As little as 20 years ago, the idea that birds descended from dinosaurs seemed completely ridiculous--after all, birds are small, light, fluttery creatures, while dinosaurs were huge, plodding, and distinctly unaerodynamic. But as the evidence began to mount--dinosaurs with feathers, beaks, and other birdlike characteristics--the dinosaur/bird connection gradually trickled up into the mainstream. Today, it's the rare paleontologist who disputes the descent of birds from dinosaurs.
However, this doesn’t mean that all the technical aspects of the dinosaur/bird transition have been settled once and for all. Researchers still disagree about which dinosaurs are most closely related to modern birds, whether or not terrestrial dinos had feathers, and--perhaps most contentiously of all--how these proto-birds made the huge evolutionary leap into powered flight.
The Origin of Feathers
First things first, though. Before any dinosaur could take to the air, it had to be equipped with a key evolutionary adaptation suited to flight--that is, feathers.
Why, and how, did dinosaurs evolve feathers? It's a common mistake among people unversed in evolutionary theory to assume that feathers evolved specifically for the purpose of flight. Evolution, however, is a "blind" process—it doesn’t "know" where it's going until it gets there. For this reason, the accepted explanation among scientists is that dinosaurs evolved feathers as a form of insulation in cold climates (and, possibly, as a way to puff themselves up in the eyes of the opposite sex).
If this sounds unlikely, bear in mind that even bird species that have been flightless for millions of years still have feathers. If the purpose of feathers were solely to power flight, there'd be no reason, from an evolutionary perspective, for penguins to keep their feathers: in fact, they might be better off with thick coats of fur!
The first dinosaurs with feathers are thought to have appeared on the scene about 140 million years ago. As the aeons ground on, these primitive (that is, short and hairlike) feathers gradually evolved into the broad, flat kind we're familiar with today, which are better at trapping air (and thus insulating the skin beneath). At this point the question poses itself: how did these feathered dinosaurs make the transition to flight?
Theory #1: Dinosaurs Took a Running Leap
Extrapolating backward from the behavior of modern birds, like ostriches, it's reasonable to infer that smallish, two-footed dinosaurs could easily reach top running speeds of 30 or 40 miles per hour. Since birds are believed to have evolved from theropods--bipedal, carnivorous dinosaurs like Deinonychus and Velociraptor--it has become fashionable to portray these (and other) carnivores as coated with feathers.
Here's how the "running leap" theory goes: as they hit their top speeds chasing down prey (or trying to elude bigger carnivores), feathered theropods discovered that their coat of insulating feathers gave them an aerodynamic "bounce," helping them land their next meal or live another day. Since well-fed theropods, or those that eluded carnivores, would produce more offspring, the evolutionary trend would be toward larger feathers, which provided more "lift." From there, it was only a short step to taking actual flight for short periods of time.
In the excellent Nova episode The Four-Winged Dinosaur (about the Microraptor recently discovered in China), a scientist is quoted to the effect that the baby birds of some species may reproduce this ancient evolutionary heritage. Though they're still unable to fly, they can jump farther, and scuttle up inclined surfaces, with the aid of their feathers--the same advantages supposedly enjoyed by primitive theropods.Theory #2: Dinosaurs Fell Out of Trees
Birds aren't the only animals whose behavior can be extrapolated back to ancient dinosaurs. Flying squirrels glide across the forest, from tree to tree, by leaping off tall branches and spreading the flaps of skin attached to their arms and legs. They’re not capable of powered flight, of course, but they can glide for impressive distances, up to two-thirds of the length of a football field.
Conceivably, some species of feathered dinosaur might have lived in trees (this would entail a relatively small size and the ability to climb). They then might have followed the same evolutionary path as flying squirrels, gliding for longer and longer distances as their feathers slowly adapted to the optimum shape. At some point, the evolutionary innovation of flappable wings would have allowed them to take to the air for indefinite periods.
The main problem with this "arboreal" theory, as it’s called, is that it's easier to imagine powered flight developing in the ground-up approach (picture a terrified dino desperately flexing its vestigial wings to escape a larger carnivore) than as a result of tree-to-tree gliding. Also, to date, no flying squirrel (with the exception of Bullwinkle's pal Rocky) has achieved powered flight.
Current Thinking About Dinosaurs and Birds
The problem (if you can call it that) with feathered dinosaurs is that new species are constantly being discovered, many of them in China. Since these specimens date back to different eras, separated by tens of millions of years, it can be difficult for paleontologists to reconstruct the exact evolutionary line that led from dinosaurs to birds.
Microraptor is a good example. In the Nova documentary, this weird, four-winged creature provokes intense disagreement among paleontologists, some of whom see it as an evolutionary dead end, others as an "intermediate" form between dinosaurs and birds, and yet others as not technically a dinosaur at all, but an offshoot from a point of the evolutionary tree long before dinos came on the scene.
Today, most paleontologists subscribe to the theory that a) birds are descended from dinosaurs, and b) flight developed from the ground up, rather than from the trees down. But as with all things prehistoric, these views may change with the next spectacular fossil find.
