Feathers are one of the most remarkable structural engineering feats of nature. They are extremely light weight, while incredibly strong and durable. They can insulate against cold and create aerodynamic surfaces to help facilitate flight. And feathers come in a dazzling array of colors!
Feathers are unique to birds and serve as one of their defining characteristics. However, most scientists argue this was not always the case. A number of small fossil dinosaurs with distinct impressions of feathers in their skin have been discovered in parts of China and Siberia. These fossils date back 235-240 million years ago, roughly 90 million years before the first birds appeared. It has long been believed that modern birds are direct descendants of a group of two-legged dinosaurs called therapods, which included T. rex and the velociraptors made famous by the movie Jurassic Park. In fact, many evolutionary biologists believe that birds are actually small dinosaurs.
The feathered dinosaurs never contemplated flight; they were covered with feathers to help retain body heat. Like mammals, many species of dinosaurs were warm-blooded and the insulating properties of feathers helped them conserve energy.
Although the feathered dinosaurs didn’t fly, feathers were a prerequisite for flight and it makes perfect sense that birds descended from feathered dinosaurs. How they made the transition from ground to sky is a matter of conjecture, but one theory suggests the development of elongated feathers on the forelimbs helped the precursors of flying birds capture flying insects.
Whatever the evolutionary process was that created modern birds, feathers were a central aspect of their ascendency from dinosaurs to every avian species we see today.
The basic structure of a typical feather includes the calamus (or quill, as it is also called), a central rachis (also called the shaft) which branches into barbs, and then into barbules with small hooks that interlock with nearby barbules. While there are evolutionary variations on this anatomical theme, all modern feathers are based on this fundamental design.
Thus, feathers are often grouped into the following seven types: wing (also called flight feathers), tail, contour, down, semiplume, filoplume, and bristle.
The function of wing and tail feathers is self explanatory, while contour feathers cover most of a bird’s body to give it an aerodynamic shape. Semiplume and down feathers help insulate birds from heat loss. Filoplumes and bristle feathers are the most specialized and deviate dramatically from the basic feather design described above. Filoplumes may be located on the wings, tails, and bodies of most birds and serve a sensory function that can help birds determine temperature, wind speed, and feather movements. Bristle feathers are typically found on the head and look much like hairs or whiskers. They can help protect the eyes, or in the case of flycatchers, help capture flying insects.
Other modifications have evolved in these seven feather types to provide additional advantages for survival and reproduction. For example, the tail feathers of woodpeckers are more rigid than other birds so they can lean back and brace against them while hammering on a tree. Male Anna’s hummingbirds have specially adapted tail feathers that cause them to create a loud chirp during their courtship flight.
Courtship, in fact, seems to have driven the most extremes in feather evolution. Most birds have 10 to 12 tail feathers, but male peacocks have 150 to 200 that are long and brightly colored. The male Indonesian peafowl, a relative of the peacock, has tail feathers that measure more than 5 feet long!
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