Plumage Polymorphism Research Articles

Sexual selection and variation in reproductive strategy in male yellow warblers (Dendroica petechia)

Male yellow warblers (Dendroica petechia) exhibit extensive variation in the amount and conspicuousness of the sexually distinctive brown streaking on the breast. We investigated this intraspecific variation in degree of sexual dichromatism to see if male plumage rank (essentially the amount of brown streaking) is correlated with the amount of reproductive effort allocated to parental investment, as sexual selection theory would predict. As predicted, the average rate of feeding visits to the nest by males was negatively correlated with the plumage rank of the male in 1982 (Fig. 1) and 1983 (Fig. 2), and varied little between years or among study areas. Within one study area, the higher nest visit rate of dull (low plumage rank) males was correlated with higher nestling growth rates. But in the other two study areas, where bright (high plumage rank) males predominated, nestlings of bright males tended to have the highest growth rates despite minimal nest visit rates. Thus, overall there was no correlation between male plumage rank and nestling growth rate, a potential index of relative reproductive success, in either year (Figs. 3 and 4). This overall equal nest success likely helps to maintain the behavioral and plumage polymorphism. Since we could find no evidence that the high nestling growth rates of bright males were due to an adjustment of female parental effort to compensate for low male parental effort, there must be some other benefit of being bright which contributes to nest success. To address this, we used the relationship between parental feeding rates and nestling growth rates for each nest as an index of relative territory quality. This analysis suggests that bright males generally occupy the best territories available, possibly because they are more aggressive in obtaining and defending them than dull males. Dull males appear to be relegated to poorer territories, where an increase in male parental contribution would be necessary to achieve high nestling growth rates. We propose that different males are using alternative but evolutionarily stable reproductive strategies, in a way that is consistent with the predictions of sexual selection theory. Allocation of limited reproductive effort to parental investment decreases as sexual dichromatism increases, which probably reflects an increased investment in intermale competition for the best territories.

LIFE HISTORY STUDIES OF THE LESSER SNOW GOOSE (ANSER CAERULESCENS CAERULESCENS). IV. THE SELECTIVE VALUE OF PLUMAGE POLYMORPHISM: NET VIABILITY, THE TIMING OF MATURATION, AND BREEDING PROPENSITY.

Cooch (1961, 1963) suggested that changes in the genotypic composition of snow goose colonies comprising the Hudson Bay-Foxe Basin population could be attributed to selection favoring the blue phenotype. In the preceding paper (Cooke et al., 1985), we examined potential differences in net fecundity between the two phenotypes at La Pérouse Bay in northern Manitoba. No substantial differences in any component of fecundity were detected. In the present paper, we examined potential differences in pre-reproductive and adult viability, age of maturation, and breeding propensity. Again, no differences associated with the plumage polymorphism were detected in any of these life history characteristics. Thus, despite a thorough analysis of the complete life cycle, we were unable to uncover any evidence supporting a selection hypothesis for the persistence and dynamics of this conspicuous polymorphism. An alternate hypothesis based primarily on gene flow and assortative mating appears more plausible.

The Role of Plumage Polymorphism in Dominance Relationships of the White-throated Sparrow

Abstract We studied dominance behaviors of captive winter flocks of White-throated Sparrows (Zonotrichia albicollis) in central Pennsylvania. Preliminary tests with a small group of birds indicated that there might be differences in behaviors between color morphs if age and sexual differences were controlled. Studies of two larger groups produced considerable variation in results between the groups. One group displayed striking correlations between dominance ability and color. In this group white adult males dominated others more than tan adult males did, whereas tan immature males, adult females, and immature females were more often dominant than were white immature and female birds. The second group did not display such striking differences. Tan adult and immature females were more dominant than white females, as in the first group, but not significantly. Immature males displayed the opposite trend from the first group-white morphs were more dominant than tans. Adult males in the second group showed no clear trend. We also found differences in dominance between tan and white females that appeared to depend on season, white birds dominating tan birds in a small group in the spring, a reversal of relationships documented in the fall. Within age-sex classes, dominant females in both large groups tended to be duller in plumage brightness (scaled as an index) than were subordinate females. In one of the two large groups, duller immature males were more dominant than brighter ones, while brighter adult males tended to be more dominant than duller ones. We suggest that the relationships between immature male plumage and dominance may be influenced by the morph of adult males of specific dominance status in the flock. Spring plumage indices were more often correlated with fall dominance differences than were fall plumage conditions, suggesting a genetic or causal relationship between dominance and plumage rather than a proximate (status signaling) function. Recorded morph frequencies from this study and the literature support the hypothesis that selection is balanced between the two morphs and is dependent on the advantages or disadvantages of aggressiveness within a sex.

THE HERITABILITY OF EXTERNAL MORPHOLOGY IN DARWIN'S GROUND FINCHES (GEOSPIZA) ON ISLA DAPHNE MAJOR, GALÁPAGOS.

Ecologists use avian morphological measurements to develop and test evolutionary theories. The theories are usually based on genetic models, although little is known about the inheritance of such characters. In field studies it is commonly assumed that phenotypic variation closely reflects underlying genetic variation (Grant et al., 1976), while theoreticians sometimes assume that heritabilities are equal to one (Long, 1974). P. R. Grant and his colleagues (Grant et al., 1976; Abbott et al., 1977; Boag and Grant, 1981; Grant, 198 lb) have been studying Darwin's ground finches (Geospiza) in the Galapagos, focusing on relationships between finch morphology and food supplies. Unlike Mendelian characters such as plumage polymorphisms (Mineau and Cooke, 1979), finch morphology involves metric characters and is studied using quantitative genetics (Falconer, 1981). Quantitative genetics describes the phenotypic value of an individual (e.g., its

PREDATOR POLYMORPHISM AND APOSTATIC SELECTION.

The phenomenon of polymorphism is widespread in living organisms, but its adaptive significance remains obscure in most cases. Both genetic and environmental factors involved in the evolution of polymorphism have been studied, and in these studies it has been shown that a variety of factors influence both the extent and the type of polymorphism. Plumage polymorphism, independent of sex and age, in birds has been the subject of numerous studies, most extensively reviewed by Mayr (1942) and Huxley (1955). Huxley recorded polymorphism in 21-25 of 134 bird families; in only seven was it considered frequent. I have not compiled an exhaustive list, but it is clear that the phenomenon is really frequent in only a few families: Procellariidae, Ardeidae, Accipitridae, Falconidae, Stercorariidae and Strigidae. When present in other families it is sporadic, often occurring in only a single or a few species of a large group. Examples are the ruff (Philomachus pugnax), a strikingly polymorphic species in the otherwise monomorphic Scolopacidae; the bush-shrikes (Malaconotus) of the Laniidae; and one goose (Anser caerulescens) of the Anatidae. Polymorphisms in birds vary in extent from those that can be observed only at close range (as the ring on Uria aalge) through those that are minor but conspicuous (as the head color in Poephila gouldiae) through those that involve the entire ventral coloration (as in many birds of prey) to those that encompass the greater part or all of the plumage (as in some herons). Genetic polymorphism in any form has been considered adaptive in providing an increased diversity of genotypes, leading to reduced intraspecific competition and increased buffering against environmental changes (Mayr, 1963: 252). Thus there should be selection for variants, especially in situations in which the pressure from interspecific competition may be reduced, as on islands. Yet there is no reason for variation in external appearance to increase, and indeed there should be selection against such variation in species that recognize each other visually, as do birds. An alternate possibility is that variation in appearance may be used for individual recognition and facilititates social interactions, in which case it would be adaptive. But here a gradual range of variation would seem to be more advantageous than two or several sharply distinct phenotypes. One hypothesis for the adaptiveness of external polymorphism that has received attention recently is that of apostatic selection, or selection for variation for its own sake (Clarke, 1962a). In apostatic selection, a given phenotype is favored in direct proportion to its rarity through frequency-dependent predator pressure. Rand (1967) has suggested that this mechanism can be invoked to explain the great, and to our eyes often bizarre, diversity of tropical insects, i.e., that there has been selection for phenotypes that look as different as possible from the other phenotypes present in an area. Tinbergen (1960) and Mook, Mook and Heikens (1960) presented evidence that species of insects were taken by titmice (Parus spp.) less often than expected (by their frequency in the environment) when at low density and more often than expected when at higher densities. They interpreted this to mean that when a given prey species was more common, the birds encountered it with sufficient frequency to be able to form a search image of the prey, thus increasing their proficiency at finding it.

Isozyme polymorphism in the ruff (aves, Philomachus pugnax) A species with polymorphic plumage

1. 1. A captive flock of ruffs has been examined for isozyme variability in ten different protein systems. 2. 2. This species shows extensive polymorphism in five of the enzyme systems tested. 3. 3. Ruffs have both a polygynous mating system and male plumage polymorphism. These factors are discussed in relation to enzyme variability in this and other avian species.

Plumage Polymorphism in a Feral Population of the Rock Pigeon

A sample (n - 647) of feral rock pigeons collected in Syracuse, New York, between July, 1964 and February, 1965 revealed the presence of polymorphism of plumage color and wing pattern in the population. Five mutant phenotypes each comprised more than 2.5% of the sample, and apparently are being maintained by unknown selective advantages. Proportions of two common pheno- types changed significantly during the winter months. Estimates of mutant gene frequencies at five loci ranged from .037 to .239.