New Theory Of The Evolution Of Bird Flight Linked To Parental Care

This new theory, contradicting the two leading theories on the evolution of avian flight, appears this month in the German journal "Archaeopteryx," named after a feathered fossil with both reptilian and birdlike traits.

"The evidence indicates that a whole suite of behavioral and physical traits, including feathers and wings, evolved along with improved parenting and brood-care traits," said James Carey, a UC Davis demographer and ecologist. "Once the precursors to birds began to fly, the ecological interplay of flight and parental care may have been mutually reinforced, continuing the evolution of both traits and accelerating the rate at which the physical features of the modern bird were acquired."

The origin of bird flight is a fundamental issue in avian biology and in overall evolutionary theory. Many scientists point to the fossilized specimens of Archaeopteryx as evidence that there was a transitional vertebrate species that developed during the evolution from reptilian dinosaurs to birds. Furthermore, they suggest that the development of flight may explain why birdlike dinosaurs avoided extinction.

Until now, there have been two basic theories on the origins of bird flight. The first, the arboreal theory, is a tree-to-ground model, suggesting that birds' primitive ancestors were tree-dwellers that leapt from branch to branch. Through the ages, the ability to first glide and later fly developed because gliding slowed their fall to earth when they missed a branch.

The second, the cursorial theory, is a ground-up model that suggests that birds evolved from four-legged reptiles. According to this theory, scales on these creatures' front limbs gradually developed into feathers that gave them upward thrust when they ran and eventually enabled them to fly.

Carey maintains that both of these theories have major flaws. If the arboreal theory were accurate, birds' early ancestors would more likely have been four-legged creatures that developed membranes between their front and back legs, much like flying squirrels or bats, he asserts.

And, if the cursorial theory were true, there should have been obvious useful advantages to each form that developed between the four-legged reptile and the bird, he maintains. He is not persuaded by the suggestion that the intermediate stages of this line used their feathers, first developed to conserve heat, to swat insects from the air.

On the other hand, the parental-care theory is consistent with both the physical and behavioral changes that appear to have occurred as reptiles evolved into winged dinosaurs and finally into modern birds, Carey says.

He suggests that modern birds' very early ancestors were reptiles that established and guarded their nests on the ground, much like crocodiles. Over time, these creatures developed hard-shelled rather than leathery eggs and the ability to modulate their own body temperature in order to provide a more constant environment for their developing young. Scales evolved into feathers, better camouflaging and insulating the parents.

In time, these early ancestors of birds developed more advanced techniques for caring for their young. They started to feed their young in the nest, pumping liquid food or placing small food items in their mouths. They also began to produce fewer and more dependent offspring and smaller eggs, and began nesting in bushes and then small trees to better protect their offspring from predators.

Gradually the forelimbs of these creatures became feathered and even more elongated, enabling them to better manipulate their eggs and to "parachute" from their tree nests to a soft landing. Later they would develop the ability to glide and eventually fly by flapping their wings.

Carey hypothesizes that bird beaks also evolved in the context of parental care. The beak, he suggests, serves both as a point source