Birds of a feather

StoryImage( ‘/Images/Story//Auto-img-112914932328256.jpg’, ‘Photo and diagram by Dr. Robert A. Hedeen’, ‘The basic parts of a bird's contour feather.’);

From a standpoint of evolutionary success, most zoologists agree that in the competition for being the most successful group of vertebrate animals (fish, amphibians, reptiles, birds, and mammals), birds win the prize. Of the many evolutionary advances they have developed over the eons of time, the acquisition of feathers is perhaps the most important.

There is no doubt that birds evolved directly from reptiles, and in recent times there is overpowering evidence that this ancestral reptilian stock was a group of small carnivorous dinosaurs.

The first bird in the Earth’s long history was discovered in fossilized form in a stone quarry in Solenhofen, Germany, in 1861. It was given the scientific name of Archaeopteryx, and can be viewed in the British Museum. This creature was decidedly more reptilian than avian, but it did possess one salient trait of all birds: feathers. Scales covering the integument are characteristic of reptiles whose transformation into feathers is well documented.

An important factor in the evolutionary success of birds was the ability for most of them to fly. Hollow bones and other modifications give birds less weight, and feathered wings provide the equipment for solving the problems associated with aerodynamics.

A feather is very light-weight (“as light as a feather”), yet has remarkable toughness and tensile strength. A typical contour feather consists of a hollow quill, or calamus, thrust into the skin, and a shaft, or rachis, which is a continuation of the quill and bears numerous barbs. The barbs are arranged in closely parallel fashion, spread diagonally outward from both sides of the central shaft to form a flat, expansive, webbed surface called the vane. There may be several hundred barbs in the vane.

There are several types of feathers serving different functions. Contour feathers give the bird its outward form and are the type represented in the accompanying photograph. Contour feathers may extend beyond the body, and, if used in flight, are called flight feathers. Down feathers are soft tufts and are mainly hidden beneath the contours. Down feathers are abundant on the breasts of water birds and the young of other birds, and serve mainly to conserve heat. Many is the time I have watched a Canada goose pluck down from her breast to line her nest so her eggs may be kept warm when she infrequently leaves it to feed or to take a bath.

Filoplume feathers are hairlike, degenerate feathers, each with a weak shaft and short branches at the end. These are the “hairs” that remain on a plucked fowl and may be singed off if desired before preparing the bird for the dinner table.

A fourth type is called powder-down feathers, and is highly modified. These are found on herons, bitterns, hawks, and parrots. The tips of this type of feather disintegrate as they grow, and a powder is released that helps to waterproof the other feathers and give them a metallic luster.

Feathers are epidermal structures that evolved from the reptilian scale. Indeed, a developing feather closely resembles a reptile scale when growth is just beginning. We can hypothesize that the scale elongated and its edges frayed outward until it became the complex feather of birds. Also attesting to the relationship of birds to reptiles is that modern birds possess both scales (especially on their feet) and feathers.

When fully grown, a feather, like a hair, is a dead structure and is eventually cast off. The shedding, or molting, of feathers is an orderly process. Feathers are discarded gradually to avoid the appearance of bare spots. An exception to this is penguins, which shed their feathers all at once. (As I watched the delightful recent movie The March of the Penguins, I kept waiting for a sequence showing the birds without feathers, but this educational production did not oblige in that respect). Flight and tail feathers in other birds are discarded in exact pairs, one from each side in an order that balance is maintained. Almost all birds molt at least once a year, usually in late summer after the nesting season.

Almost every time I see a bird now, I realize that this usually small, delicate, and frequently beautiful creature developed from a creature similar to the horrific monster JANE in Rockford’s Burpee Museum of Natural History, and that is really fantastic!

Dr. Robert Hedeen is a former resident of Maryland’s eastern shore and resided in the Chicago area from 1960 to 1971. He is a retired professor emeritus of biological sciences in the University of Maryland system. He has published more than 30 scientific papers, has written numerous magazine articles, and is the author of two books on the natural history of the Chesapeake Bay.

From the Oct. 12-18, 2005, issue

Enjoy The Rock River Times? Help spread the word!