THE SOCIAL SIGNIFICANCE OF AVIAN WINTER PLUMAGE VARIABILITY.

Abstract

Animal species vary enormously in the degree to which individuals are similar or different from one another in appearance. In some birds, such as juncos and Harris sparrows, most individuals look different from each other, while in others, such as chickadees and song sparrows, there is little individual variation. These differences in appearance among species require several levels of interpretation. First, an understanding of their social significance is required for us to know what is being communicated by these differences. Second, we should understand the proximate mechanisms by which a correspondence between signal and message is achieved. Finally, we must understand the ecological factors which are ultimately responsible for the social systems and their associated plumage patterns. This paper is addressed only at the social significance of winter plumage variability versus monomorphism. Ordering principles pertaining to animal coloration-theories relating appearance to camouflage, advertisement, physiology, or the formation of search images-are of little help in explaining why some winter birds are highly variable and others monomorphic. The first three of these theories invoke selective principles which are expected to produce monomorphic patterns within populations and, thus, do not relate to plumage variability. Camouflage coloration is expected to select for monomorphism achieved through mimicry of environmental background, substrate color or specific objects. Likewise, advertisement or poster coloration (Lorenz, 1967), evolved by selection for warning coloration, mimicry, species or sex recognition, or advertisement of territorial possession, is not expected to lead to intra-population variability. In some cases of mimicry this is not true, but these cases always select for discontinuous variation, with morphs mimicking various discrete models. Finally, selection upon color pattern for physiological efficiency depends upon assuming one of the radiation colors, generally either black or white (Hamilton, 1973). Small temperate land birds are rarely uniformly white or black, suggesting that physiological needs have probably not been primal in determining their color patterns. Formation of search images does anticipate variability in populations and is sometimes raised to explain the highly variable plumages of certain birds. Clarke (see 1969, and references cited therein) originated the idea to explain the Cepaea snail polymorphisms through apostatic predation, and Croze (1970) has done much