Avian Plumages and Molts

Abstract

The primary function of feathers is flight, but th'ey also insulate and protect the body from the physical environment. The reptilian scales from which they evolved have this as their principal function. Perhaps a reptile that was becoming warm blooded evolved frayed scales to provide better insulation and these in turn proved to be preadapted for flight. In birds that long ago lost the ability to fly such as the kiwis (Apteryx) the vaned structure of the feathers of flying birds has deteriorated. Feathers are collectively called just as the hairs of a mammal make up its pelage or fur. The term is also used for particular ensembles of feathers such as downy and male breeding plumage. Recently Humphrey and Parkes (1959) have proposed a new usage for the word plumage, and the entire concept of plumages is discussed at greater length later in this paper. If all birds had uniformly black feathers like a crow and if selection for flight and insulation were the only influencing factors, what direction would the study of feathers take? The adaptations, especially of the wing and tail feathers, for flight would be of paramount interest: first, the general adaptations and then such specialization as notched outer primaries and forked tails, which aid aerial stability or maneuverability. Then one would turn to the original function of feathers and other epidermal structures, that of protecting the animal from the environment. This function has been retained in feathers, and, as suggested, may have preadapted them for flight. But not all birds are black. Feathers often have complex patterns, culminating in such amazing structures as the vanes of the peacock (Pavo), which are, as Darwin noted, one of the problems posed by nature to the theory of natural selection. This variety in color pattern extends to different parts of the bird's body and to the individual bird at different ages and seasons. That the shape and texture of feathers is as adaptable as their color scheme is shown by the extravagantly modified plumes of egrets and birds of paradise. Even the normal structure of the feather may be lost, as in the celluloid-like head feathers of the Curl-crested Araqari (Pteroglossus beauharnaesii) or the plumes of the King of Saxony's Bird of Paradise (Pteridophora alberti). Feathers basically adapted for flight maneuverability such as a forked tail may secondarily be modified for display, as in the Greater Racket-tailed Drongo (Dicrurus paradiseus). In several instances, for example the tail feathers of snipe (Gallinago), feathers have become modified for producing sounds used in sexual display. It is clear, then, that feathers and plumage are extremely responsive to selection. What are the secondary functions they have acquired, over and above the primary ones of flight and insulation? Camouflage. Streaked grassland birds, sand-colored desert birds, and white arctic birds are well-known examples of concealing coloration. Sometimes the camouflage is heightened by behavior as when a bittern (Botaurus) stretches head and neck skyward, thus blending with the marsh vegetation. The dark hues of forest birds (Gloger's Rule), the white of arctic birds, and the sandy hues of desert birds are probably primarily adaptations for concealment. Display. The important relation of feathers to display cannot be overemphasized. (a) Displays related to reproduction. We do not need to outline the formal classifica-