Feathers
White-fronted Bee-eater, Merops bullockoides
There is one characteristic that defines a bird and only a bird: feathers. Birds are the only living creatures with feathers, but several small dinosaurs also possessed them. Why? Many clues tell us that some dinosaurs were warm-blooded (homeothermic), needing a constant warm body temperature, and therefore needing insulation. So through evolution, scales elongated, became thinner, and feather-like structures came into being. The so called “first bird,” Archeopteryx, was skeletally a reptile, but it was covered with well-developed feathers. As the feathers elongated, they enabled these animals to glide from trees and lift off the ground at least a bit. Eventually the arms and hands became functional wings and tails that allowed early birds to fly.
Ultimately, feathers covered the entire bird except for the beak and feet, serving to insulate, propel, and make the body aerodynamic. Feathers are important enough to comprise 15-20 percent of the weight of a bird. In numbers a hummingbird’s small body might need only 1000 feathers while a swan would require 25,000.
Feathers are specialized for different functions. There are the flight feathers of the wings and tail to provide propulsion and lift. Contour feathers cover the body serve to make the bird aerodynamic. Under the body feathers we find down feathers for insulation. Between the down and contour feathers are semiplumes that fill in the gaps and also smooth the body. Modified, bristlelike filoplumes surround each body feather and monitor its position.
Every contour feather consists of a main shaft with barbs on opposite sides. Barbs can be separated and rejoined by running your fingers down the feather shaft; birds do this with their beaks to keep their feathers in good shape and remove dirt and parasites, and, squeezing oil from their tail’s oil gland with their beak, waterproof their plumage; this is called preening. But feathers suffer wear and tear and so are replaced by a regular molt, usually twice a year. Old feathers fall out and new ones grow in, usually in a regular pattern so the bird does not lose its flying ability in the process.
What we first notice when we see a bird is its shape, formed by the feathers, and the color and pattern of the plumage. The most obvious role of feathers is advertisement. Birds recognize others of their own species, tell sexes apart, and sometimes distinguish ages. Summer plumages are generally the brightest and most elegant as the generally more attractive male tries to entice a mate. Winter plumages serve to make the bird less obvious to predators. Special feathers like the showy tails of a peacock, the plumes of a heron, and the iridescent colors of hummingbirds enhance their chances of passing on their genes.