Zealand, the Dodo Bird of Mauritius (a small, remote island in the Indian Ocean), and the North American Passenger Pigeon.
New Zealand's moas formed a rich ecological community all by themselves: among them were the Giant Moa (Dinornis), the tallest bird in history at a height of 12 feet, the smaller Eastern Moa (Emeus), and assorted other picturesquely named genera such as the Heavy-Footed Moa (Pachyornis) and the Stout-Legged Moa (Euryapteryx). Unlike other flightless birds, which at least retained rudimentary stumps, moas completely lacked wings, and they seem to have been devoted vegetarians. You can figure out the rest for yourself: these gentle birds were completely unprepared for human settlers, and didn't know enough to run away when threatened--the result being that the last moas went extinct about 500 years ago. (A similar fate befell a similar, but smaller, flightless bird, New Zealand's Great Auk.)
The Dodo Bird (genus name Raphus) wasn't nearly as big as the typical moa, but it evolved similar adaptations to its isolated island habitat. This small, plump, flightless, plant-eating bird led a pretty much care-free existence for hundreds of thousands of years, until Portuguese traders discovered Mauritius in the 15th century. The Dodos that weren't easily picked off by blunderbuss-wielding hunters were torn apart by (or succumbed to diseases carried by) the traders' dogs and pigs, making them the poster birds for extinction down to the present day.
Reading the above, you might get the mistaken impression that only fat, flightless birds can be hunted to extinction by humans. Nothing could be further from the truth, a case in point being the Passenger Pigeon (genus name Ectopistes, for "wanderer.") This flying bird used to traverse the North American continent in flocks of literally billions of individuals, until overhunting (for food, sport and pest control) rendered it extinct. The last known passenger pigeon died in 1914 at the Cincinnati Zoo, despite belated attempts at preservation.