Wednesday, November 07, 2007

Bird Claws and Evolution

One of the interesting questions in bird evolution is whether early bird ancestors were primarily ground-based or tree-based. That is, did birds start on the ground and move up, or start in trees and move down?

A new paper in Current Biology proposes to answer the question by looking at the shape of birds' feet. The researchers first classed 249 bird species from seven orders according to their feeding behavior. Birds in group GB - such as herons, rails, and pheasants - feed on the ground and fly only to roost or escape predators. The other extreme, group V, contains vertical climbers like woodpeckers that forage only in trees. The middle groups - comprised of pigeons and cuckoos - have species that show both behaviors.

They then measured the claw angle on each bird's middle toe. The shape of a bird claw conforms to a circular arc. "Claw angle" refers to the angle between the radii extending from the center of this arc to the base and tip of the claw. What they found was that birds with a greater claw angle spent more of their time in trees than on the ground. This was true even within the pigeons and cuckoos.

The study suggests that claw angle can be a predictor of behavior in early birds and bird-like dinosaurs in addition to modern birds. Measuring the claw angle for extinct species presents challenges because of incomplete preservation. In those species where it can be measured or estimated from fossil remains, ancestral birds show greater similarity to modern ground-dwelling birds.

According to this indicator, early bird ancestors probably spent most of their time on the ground. Specializations for life in trees would have come later.

(Via Evolving Thoughts)