Feathers are the most complex of all outgrowths of skin. The number of interlocking parts—barbs, barbules and barbicels—in a feather smaller than your little fingernail runs to an astronomical figure. In relation to their weight, feathers are the strongest and most elastic of all organic structures, and as an insulating covering they are without peer. When it comes to the second important function of "clothing," namely adornment, feathers can easily hold their own with mink and sable, as well as with the jeweled integument of butterfly wings.
A bird's plumage owes its charm to both pattern and color. The latter springs from two different sources, one of which might be called physical, the other chemical. Hold a feather from a bluebird's back against the sky and what do we see? Nothing but a dull leaden screen. Now move it so that it is viewed by reflected instead of transmitted light and it magically turns azure. The reason is that the blue is a prismatic hue rather than a pigment. It is merely splintered light, which aging can neither change nor fade.
Most of the other hues of feathers are due to pigments, colored substances such as a painter might spread upon canvas. Many of these are also impermanent in the sense that sunlight can decompose them. In the venerable National Museum of Natural History in Paris there is a panel of mounted South American hummingbirds that have been on exhibition since the 18th century. Decades of unshaded daylight have bleached out nearly all the pigment colors, whereas the prismatic colors still flash forth iridescence. The specimens are now only shimmering ghosts of hummingbirds.
Birds, like men, vary their garb according to age, sex and season, and refurbish it frequently enough to maintain a high sartorial standard. In adult birds the plumage of the two sexes may be either identical, similar, or extraordinarily different. Wherever it is alike, as in most sea birds and many others, the family responsibilities of the mated pair are likely to be shared in common. Father as well as mother helps in the incubation of the eggs and the defense and feeding of the young. At the other extreme are the birds of paradise, in which the indescribably ornate cocks have no reproductive function other than to court and fertilize the demurely inconspicuous females. In one Old World sandpiper known as the ruff, we find the last word in sexual dissimilarity because no two male ruffs disport the same breeding plumage. Each is an "original creation," as though it were an item of costly millinery.
In the same wader or sandpiper group are even more remarkable tricks of feathering. The three species of phalaropes, for example, exhibit plumage reversal, the female possessing stronger color and pattern than her mate. This is concomitant, moreover, with a transposition of reproductive behavior. The female phalarope takes the initiative in courtship and in selection of the nest site, whereas the male does all the setting and rearing of young.
Among many of our familiar birds the plumage changes of season or lifetime, or both, are altogether spectacular. Some kinds adopt their final styles while they might yet be called children. Others require years to attain the ultimate adult dress. Some of our most resplendent summer songbirds, such as the scarlet tanager, indigo bunting, bobolink and numerous warblers, have become by autumn strikingly transformed. Usually the postnuptial molt makes the self-advertising male assume some degree of resemblance to the modestly colored female. During the transition male tanagers become a motley patchwork of scarlet and olive green. After completion of the process they can still be distinguished by their retention of black wings, which the female never has.
But although plumage is for dress up and display, we must not forget its many other functions. It keeps the skin of a bird cool in the heat and warm in the cold. When feathers are fluffed out by superficial muscles at the base of each, the entrapped air (an excellent nonconductor) can be trebled in thickness. Feathers on a living bird wash without becoming waterlogged. Preening, and the application of pomade from a little compact called the oil gland which most birds carry just above the tip of the tail, keep them in bandbox condition. The eel-catching bittern even manufactures a glandular dry shampoo from friable tufts of feathers known as powder-down patches. Wing or tail quills in many kinds of birds "sing" or, more properly, make instrumental music. And, finally, let us remember that all the refinements of man-made flight, such as camber, torque, wing-slotting, and adjustable angle of attack, had been worked out in feathers eons before the cave man first sketched a bird on the wall of his dark retreat.