Tom stepped into another scientific controversy when he proposed that the small dinosaurian ancestors of birds evolved wing feathers, not to fly but to brood their young. Flying, according to his theory, came later. Although Tom's theory has yet to gain wide acceptance in the field of paleontology, it was endorsed by John Ostrom, the chief proponent of the concept that birds evolved from ground-dwelling dinosaurs. With Ostrom's encouragement, Tom's theory was included in the book Feathered Dragons: Studies of the Transition from Dinosaurs to Birds published in 2004. Click these links for a copy of the article in pdf format, either as a self-archived, text-searchable, open access postprint version, the article as it appeared in the book, or a short abstract.
Although the relationship between birds and dinosaurs is obvious in light of fossils showing their similarities, the innovation of flight in birds was not clear. Around 100 million years ago animals existed that were feathered, but lacked the long wing feathers modern birds use for flight. Theorists were divided into two camps over how non-flying dinosaurs evolved the long feathers that enabled them to take to the air. On one side, the "arborial" theory had it that tree-dwelling dinosaurs evolved flight by gliding down from trees. Ostrom's "cursorial" theory postulated that flight arose when small, two-legged running dinosaurs somehow evolved feathers capable of lifting them off the ground.
Each theory had its merits and detractors. Key weaknesses in both theories involved the first stages: how could animals without wings begin jumping from trees without being injured or killed? On the other hand, why would ground-dwelling creatures begin flapping their arms when their feathers were too short for liftoff?
Tom weighed both arguments and after thoroughly researching knowledge in the field, he and scientific illustrator Mark Orsen developed a new version of the old cursorial theory: the Brooding-To-Flight Hypothesis.
In this concept, small, two-legged running dinosaurs, which are known from fossil evidence to have had short feathers covering their bodies, evolved long feathers on their forelimbs, not to fly at first, but to brood their young. Over time, longer and longer feathers evolved to improve brooding abilities. Then, once feathers were long enough, they could augment the animal's airborn time when it leaped from the ground to avoid enemies or while hunting.
This theory has attracted interest from paleontologists because it is the first to offer a rational scenario for gradual feather lengthening: any small improvement to the ability to shelter offspring would offer a survival advantage. Neither the arboreal or cursorial flight hypotheses could explain the early stages, because short feathers are useless for flight. Now, after more than a century of bickering among scientists, Tom and Mark's new concept offers hope a new and better understanding of the evolution of birds from their dinosaurian forebears.