Plumage Polymorphism Research Articles

Sequence variation in the coding region of the melanocortin-1 receptor gene (MC1R) is not associated with plumage variation in the blue-crowned manakin (Lepidothrix coronata)

Avian plumage traits are the targets of both natural and sexual selection. Consequently, genetic changes resulting in plumage variation among closely related taxa might represent important evolutionary events. The molecular basis of such differences, however, is unknown in most cases. Sequence variation in the melanocortin-1 receptor gene (MC1R) is associated with melanistic phenotypes in many vertebrate taxa, including several avian species. The blue-crowned manakin (Lepidothrix coronata), a widespread, sexually dichromatic passerine, exhibits striking geographic variation in male plumage colour across its range in southern Central America and western Amazonia. Northern males are black with brilliant blue crowns whereas southern males are green with lighter blue crowns. We sequenced 810 bp of the MC1R coding region in 23 individuals spanning the range of male plumage variation. The only variable sites we detected among L. coronata sequences were four synonymous substitutions, none of which were strictly associated with either plumage type. Similarly, comparative analyses showed that L. coronata sequences were monomorphic at the three amino acid sites hypothesized to be functionally important in other birds. These results demonstrate that genes other than MC1R underlie melanic plumage polymorphism in blue-crowned manakins.

Predator-prey relationships and the evolution of colour polymorphism: a comparative analysis in diurnal raptors

Genetically based variation in coloration occurs in populations of many organisms belonging to various taxa, including birds, mammals, frogs, molluscs, insects and plants. Colour polymorphism has evolved in raptors more often than in any other group of birds, suggesting that predator–prey relationships was a driving evolutionary force. Individuals displaying a new invading colour morph may enjoy an initial foraging advantage because prey have difficulties in learning the colour of a rare morph (apostatic selection), or because morphs provide alternative foraging benefits allowing differently coloured individuals to exploit distinct food niches (disruptive selection). Plumage polymorphism should therefore have evolved in species that prey upon animals having the physiological ability to distinguish between differently coloured predators but also to flee once a predator has been detected. From this assumption, we can predict that closely related polymorphic and monomorphic species prey upon different animals. They may also differ in morphology, because foraging upon different prey may require different foraging modes, and in turn different morphological structures. We tested these two predictions in a comparative study of raptors. As expected, polymorphic and monomorphic species had a different diet, and there was a difference in wing length between polymorphic and monomorphic species within two genera (Buteo and Accipiter). Across all raptors for which phylogenetic relationships are known, polymorphic species preyed more often upon mammals than did monomorphic ones. These two types of raptor did not differ in the frequency of birds, insects and reptiles in their diets. We discuss these results in the light of the hypothesis that predator–prey relationships played a role in the evolution of colour polymorphism.

Conserved Genetic Basis of a Quantitative Plumage Trait Involved in Mate Choice

A key question in evolutionary genetics is whether shared genetic mechanisms underlie the independent evolution of similar phenotypes across phylogenetically divergent lineages. Here we show that in two classic examples of melanic plumage polymorphisms in birds, lesser snow geese (Anser c. caerulescens) and arctic skuas (Stercorarius parasiticus), melanism is perfectly associated with variation in the melanocortin-1 receptor (MC1R) gene. In both species, the degree of melanism correlates with the number of copies of variant MC1R alleles. Phylogenetic reconstructions of variant MC1R alleles in geese and skuas show that melanism is a derived trait that evolved in the Pleistocene.

The evolution of plumage polymorphism in birds of prey and owls: the apostatic selection hypothesis revisited.

Co-evolution between phenotypic variation and other traits is of paramount importance for our understanding of the origin and maintenance of polymorphism in natural populations. We tested whether the evolution of plumage polymorphism in birds of prey and owls was supported by the apostatic selection hypothesis using ecological and life-history variables in birds of prey and owls and performing both cross taxa and independent contrast analyses. For both bird groups, we did not find any support for the apostatic selection hypothesis being the maintaining factor for the polymorphism: plumage polymorphism was not more common in taxa hunting avian or mammalian prey, nor in migratory species. In contrast, we found that polymorphism was related to variables such as sexual plumage dimorphism, population size and range size, as well as breeding altitude and breeding latitude. These results imply that the most likely evolutionary correlate of polymorphism in both bird groups is population size, different plumage morphs might simply arise in larger populations most likely because of a higher probability of mutations and then be maintained by sexual selection.

The development of plumage polymorphism in male Madagascar paradise flycatcher Terpsiphone mutata

AbstractThe development of plumage polymorphism in male Madagascar paradise flycatcher Terpsiphone mutata (Linnaeus, 1766) was investigated in north‐western Madagascar. Four distinct morphs were observed, namely, white‐coloured with long tails (WL), rufous‐coloured with long tails (RL), rufous‐coloured with tails of middle length (RM), and rufous‐coloured with short tails (RS). Females were rufous and had short tails. Indivudual males were marked and re‐observed during 1994–98. RS males changed to RM in the following seasons. Among RM males, some individuals retained RM, others changed to RL or WL. RM males which changed to WL in the next season had possessed white patches on their belly, whereas RM males which changed to RL had not had such patches. Neither WL nor RL males changed their morphs. Nestlings were rufous regardless of their father's morph. One nestling changed to RS in the next season. From these results and the difference of body size among four morphs, it is deduced that RS males are one year old, and change to RM males in the next season. RM males change to RL or WL males after one or two years. RL and WL are two stable terminal plumages, and they may be genetically determined morphs.

Evolutionary genetics of the melanocortin-1 receptor in vertebrates.

The molecular genetic basis of adaptive change in phenotype is a major outstanding issue in evolutionary biology. Evolutionary change in coat and plumage color is a promising system for making progress in this field. Most notably, recent work on the molecular genetic basis of hair and feather color has identified several genes which are candidates for involvement in evolutionary color change in mammals and birds. We have investigated the evolution of one of these candidate genes, the melanocortin-1 receptor (MC1R) gene, in relation to changes in melanin distribution among a wide variety of primate species, and in bananaquits (Coereba flaveola), which are a classic case of melanic plumage polymorphism in birds. In primates, a role of the MC1R coding region in coat color evolution can be ruled out in several cases in which closely related species have drastically different distributions of eumelanin and/or pheomelanin. However, reconstruction of MC1R sequences over primate evolution shows the presence of mutations at important functional sites in several lineages. Most notably, the lion tamarins (Leontopithecus) show a striking pattern of MC1R evolution, including deletions and several nonconservative amino acid changes. In the bananaquit, an E92K substitution in the MC1R is strongly associated with melanism, and this is likely to be the causative mutation. Reconstruction of the evolution of bananaquit MC1R alleles shows that melanism is a derived trait in this species. These results confirm the utility of a candidate gene approach to color evolution in vertebrates and open the way for extensive future research.

Diverse processes maintain plumage polymorphisms in birds

In a small number of species of birds, pedigree data suggest the persistence of Mendelian genetic polymorphisms for plumage characteristics, which we would expect a priori to have significant ecological or behavioral effects (Table 1). The processes leading to the maintenance of stable genetic polymorphisms were a focus of research among evolutionary ecologists during the school days of the 1950 and 60s (Dobzhansky 1970), prior to the recognition of the extensive amount of genetic variation that was neutral at the organism level. How could genetic variation be maintained in a characteristic with significant ecological effects? Was the polymorphism stable (protected by balancing selection) or a transient historical phenomenon? If stable, what mechanisms allowed competing forms to coexist? What determined the equilibrium frequency? Was spatial or temporal variation in external environments involved, or was an intrinsic balance achieved by negative frequency-dependent selection? Was assortative mating involved, and if so, were these preferences adaptive? The evolutionary dynamics and ecological mechanisms involved for plumage polymorphisms in the species studied thus far turn out to be diverse variations on the theoretical possibilities envisioned 60 years ago. The simplest case is that of lesser snow geese, Chen caerulescens caerulescens (Cooke et al. 1995), which appears to be an unstable transient historical phenomenon resulting from secondary contact. Light and dark plumage morphs evolved in allopatry and came into secondary contact about 100 years ago. Despite decades of intensive field work, no functional differences have been associated with the dramatic difference

Maladaptive mate choice maintained by heterozygote advantage.

Common buzzards (Buteo buteo) show a plumage polymorphism that appears to be maintained by heterozygote advantage and allows a maladaptive form of mate choice to persist. The light and dark morphs have a much lower fitness than the presumed heterozygous intermediate morph, but are replenished through Mendelian segregation in intermediate-intermediate pairs. Light and dark morphs could maximize their fitness by mating light with dark to produce all intermediate offspring, but instead choose partners of their own color, thereby producing broods of minimally fit homozygotes. Such maladaptive behavior argues forcefully against mate choice based on "good genes," and its persistence is best explained by heterozygote advantage maintaining the polymorphism coupled with nongenetic mate choice based on sexual imprinting. Modeling different patterns of mate choice shows that random mating and preference for own morph fit our data poorly, whereas preference for mother's morph yields a good fit.

The molecular basis of an avian plumage polymorphism in the wild: a melanocortin-1-receptor point mutation is perfectly associated with the melanic plumage morph of the bananaquit, Coereba flaveola.

Evolution depends on natural selection acting on phenotypic variation, but the genes responsible for phenotypic variation in natural populations of vertebrates are rarely known. The molecular genetic basis for plumage color variation has not been described in any wild bird. Bananaquits (Coereba flaveola) are small passerine birds that occur as two main plumage variants, a widespread yellow morph with dark back and yellow breast and a virtually all black melanic morph. A candidate gene for this color difference is the melanocortin-1 receptor (MC1R), a key regulator of melanin synthesis in feather melanocytes. We sequenced the MC1R gene from four Caribbean populations of the bananaquit; two populations of the yellow morph and two populations containing both the yellow morph and the melanic morph. A point mutation resulting in the replacement of glutamate with lysine was present in at least one allele of the MC1R gene in all melanic birds and was absent in all yellow morph birds. This substitution probably causes the color variation, as the same substitution is responsible for melanism in domestic chickens and mice. The evolutionary relationships among the MC1R haplotypes show that the melanic alleles on Grenada and St. Vincent had a single origin. The low prevalence of nonsynonymous substitutions among yellow haplotypes suggests that they have been under stabilizing selection, whereas strong selective constraint on melanic haplotypes is absent. We conclude that a mutation in the MC1R is responsible for the plumage polymorphism in a wild bird population and that the melanic MC1R alleles in Grenada and St. Vincent bananaquit populations have a single evolutionary origin from a yellow allele.

MALADAPTIVE MATE CHOICE MAINTAINED BY HETEROZYGOTE ADVANTAGE

Common buzzards (Buteo buteo) show a plumage polymorphism that appears to be maintained by heterozygote advantage and allows a maladaptive form of mate choice to persist. The light and dark morphs have a much lower fitness than the presumed heterozygous intermediate morph, but are replenished through Mendelian segregation in intermediate-intermediate pairs. Light and dark morphs could maximize their fitness by mating light with dark to produce all intermediate offspring, but instead choose partners of their own color, thereby producing broods of minimally fit homozygotes. Such maladaptive behavior argues forcefully against mate choice based on “good genes,” and its persistence is best explained by heterozygote advantage maintaining the polymorphism coupled with nongenetic mate choice based on sexual imprinting. Modeling different patterns of mate choice shows that random mating and preference for own morph fit our data poorly, whereas preference for mother's morph yields a good fit.Corresponding Editor: B. Crespi

Plumage polymorphism in Terpsiphone and the rule of island drabness

Plompen, W. 2000. Plumage polymorphism in Terpsiphone and the rule of island drabness. Ostrich 71 (1 & 2): 316. Many related bird species differ through their conspicuous plumage coloration characteristics. On islands, birds often have a drabber plumage coloration than their mainland counterpart. This rule is regarded as the main evidence for the species recognition hypothesis, because on islands mostly only one species per genus is present. In the absence of congeners there may be no benefit from a species specific badge, with drabness as a result. However drabness as such is not predicted by this species recognition hypothesis. In fact the only selective pressure caused by a species recognition mechanism, works in sympatry. Therefore on an island with no related species present, the elaborate coloration that originated in the mainland situation, is allowed to evolve in any sense, or may even not evolve at all. Taking this all together, if under the. species recognition hypothesis island taxa may evolve at random with respect to colouration, and island drabness is a well known phenomenon, then an additional mechanism inherent to the island situation is needed, that causes the degradation of any bright plumage colouration. As a consequence, island drabness in itself does not provide any information at all on the original mechanism that caused plumage elaboration on the mainland.

Plumage polymorphism of red‐footed boobies (Sula sula) in the western Indian Ocean: an indicator of biogeographic isolation

AbstractThe pantropical red‐footed booby Sula sula is one of the most polymorphic seabirds. In the Indian Ocean most extant colonies hold white morph adults except on Europa Island (southern Mozambique Channel), where adults are of the white‐tailed brown morph and on Tromelin Island (western Indian Ocean), where one‐third of the birds are of the white‐tailed brown morph and two‐thirds are of the white morph. On Tromelin, the morph ratio has remained constant over the last 40 years. An extinct colony (Glorieuses Islands, northern Mozambique Channel) once supported a white‐tailed brown morph population. These results suggest that the colonies of the western Indian ocean do not constitute one great gene pool where exchanges occur, but are isolated at various degree from each other. Particularly, the population of Europa is the only current population of the Indian Ocean constituted almost entirely of white‐tailed brown morph, suggesting that it is isolated from other nearby colonies. Possible causes of isolation are discussed in the light of oceanic features of the Mozambique Channel. Although further studies are needed to understand the adaptive significance of plumage coloration in red‐footed boobies, I suggest that white‐tailed brown morph at Europa may act as a defensive camouflage against kleptoparasitism by great frigatebirds Fregata minor and brown skuas Catharacta antarctica.

Plumage polymorphism of red-footed boobies (Sula sula) in the western Indian Ocean: an indicator of biogeographic isolation

The pantropical red-footed booby Sula sula is one of the most polymorphic seabirds. In the Indian Ocean most extant colonies hold white morph adults except on Europa Island (southern Mozambique Channel), where adults are of the white-tailed brown morph and on Tromelin Island (western Indian Ocean), where one-third of the birds are of the white-tailed brown morph and two-thirds are of the white morph. On Tromelin, the morph ratio has remained constant over the last 40 years. An extinct colony (Glorieuses Islands, northern Mozambique Channel) once supported a white-tailed brown morph population. These results suggest that the colonies of the western Indian ocean do not constitute one great gene pool where exchanges occur, but are isolated at various degree from each other. Particularly, the population of Europa is the only current population of the Indian Ocean constituted almost entirely of white-tailed brown morph, suggesting that it is isolated from other nearby colonies. Possible causes of isolation are discussed in the light of oceanic features of the Mozambique Channel. Although further studies are needed to understand the adaptive significance of plumage coloration in red-footed boobies, I suggest that white-tailed brown morph at Europa may act as a defensive camouflage against kleptoparasitism by great frigatebirds Fregata minor and brown skuas Catharacta antarctica.

Plumage polymorphism of the Feral Pigeon (Columba livia)

Strasentauben zeigen in ihrer Gefiederfarbung die wohl groste bekannte Variabilitat unter den frei lebenden Vogeln. Der groste Teil der von der Wildform abweichenden Gefiederfarbungen last sich auf die Haustaubenvorfahren unserer Strasentauben zuruckfuhren. Von den 60 bekannten und 52 in Mitteleuropa bei Strasentauben moglichen Erbfaktoren traten bei Wiener Strasentauben 23 in 100 Erbfaktorenkombinationen und bei Basler Strasentauben 15 in 30 Kombinationen auf. Fur die grosere Zahl an Erbfaktoren und Erbfaktorenkombinationen in Wien konnte die absolute Grose der Bestande, das unterschiedliche rezente Einfliesen an Haustaubengenen in Wien und Basel sowie die relativ kleine Stichprobe in Basel verantwortlich sein. In Basel traten signifikant mehr Tiere mit der Grundfarbe Dominant Rot auf als in Wien. Dazu werden verschiedene Erklarungen diskutiert. In der Verteilung der Zeichnungsanlagen und der Gruppen der Blaubindigen, Melanisierten und Anderen traten zwischen Wien und Basel keine signifikanten Unterschiede auf. In der Verteilung der Zusatzfaktoren hingegen unterscheiden sich die beiden Populationen signifikant voneinander. Der in Basel haufigere Scheckfaktor konnte auf einen starkeren Brieftaubeneinflus, der in Wien haufigere Schimmelfaktor auf entflohene Tummler zuruckgehen. Die unterschiedlichen Selektionsregimes in Wien und Basel, vor allem der Feinddruck und die Ernahrungssituation, werden in ihren moglichen Auswirkungen auf die Verteilung der Gefiederfarbungen diskutiert.

MITOCHONDRIAL DNA HOMOGENEITY IN THE PHENOTYPICALLY DIVERSE REDPOLL FINCH COMPLEX (AVES: CARDUELINAE:CARDUELIS FLAMMEA-HORNEMANNI)

Breeding redpoll finches (Aves: Carduelinae) show extensive plumage and size variability and, in many cases, a plumage polymorphism that is not related to age or sex. This has been ascribed to extreme phenotypic variation within a single taxon or to moderate variability within distinct taxa coupled with hybridization. The predominant view favors the recognition of two largely sympatric species: Carduelis flammea, comprised of four well-marked subspecies-flammea, cabaret, islandica, and rostrata; and C. hornemanni, comprised of two subspecies-hornemanni and exilipes. We studied representative samples of these putative subspecies (except islandica) for variation in mitochondrial DNA (mtDNA). Using 20 informative restriction enzymes that recognized 124 sites (642 base pairs [bp] of sequence or ≈ 3.7% of the molecule), we identified 17 RFLP haplotypes in the 31 individuals surveyed. The haplotypes formed a simple phylogenetic network with most clones diverging by a single site difference from a common haplotype found in almost half of the individuals. Within populations and taxa, levels of mtDNA diversity were similar to those observed in other avian species. The pattern of mtDNA divergence among populations was statistically unrelated to their geographic or traditional taxonomic relationships, and the estimated distance between the two traditionally recognized species was very small relative to those typically observed among avian sister species.

Inheritance of Egg-Color Polymorphism in the Village Weaver (Ploceus cucullatus)

--Eggs collected from an aviary colony of Village Weavers (Ploceus cucullatus cucullatus) over a 14-year period varied in color among different females from white to emerald or turquoise, and from many spots to few or none. Color and amount of spotting of the eggs of a given female were constant throughout her life (based on 815 eggs from 37 females). Color was judged from the Villalobos Color Atlas. Mendelian analysis of five different hypotheses showed that inheritance of background color of eggs in 20 different crosses was consistent only with a hypothesis of two independent pairs of autosomal alleles for hue. Received 14 September 1992, accepted 25 November 1992. THE EDITORS of a recent book on avian genetics pointed out that, despite widespread interest among ornithologists in plumage polymorphism, evidence of the genetic basis for most of these polymorphisms is sadly lacking (Cooke and Buckley 1987:202). Even less is known about the genetic control of egg-shell-color polymorphisms. My objective is to present evidence on Mendelian inheritance of egg-shell-color polymorphism in a passerine bird. In the domestic fowl (Gallus gallus) for which the first demonstration of Mendelian heredity in the animal kingdom was made by William Bateson (Punnett 1923), a Mendelian analysis of egg-shellcolor polymorphism was done long ago (Punnett and Bailey 1920, Punnett 1923, 1933a), and this classic work is still one of the few published records dealing with this problem (Washburn 1990). There is no reference in the literature giving comparable evidence for any passerine bird. In North America the color of eggs is generally consistent for a given species of bird, and this is particularly true of the passetines (Harrison 1978). This is in strong contrast to the genus Ploceus (Ploceidae, weaverbirds) in which variation in egg color within the species is higher than in any other group of birds (Moreau 1960:446). In the Village Weaver (Ploceus cucullatus) of sub-Saharan Africa, the eggs are among the most variable in color and pattern of any ploceid species (Meise 1983:521). The different races of the Village Weaver have similar variations in egg color and pattern, in which ground color varies from white through blues to greens, and from plain eggs to eggs spotted to varying degrees usually with brown or reddish brown. This pattern of variability is seen in eastern (Mackworth-Praed and Grant 1960), central (Chapin 1954), western (Bannerman 1949), and southern (Maclean 1985) Africa. Bannerman (1949:94) further noted for the Village Weaver: Only one type of coloring ap-

Morphometric Variability in Redpolls from Churchill, Manitoba

Two plumage forms were observed in redpolls from Churchill, Manitoba, which correspond to the putative taxa Carduelis fiammea fiammea (dark-plumaged birds) and C. hornemanni exilipes (pale-plumaged birds). in a sample of breeding adults (n = 277), we examined whether morphometric differentiation parallels this plumage polymorphism. Males and females were analyzed independently, and age, date of capture, and status at measurement (study skin vs. live bird) were considered as covariates in univariate analyses. We used a separate phenetic analysis of plumage characters, rather than our subjective field identifi- cations, to establish the groups to be compared statistically; this ensured that mensural characters (e.g. bill shape) were not utilized as classification criteria for the present investi- gation of metric traits. in both sexes, redpoll plumage forms differed significantly in three of seven external mensural characters. Discriminant-function analyses, based on the same characters, showed that the forms can be distinguished morphometrically with great confi- dence (jackknifed estimate of correctly classified individuals was 87% in both sexes). in males and females, the distribution of discriminant scores of typical individuals and of a set of unidentified birds is bimodal. in a bivariate-reduced space of plumage and morphometric variability, pale- and dark-plumaged ASY (after-second-year) males form distinct groups. in SY (second-year) males and in females, plumage forms are not strictly distinct, but in no case was there an abundance of intermediates as predicted under the hypothesis that the forms are distinct species that frequently interbreed. Redpoll types may be specifically distinct, as has frequently been suggested, but they also may be examples of intraspecific genetic or ecophenotypic polymorphism. Experimental breeding and an assessment of mating patterns in the field are required to test these possibilities. Received 3 October 1991, accepted 5 September 1992.

Aggressive Performance as a Behavioral Correlate of Plumage Polymorphism in the White-Throated Sparrow (Zonotrichia Albicollis)

Abstract Morph and sex-specific differences in aggressive performance were examined in the white-throated sparrow. Among the four morph/sex classes, white-striped males had the strongest response to white-striped models and were the most likely to approach and attend song playbacks independently. The slightly weaker responses of tan-striped males and white-striped females were similar to each other. Tan-striped females were the least aggressive. Aggression by white-striped females enabled tan-striped male x white-striped female pairs to perform as well, or better, than white-striped male x tan-striped female pairs. Tan-striped males paired earlier than white-striped males. An increased prevalence of territorial tan-striped males corresponded to the arrival of females on the breeding grounds. The data are consistent with the hypothesis that negative assortative mating benefits tan-striped males through the supplemental aggression of white-striped females. However, the apparent preference of white-striped females for tan-striped males suggests that there are benefits to negative asortative mating other than those that might arise from differences in aggressive performance.

Plumage Variability in Redpolls from Churchill, Manitoba

-Redpolls (Carduelis flammea and C. hornemanni) are well known for their extensive phenotypic variability. Various explanations have been proposed for the phenomenon, ranging from extreme variability within a single inclusive taxon, to more modest variation within frequently hybridizing or strictly isolated taxa. The predominant view favors the recognition of two largely sympatric and only slightly differentiated species: the Common Redpoll (C. flammea), comprised of the four subspecies flammea, cabaret, islandica, and rostrata; and the Hoary Redpoll (C. hornemanni) comprised of the two subspecies hornemanni and exilipes. We investigated the possible existence of sympatric redpoll forms in a large sample (n = 209) of breeding individuals from Churchill, Manitoba, where the putative forms flammea and exilipes were reported to co-occur. Groups of individuals of specific age and sex were analyzed independently. The sample was not divided a priori into flammea and exilipes subgroups. The pattern of variation within each age-and-sex group is described, and the differences between these groups are analyzed. We show that several pigmentation characters in specific age-andsex groups are not distributed normally, and that pale and dark subgroups of second-year (SY) and after-second-year (ASY) males can be identified in a principal-components analysis (PCA) of seven characters reflecting the extent of melanin and carotenoid pigmentation. The pinkand red-breasted subgroups of ASY males match almost perfectly the pale and dark groups found in the PCA analysis. Further, when ASY males of different breast color (pink vs. red) are analyzed independently, all plumage characters are distributed normally. The analyses of SY and ASY females failed to provide good evidence for a plumage polymorphism. Our results support the idea that two relatively distinct redpoll forms breed at Churchill, which differ in several plumage characteristics involving both melanin and carotenoid pigmentations. These forms are more clearly differentiated in males than in females. They may be specifically distinct (C. f. flammea and C. hornemanni exilipes), as has frequently been suggested, but they also could be the product of different types of intraspecific genetic or ecophenotypic polymorphisms. Preliminary observations are presented that support the idea that plumage variants are largely genetically determined, but experimental work is needed to confirm this suggestion. Received 28 May 1991, accepted 19 February 1992. ORNITHOLOGISTS HAVE long been intrigued by the extensive plumage variability seen in redpolls (Carduelis [flammea] spp.). Based roughly on the same type of data and analytical approach (i.e. colorimetric and morphometric studies), different investigators have described that variation as continuous (e.g. Harris et al. 1965, Troy 1985) or dimorphic (e.g. Todd 1963, Molau 1985, Knox 1988), and have divided the complex into one to four species (see Knox 1988:table 1). The taxonomic disagreement comes mainly from the different interpretations given to the origin of birds having intermediate attributes in sympatric populations. Some re1 Present addresses: Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA; and Smithsonian Tropical Research Institute, Unit 0948, APO AA 34002, USA. searchers hold that intermediate birds are common in many populations and represent either hybrids and backcross individuals (e.g. Williamson 1961, Harris et al. 1965), or merely central points in a homogeneous distribution of phenotypic variants within a single highly variable species (Troy 1985). Other investigators (e.g. Molau 1985, Knox 1988) have suggested that, when sexual and age dimorphisms are carefully taken into account, very few redpolls qualify as intermediate, and these probably are convergent variants of distinct sympatric species. The most commonly encountered classification of redpolls is one in which two polytypic species are recognized, the Common Redpoll (C. flammea) comprised of the subspecies cabaret, flammea, and rostrata, and the Hoary Redpoll (C. hornemanni) comprised of the subspecies exilipes and hornemanni (Knox 1988). The taxonomic affinities of redpolls breeding in Iceland are un-

Geographical Variation of the Plumage Polymorphism in the Eastern Reef Heron (Egretta sacra)

I investigated the distribution of white and dark morphs of the Eastern Reef Heron (Egretta sacra) on several coasts and islands in central to southwestern Japan, eastern Australia, and French Polynesia. Both morphs preferred white beaches to dark beaches. Every local population contained some dark herons, however, the range of the white morph was restricted. The white morph population increased in proportion to the distance from the equator in Japan. It was plentiful in the Great Barrier Reef in Australia and was common in Polynesian islands. The distribution of the white morph coincides with the presence of coral reefs, from which white beaches are derived. Only the dark morph was seen outside this range. None of several hypotheses proposed so far explains the observed polymorphism and its geographic variation.