Conspecific versus heterospecific gene exchange between populations of Darwin's finches

This study addresses the extent and consequences of gene exchange between populations of Darwin''s finches. Four species of ground finches (Geospiza) inhabit the small island of Daphne Major in the centre of the Galápagos archipelago. We undertook a study of microsatellite DNA variation at 16 loci in order to quantify gene flow within species owing to immigration and between species owing to hybridization. A combination of pedigrees of observed breeders and assignments of individuals to populations by the program Structure enabled us to determine the frequency of gene exchange and the island of origin of immigrants in some cases. The relatively large populations of Geospiza fortis and G. scandens receive conspecific immigrants at a rate of less than one per generation. They exchange genes more frequently by rare but repeated hybridization. Effects of heterospecific gene flow from hybridization are not counteracted by lower fitness of the offspring. As a result, the standing genetic variation of the two main resident populations on Daphne Major is enhanced to a greater extent by introgressive hybridization than through breeding with conspecific immigrants. Immigrant G. fuliginosa also breeds with G. fortis. Conspecific immigration was highest in the fourth species, G. magnirostris. This species is much larger than the other three and perhaps for this reason it has not bred with any of them. The source island of most immigrants is probably the neighbouring island of Santa Cruz. Evolutionary change may be inhibited in G. magnirostris by continuing gene flow, but enhanced in G. fortis and G. scandens by introgressive hybridization.     

Divergence with gene flow as facilitated by ecological differences: within-island variation in Darwin's finches

Divergence and speciation can sometimes proceed in the face of, and even be enhanced by, ongoing gene flow. We here study divergence with gene flow in Darwin''s finches, focusing on the role of ecological/adaptive differences in maintaining/promoting divergence and reproductive isolation. To this end, we survey allelic variation at 10 microsatellite loci for 989 medium ground finches (Geospiza fortis) on Santa Cruz Island, Galápagos. We find only small genetic differences among G. fortis from different sites. We instead find noteworthy genetic differences associated with beak. Moreover, G. fortis at the site with the greatest divergence in beak size also showed the greatest divergence at neutral markers; i.e. the lowest gene flow. Finally, morphological and genetic differentiation between the G. fortis beak-size morphs was intermediate to that between G. fortis and its smaller (Geospiza fuliginosa) and larger (Geospiza magnirostris) congeners. We conclude that ecological differences associated with beak size (i.e. foraging) influence patterns of gene flow within G. fortis on a single island, providing additional support for ecological speciation in the face of gene flow. Patterns of genetic similarity within and between species also suggest that interspecific hybridization might contribute to the formation of beak-size morphs within G. fortis.     

Heat Loss May Explain Bill Size Differences between Birds Occupying Different Habitats

Background

Research on variation in bill morphology has focused on the role of diet. Bills have other functions, however, including a role in heat and water balance. The role of the bill in heat loss may be particularly important in birds where water is limiting. Song sparrows localized in coastal dunes and salt marsh edge (Melospiza melodia atlantica) are similar in size to, but have bills with a 17% greater surface area than, those that live in mesic habitats (M. m. melodia), a pattern shared with other coastal sparrows. We tested the hypotheses that sparrows can use their bills to dissipate “dry” heat, and that heat loss from the bill is higher in M. m. atlantica than M. m. melodia, which would indicate a role of heat loss and water conservation in selection for bill size.

Methodology/Principal Findings

Bill, tarsus, and body surface temperatures were measured using thermal imaging of sparrows exposed to temperatures from 15–37°C and combined with surface area and physical modeling to estimate the contribution of each body part to total heat loss. Song sparrow bills averaged 5–10°C hotter than ambient. The bill of M. m atlantica dissipated up to 33% more heat and 38% greater proportion of total heat than that of M. m. melodia. This could potentially reduce water loss requirements by approximately 7.7%.

Conclusions/Significance

This >30% higher heat loss in the bill of M. m. atlantica is independent of evaporative water loss and thus could play an important role in the water balance of sparrows occupying the hot and exposed dune/salt marsh environments during the summer. Heat loss capacity and water conservation could play an important role in the selection for bill size differences between bird populations and should be considered along with trophic adaptations when studying variation in bill size.     

Colour also matters for nocturnal birds: owlet bill coloration advertises quality and influences parental feeding behaviour in little owls

Chromatic signals of offspring quality have been shown to play a role in parent–offspring communication in diurnal birds, but are assumed to be useless in dim light conditions because colour-based discrimination probably requires more light. A major ecological and evolutionary conundrum in this scenario is why the nestlings of some nocturnal owls display colourful beaks. Here, we test the hypothesis that yellow bill coloration of owlets of the nocturnal little owl Athene noctua may function as a chromatic signal revealing to parents aspects of quality of their offspring. In a first step, we examined physical variation in bill coloration and its covariation with owlet quality. Secondly, we studied parental provisioning in relation to an experimental manipulation of bill coloration of owlets. Bills of owlets showed higher within-nest variation in yellow–red chroma than in brightness. Plasma carotenoid concentration and nestling immunological status were not associated with chromatic or achromatic features of the bill. Interestingly, however, heavier owlets displayed more yellow bills than lighter ones. The effect of bill coloration on parental favouritism changed with brood size. Parents holding large broods preferentially fed owlets with enhanced over reduced yellow bill coloration, whereas those with small broods did not significantly bias feeding in relation to owlet bill coloration. Our results, based on integration of objective spectrophotometric assessment of colour and experimental procedures, confirm that parent little owls use bill coloration to reveal information on owlet body mass to adjust their feeding strategies, thus highlighting the importance of considering potential chromatic signals for a full comprehension of parent–offspring communication processes in nocturnal bird species.     

Evolution of sexual dimorphism in bill size and shape of hermit hummingbirds (Phaethornithinae): a role for ecological causation

Unambiguous examples of ecological causation of sexual dimorphism are rare, and the best evidence involves sexual differences in trophic morphology. We show that moderate female-biased sexual dimorphism in bill curvature is the ancestral condition in hermit hummingbirds (Phaethornithinae), and that it is greatly amplified in species such as Glaucis hirsutus and Phaethornis guy, where bills of females are 60 per cent more curved than bills of males. In contrast, bill curvature dimorphism is lost or reduced in a lineage of short-billed hermit species and in specialist Eutoxeres sicklebill hermits. In the hermits, males tend to be larger than females in the majority of species, although size dimorphism is typically small. Consistent with earlier studies of hummingbird feeding performance, both raw regressions of traits and phylogenetic independent contrasts supported the prediction that dimorphism in bill curvature of hermits is associated with longer bills. Some evidence indicates that differences between sexes of hermit hummingbirds are associated with differences in the use of food plants. We suggest that some hermit hummingbirds provide model organisms for studies of ecological causation of sexual dimorphism because their sexual dimorphism in bill curvature provides a diagnostic clue for the food plants that need to be monitored for studies of sexual differences in resource use.     

High summer temperature explains bill size variation in salt marsh sparrows

Physiological factors are rarely proposed to account for variation in the morphology of feeding structures. Recently, bird bills have been demonstrated to be important convective and radiant heat sinks. Larger bills have greater surface area than smaller bills and could serve as more effective thermoregulatory organs under hot conditions. The heat radiating function of bills should be more important in open habitats with little shade and stronger convective winds. Furthermore, as a means of dumping heat without increasing water loss through evaporation, bills might play a particularly important thermoregulatory role in heat loss in windy habitat where fresh water is limited. North American salt marshes provide a latitudinal gradient of relatively homogeneous habitat that is windy, open, and fresh‐water limited. To examine the potential role of thermoregulation in determining bill size variation among ten species or subspecies of tidal marsh sparrows, we plotted bill size against maximum summer and minimum winter temperatures. Bill surface areas increases with summer temperature, which explained 82–89% of the variance (depending upon sex) when we controlled for genus membership. Latitude alone predicted bill surface area much more poorly than summer temperature, and winter temperatures explained < 10% of the variance in winter bill size. Tidal marsh sparrow bill morphology may, to a large degree, reflect the role of the bill in expelling excess body heat in these unbuffered, fresh‐water‐limited environments. This new example of Allen's rule reaffirms the importance of physiological constraints on the evolution of vertebrate morphologies, even in bird bills, which have conventionally been considered as products of adaptation to foraging niche.     

Heterocarpy in Salvia roemeriana (Lamiaceae)

Although Salvia roemeriana has long been known to produce both chasmogamous and cleistogamous flowers, the mericarps resulting from those flowers have received little attention. We germinated seeds from chasmogamous and cleistogamous flowers, recorded germination times, and fit time‐to‐germination, three‐parameter log‐logistic regressions to analyze differences in germination progress. Additionally, we compared the mass and size of mericarps from both kinds of flowers. Our results show that the mericarps produced from chasmogamous flowers are larger and heavier than those from cleistogamous flowers. In addition, seeds from chasmogamous flowers had a longer dormancy than those from cleistogamous flowers. This is the first report of heterocarpy in Salvia and in the family Lamiaceae. Together, cleistogamy and heterocarpy are a multiple strategy that may be advantageous in heterogeneous environments.     

Population Variation in Continuously Varying Traits as an Ecological Genetics Problem

The niche variation hypothesis is an adaptive explanation forvariation within populations and for, the differences in variationbetween populations in morphological, physiological or behavioraltraits. It has received only partial support from empiricaltests and has been criticized on theoretical grounds. Recentquantitative genetic models have made an advance by exploringthe effects of mutation, migration, mating pattern and selectionon phenotypic variance. These models are reviewed and theirmost important features are integrated in a new model. In thismodel population variation is in a state of balance betweenthe opposing forces of mutation and immigration, which tendto elevate variation, and selection and possibly genetic drifttending to decrease it. Populations exhibiting different levelsof variation are interpereted as having different equilibriumpoints, and it is the task of empirical studies to determinethe relative magnitudes of the opposing factors. An exampleis given from studies of Darwin's finches. Geospiza fortis variesmore than G. scandens on Isla Daphne Major, Galápagos,in several morphological traits including beak and body size.This is explained, assuming equal mutation rates in the twospecies, as the result of more frequent genetic input to theG. fortis population, through occasional hybridization withimmigrant G. fuliginosa, and relaxed stabilizing selection.Stabilizing selection is less intense on G.fortis than on G.scandens because the G. fortis population has a broader niche;there is both a within-phenotype and betweenphenotype componentto the broad niche of G. fortis. The success of theory in explainingpopulation variation is discussed, and it is concluded thatempirical studies lag far behind theory.     

Evolution of island warblers: beyond bills and masses

In this paper, we take a closer look into the evolution of Acrocephalus warblers on islands in the Atlantic, Indian and Pacific Oceans. The shape‐related morphological evolution of island species is characterized by changes in the hind limb, flight, and feeding apparatus. Birds on islands converged to a morphology with strong legs, shorter rictal bristles, and rounder, more slotted and broader wings. Because of their high variance among islands, body size and bill dimensions did not contribute to the separation of continental and island forms, although bills tend to be longer on islands. The wings of island birds hardly vary among islands, unsurprisingly due to a lack of the adaptive features associated with long distance flights. The tendency towards shorter rictal bristles in the island warblers can be explained by the diminished role of aerial feeding, and closer contact with various substrates in the course of extractive foraging. The shift towards stronger legs in several insular species is remarkable because reed warblers on continents have even stronger legs than other passerines of comparable size. This trait correlates with diverse, acrobatic feeding techniques that are typically associated with broad habitat use. Bills reach extreme lengths on some islands. However, short bills occur as frequently, rendering this character highly variable among islands. Short bills indicate gleaning feeding techniques, whereas long bills are typical for species that pursue hidden and difficult‐to‐access prey. Body sizes differ greatly from island to island. On average, the sizes of island birds do not differ from continental ones, however. We suggest that vegetation clutter is the major driving force for this variation. The main conclusion following from our results is that evolution on islands pertains to all functional complexes, and not only the hitherto studied body size and bill dimensions.     

Natural selection on body size traits in a long‐lived bird,the snow petrel Pagodroma nivea

I consider the possibility of selection favouring large body size in a population of snow petrels (Pagodroma nivea), a long‐lived seabird species. I measured natural selection on body size traits in a population from 1987 to 1998. There was evidence of selection on body size associated with fecundity and survival. Directional selection on bill length and stabilizing selection on tarsus length associated with reproductive success were detected among males. Selection associated with survival favoured males with longer bills. However, selection was weak in all cases. No evidence of selection acting on female body size traits was detected. Offspring–parents regression suggested that bill length and tarsus length were heritable. Although I was able to identify the targets of selection in this population, I could not demonstrate the ecological implications of both tarsus length and bill length variation. The selection on male, but not on female, body size traits suggests factors such as intrasexual competition for nests and/or mates rather than factors such as feeding efficiency as mechanisms of selection on bill size.     

SELECTION ON BILL CHARACTERS IN A POPULATION OF DARWIN'S FINCHES: GEOSPIZA CONIROSTRIS ON ISLA GENOVESA,GALÁPAGOS

Lande and Arnold's (1983) technique for measuring selection on correlated quantitative traits was used to identify the targets of selection and to reveal the direction of selection on three bill dimensions, during different stages of the life cycle in a population of Darwin's finches, Geospiza conirostris, on Isla Genovesa, Galápagos. There was a tendency towards disruptive selection during dry conditions, arising from differential survival. In terms of longevity and breeding success of females, the direction of selection was to increase bill length. For males competing for territories, selection acted to increase bill depth and bill length. The effects of male-male interactions were separated from those of female choice. Male-male interactions selected for deep and long bills, whereas females chose their mates on the basis of a male's territory position and plumage coloration. The results reveal three factors constraining changes in bill dimensions: a tendency for the mean of a dimension to shift in one direction is counteracted by selection in the opposite direction on 1) another, positively correlated, bill dimension, 2) the same dimension in the other sex, and 3) the same dimension at another stage of the life cycle. If these factors are overcome by strong directional selection at one stage of the life cycle and relaxation at another, there can be an evolutionary response because the bill dimensions in this population are known to be heritable. The results complement those found in studies of G. fortis on another island and strengthen the view that these populations of Darwin's finches are frequently subjected to natural selection.     

Becoming Different But Staying Alike: Patterns of Sexual Size and Shape Dimorphism in Bills of Hummingbirds

Hummingbirds are known for their distinctive patterns of sexual dimorphism, with many species exhibiting sex-related differences in various ecologically-relevant traits, including sex-specific differences in bill shape. It is generally assumed that such patterns are consistent across all hummingbird lineages, yet many taxa remain understudied. In this study we examined patterns of sexual size and sexual shape dimorphism in bills of 32 of 35 species in the monophyletic Mellisugini lineage. We also compared patterns of bill size dimorphism in this group to other hummingbird lineages, using data from 219 hummingbird species. Overall, the presence and degree of sexual size dimorphism was similar across all hummingbird lineages, with the majority of Mellisugini species displaying female-biased sexual size dimorphism, patterns that remain unchanged when analyzed in a phylogenetic context. Surprisingly however, we found that sexual dimorphism in bill shape was nearly absent in the Mellisugini clade, with only 3 of the 32 species examined displaying bill shape dimorphism. Based on observations in other hummingbird lineages, the lack of sexual shape dimorphism in Mellisugini is particularly unusual. We hypothesize that the patterns of sexual size dimorphism observed here may be the consequence of differential selective forces that result from competition for ecological resources. We further propose that an influential mechanism underlying shape dimorphism is competition and niche segregation. Taken together, the evolutionary changes in patterns of sexual shape dimorphism observed in Mellisugini suggest that the evolutionary trends of sexual dimorphism in the Trochilidae are far more dynamic than was previously believed.     

The occurrence of two bill morphs of prions Pachyptila vittata on Gough Island

Prions Pachyptila are abundant seabirds in the Southern Ocean that have been used to infer environmental change, but this relies on an understanding of their morphological diversity. Species limits among prions are largely defined by the size and structure of their bills. The broad-billed prion P. vittata, which breeds at temperate islands in the central South Atlantic Ocean and around New Zealand, is the largest species with adult bill widths averaging 21–22 mm. We report local differences in bill sizes on Gough Island, with typical adults breeding in some areas and narrower-billed birds in other areas (average bill width 18–19 mm, although at one site there was a second mode at bill widths of 16–17 mm). The narrow-billed birds have slightly shorter bills, heads and wings (averaging 1–2 % smaller than typical adult broad-billed prions), but the difference in bill width is much more marked (15 %). The small-billed birds differ from typical broad-billed prions in having blue colouration in the upper mandible and are similar in size to MacGillivray’s prion P. [salvini/vittata] macgillivrayi from Amsterdam and St Paul islands in the temperate Indian Ocean. The occurrence of two prion morphs on Gough Island raises intriguing questions about their ecology and systematics. Small-billed birds breed 3 months later than large-billed birds, suggesting that they are a separate species, not an example of bill polymorphism.     

THE ADAPTIVE RADIATION AND FEEDING ECOLOGY OF SOME BRITISH FINCHES

The habitats and foods of ten species of finches studied near Oxford, England, are described in detail, and differences in feeding habits between them are related to differences in bill- and leg-structure, body-weight and behaviour. The Hawfinch, Siskin, Redpoll and Crossbill live primarily in woodland, the Greenfinch, Linnet and Goldfinch primarily in farmland, while the Bullfinch and Chaffinch breed, and occur throughout the year, in both these habitats. Habitat differences between species were probably more marked in primeval England than now, and the problems of studying animals in man-modified environments are discussed in this context. Diet was studied by observation and by gut-analyses of live and dead finches. Before roosting, finches usually accumulate food in their gullet, which is expandable but otherwise undifferentiated (the term “crop” is thus inappropriate). The Lesser Redpoll, the smallest species studied, can store up to about 0.8 c.c. of food in its gullet, but the Greenfinch, one of the largest finches, up to 2.5 c.c. Among the Carduelis finches, the Greenfinch lives mainly on the large seeds of various trees, cereal crops and agricultural weeds; the Goldfinch on the seeds of Compositae; the Siskin on alder and conifer seeds; the Lesser Redpoll on birch and other small seeds; and the Linnet on the seeds of various weeds of farmland. The Hawfinch and Bullfinch eat a greater proportion of buds and seeds from fleshy fruits than do other finches; but the Hawfinch prefers larger ones than does the Bullfinch. The three European crossbills feed almost entirely on conifer seeds; specializing on different tree species. In summer, the Chaffinch and Brambling live largely on invertebrates but in winter on beechmast or small seeds taken from the ground. In winter, Chaffinches reaching Britain from northern Europe tend to feed in large flocks in open areas and to roost communally, while residents tend to remain near their breeding places and roost singly. The migrations and distribution of finches in Europe are described briefly. At any time of winter, closely related species with similar feeding habits are often concentrated in different parts of a common wintering range. Furthermore, in some species, such as the Goldfinch and Chaffinch, the sexes winter mainly in different areas. In Oxford, food was least readily available during hard (winter) weather and there was then complete segregation in diet between different species, but at other times their diets often overlapped considerably. The overlap was much more pronounced in the species living in farmland (which is an artificial habitat) than in those living in natural or seminatural woodland; some tentative explanations for this are offered. The fringilline finches, the Chaffinch and Brambling, feed their young entirely on invertebrates, are territorial in the breeding season and collect food mostly within their territories. The cardueline finches, on the other hand, feed their young mainly, or entirely, on seeds, often nest in “loose colonies” and usually gather food communally at some distance from their nests. The fringilline finches carry food in their bills to the young, which therefore receive small, but frequent, meals; whereas the carduelines feed their young by regurgitation and at longer intervals. Among the carduelines, the Hawfinch is exceptional in that seeds are first swallowed and regurgitated to the nestlings, whereas insects are carried in the bill. The Bullfinch develops in the breeding season special buccal pouches in which food is carried to the young. In general, most finches select diets for their young which differ from their own diets in the breeding season. The Crossbill and Linnet often feed their young on seeds alone, and some other species may do so occasionally. The breeding of these carduelines is therefore independent of a supply of invertebrates, but always coincides with an abundance of seeds. They breed at seasons when other birds do not; the most extreme cases are the three crossbills, which, given an abundance of suitable seeds, will nest at any time of year. The process of seed-husking is described; the strength of the jaws, the extent of the gape and the width of the palatal grooves limit the size and hardness of seeds which a given species can take. No two species have bills of the same shape and size, and differences in diet between species are connected with differences in the structure of their bills and the way the bill is used. In general, the deeper the bill of a finch, the more numerous the types of seeds in its diet. The Greenfinch, Linnet and Redpoll have relatively short broad bills and feed more than do other Carduelis finches from plants whose seeds are relatively exposed (e.g. grasses) or enclosed within pods or capsules; they also pick up many fallen seeds from the ground, but each species concentrates on different sizes of seeds. The Goldfinch and Siskin have reIatively long, narrow bills and obtain much of their food by probing into cones and into the seed-heads of Com-positae; unlike other finches, they also use a gaping movement while feeding. The Goldfinch has a longer bill and eats more Compositae seeds than the Siskin, while tlie latter eats more tree seeds than the Goldfinch. Further, male Goldfinches have slightly longer beaks than females, and hence can obtain teasel seeds more efficiently; females feed only rarely from these plants. The Bullfinch and Hawfinch obtain much of their food by crushing fruits, seed-heads, etc. in their bills and peeling off the outer layer to get the seeds. The methods used by the crossbill species to open cones of varying degrees of hardness are described in detail. The Chaffinch and Brambling obtain almost all their food by rapid pecks; they show none of the behaviour patterns used by carduelines to open various types of seed-heads. Species also differ in the proportion of food obtained from the ground, from vegetation, or on the wing. These differences are related to the various positions in which each species is able to feed. Those species (Goldfinch, Siskin, Redpoll and Crossbills) which are most adept at clinging and hanging feed mainly from trees and herbaceous plants, while those that are least adept at this (Chaffinch and Brambling) obtain almost all their food from the ground or from firm horizontal twigs. The remaining species are intermediate in the proportion of time spent feeding from the ground or from vegetation; and only the Chaffinch, Brambling and Bullfinch obtain an appreciable amount of food on the wing. Differences between species in the frequency with which various positions are adopted during feeding can be related to differences in their average body weights, and (to a lesser extent) in their leg-structure; the most agile species are either light in weight or have relatively short strong legs. Body-weight, leg-proportions and bill-size (and hence feeding habits) may vary considerably between different populations of the same species. In Appendix 2, the leg-proportions of certain English finches are compared with those of their German and Russian counterparts. Some finch species use their feet in coordination with their bill to obtain food. The extent to which various species do this in the wild is discussed in relation to the ease with which they can be taught experimentally to pull up food suspended on the end of a long thread. The ecology, structure and behaviour of finches is discussed in relation to some current ideas on speciation and the ontogeny of feeding behaviour. The young of several species of finches respond initially to a similar range of potential food-items (both in size and appearance) and all attempt to deal with them in similar ways. However, each individual soon learns to concentrate on those foods it can obtain most efficiently with its own particular structures and behaviour patterns. It is thus the structural and behavioural differences between closely-related species which lead the individuals of each to prefer different foods. Further, some of the behaviour patterns themselves probably develop mainly as a result of learning conditioned by structure, so that some of the behavioural differences between species that affect food-choice are in turn dependent on structural differences. This implies that, from an evolutionary viewpoint, two forms need diverge only in structure (and not necessarily “psychologically”) for some ecological segregation to be achieved. There are, however, certain differences in feeding behaviour between the fringillines and carduelines that cannot be attributed merely to structural differences between the two groups, so presumably some “psychological factor” is also important in the development of these particular behaviour patterns. A close relationship between the structure, feeding behaviour and food of an animal is usually taken for granted, but it is often difficult to decide which of these came first in evolution. Among finches, however, there are sometimes indications that a structural change was necessary before a new behaviour pattern or feeding habit could evolve, though further modification of structure may then occur through adaptation to the new foods thereby made available. Some evolutionary trends among finches are described, and the role of competition in the evolution and maintenance of ecological differences between species is discussed. The effect of social experience on feeding behaviour is also discussed, and some examples are given of the spread of novel food-habits among finches, and of presumed traditional differences in feeding behaviour between different populations of the same species. Finally, some recent changes in food, and habitat-selection by finches are discussed in relation to problems of conservation.     

Introgressive hybridization and natural selection in Darwin's finches

Introgressive hybridization, i.e. hybridization with backcrossing, can lead to the fusion of two species, but it can also lead to evolution of a new trajectory through an enhancement of genetic variation in a new or changed ecological environment. On Daphne Major Island in the Galápagos archipelago, ~1–2% of Geospiza fortis finches breed with the resident G. scandens and with the rare immigrant species G. fuliginosa in each breeding season. Previous research has demonstrated morphological convergence of G. fortis and G. scandens over a 30‐year period as a result of bidirectional introgression. Here we examine the role of hybridization with G. fuliginosa in the evolutionary trajectory of G. fortis. Geospiza fuliginosa (~12 g) is smaller and has a more pointed beak than G. fortis (~17 g). Genetic variation of the G. fortis population was increased by receiving genes more frequently from G. fuliginosa than from G. scandens (~21 g). A severe drought in 2003–2005 resulted in heavy and selective mortality of G. fortis with large beaks, and they became almost indistinguishable morphologically from G. fuliginosa. This was followed by continuing hybridization, a further decrease in beak size and a potential morphological fusion of the two species under entirely natural conditions.     

Phenotypic Variation in Mangrove Cuckoo (Coccyzus minor) across Its Geographic Range

Mangrove Cuckoo (Coccyzus minor) exhibits substantial phenotypic variation across its geographic range, but the significance of this variation for taxonomy remains unresolved. Using measurements of bill size and ventral color recorded from 274 museum specimens, I found that variation in these traits was clinal. No named subspecies was reciprocally diagnosable from all others, and none was distinguishable from the nominate form, such that previously recognized subspecific distinctions are invalid. Greatest differences in phenotype occurred between populations in Florida, the Bahamas, and the Greater Antilles–characteristically small-billed–and those in the Lesser Antilles, which had larger bills. Phenotypically intermediate individuals on the geographically intermediate islands of Barbuda and Antigua linked these two extremes. Individuals intermediate in bill size and color also characterized populations from throughout the remainder of the range in northern South America and Middle America. Mechanisms maintaining the fairly pronounced phenotypic differences between nearby populations of Greater and Lesser Antillean birds are unknown, yet the geographic proximity of these populations suggests that they probably persist despite occasional gene flow, and may be adaptive.     

Bill colour and immunocompetence in the European blackbird

The level of expression of secondary sexual characters has been suggested to signal male ability to resist parasitic infestations. To test this idea, several studies have examined the link between sexual signals and immunocompetence in birds. However, most of them have used only a single aspect of immune response to evaluate immunocompetence. We investigated the relation between bill colour and immunocompetence in captive male European blackbirds, Turdus merula, during the breeding season by assessing both cell-mediated and humoral components of the immune system. The blackbird is a sexually dimorphic species with bill colour varying from yellow to orange in males. Humoral immunity was assessed by measuring both primary and secondary responses to sheep red blood cell inoculation. Cell-mediated immunity was estimated with a delayed cutaneous hypersensitivity response to an injection of a mitogen (phytohaemagglutinin). No relation was found between male bill colour and the primary humoral response. However, males with orange bills showed a lower secondary humoral response but a higher cell-mediated immune response than males with yellow bills. Thus, the relation between immunocompetence and a secondary sexual trait may differ markedly depending on which component of the immune system is under consideration. We discuss our results in relation to mechanisms involved in sexual selection. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour      

Geographic variation in the Laysan Finch (Telespyza cantans)

Summary Significant differences in bill shape exist between the endemic Laysan population and introduced populations of the Laysan Finch (Drepanidinae:Telespyza cantans) at Pearl and Hermes Reef. Furthermore, there are significant differences in bill shape between two of four finch populations on the islands of Pearl and Hermes Reef. The variation in finch bill shape is paralleled by geographic variation in the large, hard mericarps ofTribulus cistoides, an important food item at Pearl and Hermes Reef. There are three possible explanations for the observed variation. Environmental influence may be a non-evolutionary cause of the variation, or genetic drift and natural selection may have caused the differences in morphology. Preliminary results suggest that both genetic drift and natural selection have been acting on the populations. Continuing studies, including investigation of genetic variation and cross-fostering experiments to determine heritability of morphological traits and effects of environment on morphological traits, are planned to test the various hypotheses.     

THE INFLUENCE OF THE CALIFORNIA MARINE LAYER ON BILL SIZE IN A GENERALIST SONGBIRD

The hypothesis is tested that birds in hotter and drier environments may have larger bills to increase the surface area for heat dissipation. California provides a climatic gradient to test the influence of climate on bill size. Much of California experiences dry warm/hot summers and coastal areas experience cooler summers than interior localities. Based on measurements from 1488 museum skins, song sparrows showed increasing body‐size‐corrected bill surface area from the coast to the interior and declining in the far eastern desert. As predicted by Newton's convective heat transfer equation, relative bill size increased monotonically with temperature, and then decreased where average high temperatures exceed body temperature. Of the variables considered, distance from coast, average high summer temperature, and potential evapotranspiration showed a strong quadratic association with bill size and rainfall had a weaker negative relationship. Song sparrows on larger, warmer islands also had larger bills. A subsample of radiographed specimens showed that skeletal bill size is also correlated with temperature, demonstrating that bill size differences are not a result of variation in growth and wear of keratin. Combined with recent thermographic studies of heat loss in song sparrow bills, these results support the hypothesis that bill size in California song sparrows is selected for heat dissipation.     

Sexual size dimorphism in red-necked phalaropes and functional significance of nonsexual bill structure variation for feeding performance

Sexual size dimorphism is widespread in shorebirds, yet no tests of the assumption that such size dimorphism extends to functionally significant dimensions of the bill exist. This report presents tests of: (1) the assumption that sexual size dimorphism extends to the feeding structures in sexually size dimorphic bird, and (2) the hypothesis that bill-size variation influences feeding performance in Phalaropus lobatus, the red-necked phalarope. Discriminant function analysis revealed that the sexes of this species can be distinguished on the basis of five body size/bill length variables, but with low accuracy in sexing of females because of misclassification of small females as males. In the shorebird literature, the assumption is generally made that in the absence of selection to the contrary, bill size scales to body size and hence sexual size dimorphism extends to bill size. However, discriminant function analysis of measures from red-necked phalaropes failed to separate the sexes on the basis of either external or internal bill dimensions other than length. Nonetheless, internal dimensions of the upper jaw combined with exposed culmen length explained 86% of the variance in feeding performance of phalaropes; high feeding performance depends on a wide, shallow, complex internal bill structure. This study provides evidence that internal bill dimensions determine feeding performance in a manner consistent with the mechanics of surface tension transport of prey. These results suggest that some dimensions of bill size may be constrained by performance demands and demonstrate that variation in bill morphology has functional consequences. © 1996 Wiley-Liss, Inc.