Geographic variation in male ornamentation and female mate preference in the house finch: a comparative test of models of sexual selection

I compared the mate preferences of female house finches (Carpodacusmexicanus) from populations in which males differ in both plumagecoloration and the extent of ventral pigmentation (patch size).Phylogenetic analysis indicated that the small patch of ventralcoloration displayed by males in some populations is derivedfrom a larger-patched ancestral state. Regardless of the appearanceof males in their own populations, however, females from allpopulations showed a preference for the most brightly coloredmales and males with the largest patches. A reduction in patchsize independent of a change in female mate preference is notconsistent with sensory-bias or reproductive isolation modelsof sexual selection or with general predictions of runaway modelsof sexual selection. In contrast, a lack of congruence betweenfemale mate preference and male trait expression is predictedby the honest advertisement model, with house finches respondingto variation in regional and local access to carotenoid plumagepigments.     

Paternal care as a conditional strategy: distinct reproductive tactics associated with elaboration of plumage ornamentation in the house finch

When individuals in a population differ in physiological conditionand residual reproductive value, selection should favor phenotypicplasticity in reproductive investment such that individualsare able to adopt the reproductive tactic that results in thehighest fitness under given conditions. Here we examined reproductivetactics in relation to the elaboration of condition-dependentsexual ornamentation (carotenoid breast coloration) in a Montanapopulation of the house finches (Carpodacus mexicanus). Malesused distinct reproductive tactics depending on elaborationof their sexual ornamentation. Males with red pigmentation (maximum ornament elaboration) paired with females that nestedearlier, but these males did little provisioning of incubatingfemales and nestlings. In contrast, males with yellow colorationpaired with females that nested later, but these males fedfemale and nestlings more. Consequently, for red males offspringrecruitment was primarily affected by earlier nest initiation, whereas in yellow males it was affected most by male provisioning.In males with intermediate plumage coloration, all measuredcomponents, nest initiation, provisioning of incubating female,and nestling feeding, strongly contributed to offspring recruitment.The fitness consequences of alternative reproductive tacticsof males were influenced by breeding experience and fidelityof their mates. Among first-time breeders, red males achievedthe highest fecundity because of the advantage gained throughearly nesting and pairing with more experienced females andbecause of compensation by their mates for low male provisioningof nestlings. Among experienced breeders, males with intermediateplumage coloration achieved the highest fecundity because ofthe combined benefits of relatively early pairing and high parental care. High variation in sexual ornamentation in a Montana populationof house finches may favor distinct associations of sexualdisplays with a particular set of reproductive behaviors.     

Isolation and characterization of eight microsatellite loci from the house finch (Carpodacus mexicanus)

I isolated the first set of polymorphic microsatellite markers from the house finch, Carpodacus mexicanus, a well‐studied North American bird species, as part of an effort to compare levels of genetic diversity in introduced and native populations. Here, I describe eight independently assorting microsatellite loci screened for polymorphism using 40 house finches. Polymorphism levels ranged from six to 14 alleles (mean = 10.6), making these markers a powerful tool for paternity and population level analyses of this widely distributed North American species.     

Pairing success relative to male plumage redness and pigment symmetry in the house finch: temporal and geographic constancy

Male house finches (Carpodacus mexicanus) have carotenoid-based ornamentalplumage coloration. In previous research it was shown that fora single population of house finches in a single year, malesthat paired were on average redder in plumage coloration thanmales that did not pair, and males with redder plumage tendedto nest earlier than males with less red plumage. Here we showthat these patterns continued over 6 years and at two widely separatedlocations. We also tested whether the symmetry of carotenoid-based crownpigmentation differed between paired and unpaired males andfound that paired males have, on average, more perfect symmetryof crown pigmentation than males not paired. These observationssupport the idea that expression of carotenoid-based plumagecoloration by males is a persistent and widespread criterionin female mate choice in the house finch.     

Interspecific competition between introduced house finch populations and two associated passerine species

Summary House Finches (Carpodacus mexicanus), natives of western North America, have expanded their range in the eastern United States since their 1940 release in New York City. Range and the relation of House Finch population growth to the population dynamics of House Sparrows (Passer domesticus) and Purple Finches (Carpodacus purpureus) were examined, using data from the Breeding Bird Survey and the Christmas Bird Count. The House Finch population grew exponentially throughout its eastern range. Significant negative relationships in population density, relative to spatial and temporal control populations, were found between House Finches and House Sparrows in summer and winter, and between House Finches and Purple Finches in summer. Purple Finch and House Sparrow populations outside of the House Finch range appeared to have no effect on each other throughout the study. Neither changes in 74 weather variables, nor changes in forest, field or developed habitat explained the observed trends in population density. The results indicate that House Finches compete with these two species, but winter migration complicates the picture.     

The evolution of sexual dimorphism in the house finch. I. Population divergence in morphological covariance structure

Abstract Patterns of genetic variation and covariation strongly affect the rate and direction of evolutionary change by limiting the amount and form of genetic variation available to natural selection. We studied evolution of morphological variance-covariance structure among seven populations of house finches (Carpodacus mexicanus) with a known phylogenetic history. We examined the relationship between within- and among-population covariance structure and, in particular, tested the concordance between hierarchical changes in morphological variance-covariance structure and phylogenetic history of this species. We found that among-population morphological divergence in either males or females did not follow the within-population covariance patterns. Hierarchical patterns of similarity in morphological covariance matrices were not congruent with a priori defined historical pattern of population divergence. Both of these results point to the lack of proportionality in morphological covariance structure of finch populations, suggesting that random drift alone is unlikely to account for observed divergence. Furthermore, drift alone cannot explain the sex differences in within- and among-population covariance patterns or sex-specific patterns of evolution of covariance structure. Our results suggest that extensive among-population variation in sexual dimorphism in morphological covariance structure was produced by population differences in local selection pressures acting on each sex.     

Carotenoid status signaling in captive and wild red-collared widowbirds: independent effects of badge size and color

Carotenoid-based plumage ornaments are typically consideredto be sexually selected traits, functioning as honest condition-dependentsignals of phenotypic quality, but few studies have addressedthe function of carotenoid color variation in male contestcompetition. Using two experiments, we investigated the statussignaling function of the variable (ranging from yellow tored) carotenoid throat patch (collar) in the polygynous, sexually dimorphic red-collared widowbird (Euplectes ardens). First,we tested if the red collar functions as a dominance signalby painting spectrometrically controlled collar patches ontothe brown plumage of nonbreeding males and staging dyadic malecontests over food resources. Red-collared males dominatedorange males, which in turn dominated the control brown andnovel blue collars. Red dominance persisted when the collar manipulations were reversed within dyads and also when testedagainst testosterone implanted males. In the second experimentthe collar size and color of breeding males were manipulatedin the field before and after territories were established.All males with enlarged red and most with enlarged orange orreduced red collars obtained territories, whereas most maleswith reduced orange and all with blackened (removed) collarsfailed to establish or retain territories. In addition, amongthe territorial males, those with reduced signals defendedsmaller territories, received more intrusions, and spent moretime in aggressive interactions. Redness and, to a lesser extent,size of the carotenoid ornament both seem to independently indicate male dominance status or fighting ability in male contest competition.     

The evolution of sexual size dimorphism in the house finch. V. Maternal effects

The phenotype of a mother and the environment that she provides might differentially affect the phenotypes of her sons and daughters, leading to change in sexual size dimorphism. Whereas these maternal effects should evolve to accommodate the adaptations of both the maternal and offspring generations, the mechanisms by which this is accomplished are rarely known. In birds, females adjust the onset of incubation (coincident with the first egg or after all eggs are laid) in response to the environment during breeding, and thus, indirectly, determine the duration of offspring growth. In the two house finch (Carpodacus mexicanus) populations that breed at the extremes of the species' distribution (Montana and Alabama), females experience highly distinct climatic conditions during nesting. We show that in close association with these conditions, females adjusted jointly the onset of incubation and the sequence in which they produced male and female eggs and consequently modified the growth of sons and daughters. The onset of incubation in newly breeding females closely tracked ambient temperature in a pattern consistent with the maintenance of egg viability. Because of the very different climates in Montana and Alabama, females in these populations showed the opposite patterns of seasonal change in incubation onset and the opposite sex bias in egg-laying order. In females with breeding experience, incubation onset and sex bias in laying order were closely linked regardless of the climatic variation. In nests in which incubation began with the onset of egg laying, the first-laid eggs were mostly females in Montana, but mostly males in Alabama. Because in both populations, male, but not female, embryos grew faster when exposed to longer incubation, the sex-bias produced highly divergent sizes of male and female juveniles between the populations. Overall, the compensatory interaction between the onset of incubation and the sex-biased laying order achieved a compromise between maternal and offspring adaptations and contributed to rapid morphological divergence in sexual dimorphism between populations of the house finch breeding at the climatic extremes of the species range.     

Fiery red heads: female dominance among head color morphs in the Gouldian finch

Although the evolution of genetic color polymorphisms has receivedmuch theoretical interest, few empirical studies have investigatedthe adaptive function of alternative color morphs. Furthermore,most studies have focused almost exclusively on the evolutionand adaptive expression of male coloration, leaving the roleof conspicuous female coloration largely unknown. Using thecolor polymorphic Gouldian finch (Erythrura gouldiae), thisstudy experimentally tests the status signaling function ofhead color (red, yellow, and black) among the 3 female colormorphs. In standardized dominance contests between unfamiliarfemales of different head colors, red-headed females dominatedboth black- and yellow-headed females. During contests betweenthe morphs, red-headed females passively displaced black- andyellow-headed opponents, whereas interactions between red-headeddyads were particularly aggressive and more frequent than interactionswithin dyads of the other color morphs. This effect of red dominancefurther persisted when head color was experimentally altered;red-manipulated females (of the other morphs) dominated bothblack- and yellow-headed females, whereas blackened red-headedfemales were dominated by naturally red-headed birds. Togetherwith similar dominance-related differences among male morphs,these results suggest that the 3 color morphs may display alternativestrategies in dominance behavior.     

Condition-dependent sexual traits and social dominance in the house finch

Elaboration of costly sexual traits can reduce investment inother aspects of reproduction, such as parental care or intrasexualcompetition, which may lead to the evolution of alternativemating tactics. In house finches (Carpodacus mexicanus), lesselaborately ornamented (dull) males tend to dominate more elaborated(redder) males, but redder males pair earlier and invest morein parental care. This suggests that males may pursue alternativeparental or competitive tactics, depending on the elaborationof their sexual trait. Elevation of testosterone, a hormonethat is closely associated with condition in male house finches,influences dominance and sexual behaviors but is antagonisticto parental behaviors. We tested the hypothesis that the higherdominance status of dull males reflects an alternative testosterone-dependentmating tactic. First, we experimentally manipulated the testosteronelevels of captive males and measured the effect on dominancerank, and second, we measured the association of testosteroneelevation and plumage hue in free-living males. We found that,as predicted, testosterone elevation increased dominance rankin captive males. However, in free-living males, testosteronelevels were higher in redder males, suggesting that testosteroneis dissociated from dominance status under natural circumstances.This may be because the context of social interactions and thehigher motivation of dull males to access food resources havea stronger influence on the outcome of dominance interactionsthan does the physiological effects of testosterone elevation.In turn, the strong positive correlation between testosteronelevels and plumage elaboration likely reflects the common conditiondependence of these traits.     

Carotenoid-based sexual coloration and body condition in nesting male sticklebacks

The extent, corrected for body size, and intensity of nuptial coloration of breeding male three-spined sticklebacks Gasterosteus aculeatus L. from a Scottish freshwater population were highly variable but correlated, both in the field and after a period of nest-building in the laboratory. However, marked changes in coloration occurred between the field and the laboratory screenings and the coefficients of variation amongst males decreased. In general, intensity of coloration increased between the field and laboratory screenings, but changes in size-corrected extent were not consistently directional. For both indices, there was a negative relationship between the field value and amount by which colour changed between sampling sessions, with fish that were initially the dullest gaining most in colour. Size-corrected extent and (in particular) intensity of coloration in the field sample were associated positively with body condition measured post mortem and there was a negative, though non-significant, relationship between the extent of coloration and the proportion of white cells in the blood. No such associations were found for the laboratory colour screening. The concentration of astaxanthin in the lower jaw was related strongly and positively to both size-corrected extent and intensity of coloration at the laboratory screening and related negatively, though non-significantly, to proportional white cell counts. These data have implications both for the interpretation of previous laboratory studies and for the design of future experiments, and suggest that coloration is only a true indicator of male quality when measured in the field.     

Red coloration and male parental behaviour in the threespine stickleback

In an anadromous population of the threespine stickleback, male red coloration was correlated with agonistic and egg care behaviour of parental males, but not with hatching success. When one male took over a neighbour's territory, the dominant male had more intense red coloration.     

Complexity and integration in sexual ornamentation: an example with carotenoid and melanin plumage pigmentation

Sexual ornaments often consist of several components produced by distinct developmental processes. The complexity of sexual ornaments might be favoured by mate choice of individual components in different environments which ultimately results in weak interrelationships (integration) among the developmental processes that produce these components. At the same time, sexual selection for greater exaggeration of individual components favours their stronger co-dependence on organismal resources. This should ultimately produce stronger condition-mediated integration among ornaments' components in individuals with the most exaggerated ornamentation. Here we distinguish between these two sources of integration by examining the relationship between integration and elaboration of sexual ornamentation in three bird species: two with carotenoid-based sexual ornamentation (the house finch, Carpodacus mexicanus and common redpoll, Carduelis flammea) and a species with melanin-based sexual ornamentation (house sparrow, Passer domesticus). We found that integration of components varied with elaboration of carotenoid-based ornamentation but not of melanin ornamentation. In the house finches, integration was the highest in individuals with small ornaments and decreased with ornament elaboration whereas the pattern was the opposite in common redpolls. These results suggest that in these species integration and complexity of carotenoid-based ornamental components are due to shared condition-dependence of distinct developmental pathways, whereas integration and complexity of the melanin ornamentation is due to organismal integration of developmental pathways and is largely condition- and environment-invariant. Thus, functionally, ornamentation of the house sparrows can be considered a single trait, whereas complexity of the house finch and redpoll ornamentation varies with ornament elaboration and individual condition.     

The evolution of sexual size dimorphism in the house finch. IV. Population divergence in ontogeny

Differences among taxa in sexual size dimorphism of adults can be produced by changes in distinct developmental processes and thus may reflect different evolutionary histories. Here we examine whether divergence in sexual dimorphism of adults between recently established Montana and Alabama populations of the house finch (Carpodacus mexicanus) can be attributed to population differences in growth of males and females. In both populations, males and females were similar at hatching, but as a result of sex-specific growth attained sexual size dimorphism by the time of independence. Timing and extent of growth varied between the sexes: Females maintained maximum rates of growth for a longer time than males, whereas males had higher initial growth rates and achieved maximum growth earlier and at smaller sizes than females. Ontogeny of sexual dimorphism differed between populations, but in each population, sexual dimorphism in growth parameters and sexual dimorphism at the time of nest leaving were similar to sexual dimorphism of adults. Variation in growth of females contributed more to population divergence than did growth of males. In each population, we found close correspondence between patterns of sexual dimorphism in growth and population divergence in morphology of adults: Traits that were the most sexually dimorphic in growth in each population contributed the most to population divergence in both sexes. We suggest that sex-specific expression of phenotypic and genetic variation throughout the ontogeny of house finches can result in different responses to selection between males and females of the same age, and thus produce fast population divergence in the sexual size dimorphism.     

Replacement female house sparrows regularly commit infanticide: gaining time or signaling status?

Although the killing of unrelated young (usually designed asinfanticide) has been typically considered a male behavior,recent research has shown that females may commit infanticideeven more frequently than do males. In rodents and primates,female infanticide represents a strategy associated to competitionfor resources or infant exploitation, but little is known aboutthe causes and reproductive consequences of the killing of conspecificsby females in other vertebrates. In the present article, I focuson infanticide committed by females that replace mates of territorialmales in a population of the house sparrow. I show that (1)replacement females regularly committed infanticide, (2) experiencedfemales committed infanticide more frequently than did novelfemales and tended to select polygynous males to take over theirnests, and (3) laying date and reproductive success after aterritory takeover did not differ between infanticidal and noninfanticidalfemales. These results seem to indicate that infanticide hasnot evolved in females because of the short-term reproductivebenefits it accrues to the perpetrator. I suggest that the killingof unrelated young by females relates to dominance status amongpotential female breeders and that this behavior benefits theperpetrators in terms of mate selection.     

Sexual dimorphism in relation to current selection in the house finch

Abstract.— Sexual dimorphism is thought to have evolved in response to selection pressures that differ between males and females. Our aim in this study was to determine the role of current net selection in shaping and maintaining contemporary sexual dimorphism in a recently established population of the house finch ( Carpodacus mexicanus ) in Montana. We found strong differences between sexes in direction of selection on sexually dimorphic traits, significant heritabilities of these traits, and a close congruence between current selection and observed sexual dimorphism in Montana house finches. Strong directional selection on sexually dimorphic traits and similar intensities of selection in each sex suggested that sexual dimorphism arises from adaptive responses in males and females, with both sexes being far from their local fitness optimum. This pattern is expected when a recently established population experiences continuous immigration from ecologically distinct areas of a species range or as a result of widely fluctuating selection pressures, as found in our study. Strong and sexually dimorphic selection pressures on heritable morphological traits, in combination with low phenotypic and genetic covariation among these traits during growth, may have accounted for close congruence between current selection and observed sexual dimorphism in the house finch. This conclusion is consistent with the profound adaptive population divergence in sexual dimorphism that accompanied very successful colonization of most of the North America by the house finch over the last 50 years.     

Winter male plumage coloration correlates with breeding status in a cooperative breeding species

The function of colored ornaments is usually related to thesignaling of individual quality in intra- and intersexual interactions.In cooperative breeding species, where only a fraction of themale population access the breeding status and the other fractionhas the option to help breeding pairs, colored traits mightprovide the females with a reliable information on the qualityof potential mate. Males of the cooperative breeding azure-wingedmagpies (Cyanopica cyanus) display conspicuous blue plumagecoloration. Here we explored the role played by structural bluecoloration of males and the probability of becoming a breederor a helper. Birds were trapped during 4 consecutive years,and feather coloration was measured with a spectrometer. Malesthat became breeders had a more brilliant and saturated bluecoloration and showed a more violet hue in the nonbreeding periodcompared with birds that became helpers. Breeding males alsoshowed a seasonal decline in blueness, whereas the color propertiesof helpers were constant throughout the year. Blueness of individualstrapped in the nonbreeding period was positively correlatedwith body size and condition. These findings are consistentwith a scenario in which nonbreeding blue plumage colorationmay function as a signal of individual quality in the azure-wingedmagpie at the pair formation time and add to growing evidencesuggesting that the nonbreeding season appears particularlyimportant in impacting breeding roles in cooperative breedingbirds.