POPULATION DIVERGENCE IN SEXUAL ORNAMENTS: THE WHITE FOREHEAD PATCH OF NORWEGIAN PIED FLYCATCHERS IS SMALL AND UNSEXY

Models of sexual selection suggest that populations may easily diverge in male secondary sexual characters. Studies of a Spanish population of the pied flycatcher, Ficedula hypoleuca, and a Swedish population of the closely related collared flycatcher, F. albicollis, have indicated that the white forehead patch of males is a sexually selected trait. We studied the white forehead patch of male pied flycatchers (n = 487) in a Norwegian population over seven years. Males with large forehead patches were in general more brightly colored, but patch height was not correlated to body mass, body size, or parasite loads. Conditions during the nestling period did not seem to influence patch height as an adult. Patch height increased slightly from the first to the second year as adults, but then remained relatively constant at higher ages. Patch height was not related to survival. Year-to-year changes showed that males who increased in patch height also increased in body mass, suggesting that expression of the forehead patch may be partly condition dependent. However, changes in body mass explained only a small proportion of the variance in patch height between males. Thus, patch height would not be a good indicator of male quality. Furthermore, patch size was also not related to male ability to feed nestlings, indicating that females would not obtain direct benefits by choosing males with large patches. However, patch height could be a Fisher trait, but this requires heritability and there was no significant father-son resemblance in patch height. Comparisons of the males visited by each female during the mate sampling period indicated that chosen males did not have larger forehead patches than rejected males. Experimental manipulation of patch height did not affect male mating success. These results indicate that females do not use patch size as a mate choice cue. Finally, patch height did not predict the outcome of male contests for nestboxes, suggesting that the forehead patch is not an intrasexually selected cue of status. Norwegian pied flycatchers have smaller forehead patches than both Spanish pied flycatchers and Swedish collared flycatchers. We suggest that this pattern may be explained by the lack of sexual selection on the forehead patch in the Norwegian population as compared to the other populations, where the patch is apparently sexually selected. We discuss possible reasons for these population divergences, such as female choice on an alternative secondary sexual character (general plumage color) and speciation among Ficedula flycatchers.     

Causes of reduced clutch size in a tidal marsh endemic

We tested three hypotheses of clutch size variation in two subspecies of the swamp sparrow (Melospiza georgiana georgiana and M. g. nigrescens). Swamp sparrows follow the pattern of other estuarine endemics, where clutch size is smaller among tidal salt marsh populations (M. g. nigrescens) than their closest inland relatives (M. g. georgiana). Our results support predation risk and temperature, but not adult survival, as explanations of this pattern in swamp sparrows. Coastal nests were twice as likely to fail as inland nests, and parental activity around the nest site was positively related to clutch size at both sites. When brood size was controlled for, coastal adults visited nests less often and females vocalized less frequently during visits than inland birds, which may decrease nest detectability to predators. Coastal parents waited longer than inland birds to feed offspring in the presence of a model nest predator, but there was no difference in their response to models of predators of adults, as would be expected if coastal birds possessed increased longevity. Additionally, coastal females laid more eggs than inland females over a single season, following a within-season bet-hedging strategy rather than reducing within-season investment. Coastal territories experienced ambient air temperatures above the physiological zero of egg development more often, and higher temperatures during laying correlated with smaller clutches and increased egg inviability among coastal birds. Similar effects were not seen among inland nests, where laying temperatures were generally below physiological zero. Both subspecies showed an increase in hatching asynchrony and a decrease in apparent incubation length under high temperatures. Coastal individuals, however, showed less hatching asynchrony overall despite higher temperatures. Both air temperatures during laying and predation risk could potentially explain reduced clutch size in not only coastal plain swamp sparrows, but also other tidal marsh endemics.     

Phenotypic plasticity in breeding plumage signals in both sexes of a migratory bird: responses to breeding conditions

Adaptive phenotypic plasticity may respond to present ambient conditions. Sexual and social signals in both sexes may express phenotype performance. Plumage signals that change discontinuously allow relating discrete variation to previous performance. Both sexes of the pied flycatcher Ficedula hypoleuca present white patches on the wings and on the forehead, which constitute sexual and social signals. Forehead patches are moulted together with body plumage in Africa, while wing patches are partly moulted in Africa and partly in the breeding area soon after breeding. We studied individual inter‐year changes (corrected for regression to the mean) in the size of forehead and wing patches of both sexes in seven years for females or six years for males in two nearby study areas in central Spain. We found that initial signal extent strongly delimits the possible subsequent changes negatively. There is a negative association of male age with forehead patch changes. Cold and rainy springs are associated in females with decreases in both patch areas and vice versa, while no association with climate is observed in male wing patch changes. Cold pre‐breeding conditions predict positive changes in female wing and male forehead patches. Breeding success is positively associated with forehead patch changes in females. Late‐breeding males experience more positive changes in forehead patch size than early‐breeding males. Some of these trends can be explained by variable costs of breeding in certain conditions for subsequent signal production and/or maintenance, while absence of trends in some cases may be explained by sex differences in costs of breeding and interactions with phenotypic quality of breeders.     

Reflectance of sexually dichromatic UV‐blue patches varies during the breeding season and between two subspecies of Gallotia galloti (Squamata: Lacertidae)

Body coloration is sexually dimorphic in many vertebrate species, including lizards, in which males are often more conspicuous than females. A detailed analysis of the relative size of coloured patches and their reflectance, including the ultraviolet (UV) range, has rarely been performed. In the present work we quantified sexual dimorphism in body traits and surface area of all lateral patches from adult females and males of two subspecies of Gallotia galloti (G. g. galloti and G. g. eisentrauti). We also analysed the magnitude of sexual dichromatism in the UV‐visible reflectance of such patches and the changes in patch size and brightness during the reproductive season (April–July). Males had significantly larger patch areas (relative to their snout‐vent length) and higher brightness (mainly in the UV‐blue range) than did females in both subspecies. The comparison of relative patch areas among months did not reach statistical significance. However, patch brightness significantly changed during the breeding season: that of the UV‐blue (300–495 nm) range from lizards of the two subspecies was significantly larger in June than in April, while brightness in the 495–700 nm range in G. g. galloti was larger in May, June, and July than in April. A different pattern of dichromatism was also detected in the two populations, with G. g. eisentrauti being more sexually dichromatic than G. g. galloti. We discuss the results in terms of possible evolutionary causes for the sexual dichromatism related to different ecological characteristics of the habitats where each subspecies live. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 556–569.     

COLOR EVOLUTION IN THE HUMMINGBIRD GENUS COELIGENA

The remarkable diversity of coloration and species present in hummingbirds has been considered the result of sexual selection. I evaluate if color differences among species in the genus Coeligena are consistent with expectations from sexual selection theory. If sexual selection on color is important for speciation, closely related species should be markedly different in the colors of feather patches associated with aggression and breeding. I evaluate this prediction through a statistical assessment of the phylogenetic signal of colors from five feather patches: crown, gorget, belly, upper back, and rump. The first two are associated with aggressive and courtship displays and are expected to be under sexual selection, whereas the others are not. Contrary to expectations, the crown and gorget were the only patches with significant phylogenetic signal. Furthermore, I assess if populations of dichromatic species are more divergent in coloration and therefore have reduced gene flow. Color distances among dichromatic subspecies were larger than among monochromatic subspecies, but the magnitude of phenotypic differentiation was not related to levels of gene flow. These results support a role for sexual selection in shaping color variation among populations, but these differences alone are not sufficient to explain speciation.     

Female ornaments in the Pied Flycatcher Ficedula hypoleuca: associations with age, health and reproductive success

Female ornamentation has received little attention in studies of sexual selection. Traditionally, female ornaments have been explained as a genetically correlated response to selection in males. However, recent findings suggest that female ornaments may be adaptive. Southern populations of Pied Flycatchers Ficedula hypoleuca are suited for studies of female ornamentation because, in addition to the white wing patch, some females also express the white forehead patch characteristic of males. We thus addressed the associations of these two ornaments with female age and with some health and breeding parameters in a Spanish population of Pied Flycatchers. Female ornament expression was not associated with haemoparasite prevalences, clutch size or parental provisioning effort. However, females expressing the white forehead patch raised more fledglings, and females with larger wing patches bred earlier, had higher number of hatchlings and showed increased levels of total serum immunoglobulins. Thus, these two unrelated epigamic ornaments may indicate some aspects of female quality. Further experimental studies could test the possibility that these plumage traits might function as signals to the males or might be used during female–female aggressive encounters in competition for nest-sites and mates.     

Phenotypic differentiation is associated with divergent sexual selection among closely related barn swallow populations

Sexual selection plays a key role in the diversification of numerous animal clades and may accelerate trait divergence during speciation. However, much of our understanding of this process comes from phylogenetic comparative studies, which rely on surrogate measures such as dimorphism that may not represent selection in wild populations. In this study, we assess sexual selection pressures for multiple male visual signals across four barn swallow (Hirundo rustica) populations. Our sample encompassed 2400 linear km and two described subspecies: European H. r. rustica (in the Czech Republic and Romania) and eastern Mediterranean H. r. transitiva (in Israel), as well as a potential area of contact (in Turkey). We demonstrate significant phenotypic differentiation in four sexual signalling axes, despite very low‐level genomic divergence and no comparable divergence in an ecological trait. Moreover, the direction of phenotypic divergence is consistent with differences in sexual selection pressures among subspecies. Thus, H. r. transitiva, which have the darkest ventral plumage of any population, experience directional selection for darker plumage. Similarly, H. r. rustica, which have the longest tail feathers of any population, experience directional selection for elongated tail feathers and disruptive selection for ventral plumage saturation. These results suggest that sexual selection is the primary driver of phenotypic differentiation in this species. Our findings add to growing evidence of phenotypic divergence with gene flow. However, to our knowledge, this is the first study to relate direct measures of the strength and targets of sexual selection to phenotypic divergence among closely related wild populations.     

Sexual Selection on a Coloured Ornament in King Penguins

Species in which the sexes equally exhibit colourful ornaments are an issue for evolutionary theory. Among several hypotheses, sexual selection for mutual mate choice and social selection for signals of behavioural dominance are most commonly supported. We examined the previously documented sex‐similar size of yellow‐orange ear patches in the king penguin, Aptenodytes patagonicus. This species is monogamous and pairs just before reproduction. Raising a chick requires considerable effort by both parents, as they alternate care of their single offspring with foraging at sea. The size of the ear patches appears to signal aggressive territoriality in the breeding colony for both sexes. However, experiments suggest that females prefer large patch size during mate choice, and males do not prefer this trait. We tested whether the size of the coloured ear patch was influenced by sexual selection for couples that had recently paired. We used analyses of covariance to compare the size of the ear patch to a measure of body size and then tested for the difference between males and females. Males were 6.2% larger in ear patch width and 7.7% larger in ear patch area than females, and the distance between the ear patches over the head was 7.5% smaller in males, with all differences highly significant. Consequently, sexual selection appears to favour larger ear patches in males, possibly because of an excess of males that promotes female choice. Social selection also appears to favour the evolutionary maintenance of ear patches of males, and thus both types of selection may contribute to enlarged ear patches.     

Variation in melanin pigmentation of a sexually selected plumage trait and its adaptive value in the Common Snipe Gallinago gallinago

There is increasing evidence that melanin‐based plumage coloration correlates with different components of fitness and that it may act as a social or sexual signal of individual quality. We analysed variation in melanin pigmentation in the outermost tail feathers of the Common Snipe Gallinago gallinago. During courtship flights, male Snipe use their outermost tail feathers to generate a drumming sound, which plays a role in territory establishment and mate choice. As the outermost tail feathers are displayed to females during these flights, we predicted that conspicuous variation in their rusty‐brown (pheomelanin‐based) coloration may act as an honest signal of individual quality. To test this prediction, we spectrophotometrically measured brightness (an indicator of total melanin content) and red chroma (an indicator of pheomelanin content) of the outermost tail feathers in 180 juvenile and adult Common Snipe. An age‐related decline in feather brightness was found exclusively in females, suggesting that melanization could have evolved by natural selection to camouflage incubating birds. In both sexes, brightness of the tail feathers was inversely correlated with their structural quality (as measured with mass–length residuals), suggesting that melanization could increase mechanical properties of feathers and, in males, enhance the quality of courtship sonation. Red chroma positively correlated with total plasma protein concentration, supporting our prediction that pheomelanin pigmentation of tail feathers may act as an honest signal of condition. Our study indicated that variation in the melanin‐based coloration of the outermost tail feathers in the Common Snipe could have evolved as a result of several different selection pressures and it emphasizes the complexity of the processes that underlie the evolution of melanin‐based plumage coloration in birds.     

Pollination biology and breeding systems in the terrestrial orchid Bletilla striata

The levels of fruit set in different habitats (three inland populations vs. three coastal populations) and the breeding system in the terrestrial orchid Bletilla striata were examined in two groups of populations in Haenam-gun, Jollanam-do Province in southern Korea. In total inland populations (mean fruit set=13.3%) showed about eight-fold higher fruit set per plant than the coastal populations (1.6%), probably due to mild climatic conditions in the inland populations during the flowering period. Honey bee workers Apis mellifera were observed in the inland populations, but no visitors were found in the coastal populations. However, a great increase in fruit set was achieved by hand-pollination: the percentage of fruit set through artificial self-pollination and geitonogamous pollination were close to 90%, indicating that B. striata is self-compatible. Agamospermy and spontaneous autogamy (automatic self-pollination) were not detected in this study. These indicate that pollinia vectors are essential for achieving fruit set in natural populations of B. striata. Except in one inland population examined in 2001, there was no significant correlation between the patch size per population and the average percentage of fruit set, which might reflect deception-pollination of B. striata and/or paucity of pollinators. Overall levels of fruit sets within and among patches in each population were homogeneous during a two-year period.     

Evidence for differential maternal allocation to eggs in relation to manipulated male attractiveness in the pied flycatcher (Ficedula hypoleuca)

We present evidence of differential maternal allocation to eggs in response to manipulated male attractiveness in the migratory pied flycatcher (Ficedula hypoleuca). We manipulated the size of a male secondary sexual trait, the white forehead patch, right after male arrival to the breeding area and before female arrival. Patch size was (1) enlarged to the maximum observed in the population, (2) reduced by 40% or (3) kept constant by painting with indelible felt markers over the natural feather patch. Male behaviour was affected by the experimental manipulation, as individuals with enlarged patches performed more approaches to the nestbox in response to song playback during the territory occupation and nest-site presentation phases. Females paired with males with reduced forehead patches laid significantly smaller eggs than those paired with males in the control and enlarged-patch treatments. Laying date and clutch size did not differ among the experimental groups. We discuss that manipulations of ornaments designed to study differential allocation at laying should reduce as well as enlarge their expression.     

Bacterial degradability of an intrafeather unmelanized ornament: a role for feather‐degrading bacteria in sexual selection?

The impact of feather‐degrading bacilli on feathers depends on the presence or absence of melanin. In vitro studies have demonstrated that unmelanized (white) feathers are more degradable by bacteria than melanized (dark) ones. However, no previous study has looked at the possible effect of feather‐degrading bacilli on the occurrence of patterns of unmelanized patches on otherwise melanized feathers. The pied flycatcher Ficedula hypoleuca Pallas, 1764 is a sexually dimorphic passerine with white wing bands consisting of unmelanized patches on dark flight feathers. These patches are considered to be a sexually selected trait in Ficedula flycatchers, especially in males, where the patches are more conspicuous (larger and possibly whiter) than in females. Using in vitro tests of feather bacterial degradation, we compared the degradability of unmelanized and melanized areas of the same feather for 127 primaries collected from the same number of individuals in a population breeding in central Spain (58 males and 69 females). In addition, we also looked for sex differences in feather degradability. Based on honest signalling theory and on the fact that there is stronger sexual selection for males to signal feather quality than in females, we predicted that unmelanized areas should be more degradable by bacteria than melanized ones within the same feather, and that these unmelanized areas should also be more degradable in males than in females. We confirmed both predictions. Microstructural differences between cross‐section dimensions of unmelanized and melanized barbs, but not differences in the density of barbs within unmelanized and melanized areas of feathers in males and females, could partly explain differences in degradability. This is the first study to show differences in bacterial degradability among markings on the same feather and among unmelanized feather patches between males and females as predicted by sexual selection theory. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 409–419.     

Climatic and temporal effects on the expression of secondary sexual characters: genetic and environmental components

Despite great interest in sexual selection, relatively little is known in detail about the genetic and environmental determinants of secondary sexual characters in natural populations. Such information is important for determining the way in which populations may respond to sexual selection. We report analyses of genetic and large-scale environmental components of phenotypic variation of two secondary sexual plumage characters (forehead and wing patch size) in the collared flycatcher Ficedula albicollis over a 22-year period. We found significant heritability for both characters but little genetic covariance between the two. We found a positive association between forehead patch size and a large-scale climatic index, the North Atlantic Oscillation (NAO) index, but not for wing patch. This pattern was observed in both cross-sectional and longitudinal data suggesting that the population response to NAO index can be explained as the result of phenotypic plasticity. Heritability of forehead patch size for old males, calculated under favorable conditions (NAO index > or = median), was greater than that under unfavorable conditions (NAO index < median). These changes occurred because there were opposing changes in additive genetic variance (VA) and residual variance (VR) under favorable and unfavorable conditions, with VA increasing and VR decreasing in good environments. However, no such effect was detected for young birds, or for wing patch size in either age class. In addition to these environmental effects on both phenotypic and genetic variances, we found evidence for a significant decrease of forehead patch size over time in older birds. This change appears to be caused by a change in the sign of viability selection on forehead patch size, which is associated with a decline in the breeding value of multiple breeders. Our data thus reveal complex patterns of environmental influence on the expression of secondary sexual characters, which may have important implications for understanding selection and evolution of these characters.     

Sexual selection and tail streamers in the barn swallow

The functional significance of elongated, narrow tips of the tail feathers of certain birds, so-called tail streamers, has recently been discussed from an aerodynamic point of view, and the effects of sexual selection on such traits have been questioned. We review our long-term field studies using observational and experimental approaches to investigate natural and sexual selection in the barn swallow, Hirundo rustica, which has sexually size-dimorphic outermost tail feathers. Experimental manipulation of the length of the outermost tail feathers has demonstrated sexual selection advantages of tail elongation and disadvantages of tail shortening, with opposite effects for natural selection in terms of foraging efficiency, haematocrit and survival. These findings are contrary to the prediction of a general deterioration from both shortening and elongation, if the tail trait was determined solely by its effects on aerodynamic efficiency and flight manoeuvrability. Patterns of sexual selection in manipulated birds conform with patterns in unmanipulated birds, and selection differentials for different components of sexual selection in manipulated birds are strongly positively correlated with differentials in unmanipulated birds. Age and sex differences in tail length, and geographical patterns of sexual size dimorphism, are also consistent with sexual selection theory, but inconsistent with a purely natural selection advantage of long outermost tail feathers in male barn swallows.     

Sexual selection resulting from extrapair paternity in collared flycatchers

Extrapair paternity has been suggested to represent a potentially important source of sexual selection on male secondary sexual characters, particularly in birds with predominantly socially monogamous mating systems. However, relatively few studies have demonstrated sexual selection within single species by this mechanism, and there have been few attempts to assess the importance of extrapair paternity in relation to other mechanisms of sexual selection. We report estimates of sexual selection gradients on male secondary sexual plumage characters resulting from extrapair paternity in the collared flycatcher Ficedula albicollis, and compare the importance of this form of sexual selection with that resulting from variation in mate fecundity. Microsatellite genotyping revealed that 15% of nestlings, distributed nonrandomly among 33% of broods (N=79), were the result of extrapair copulations. Multivariate selection analyses revealed significant positive directional sexual selection on two uncorrelated secondary sexual characters in males (forehead and wing patch size) when fledgling number was used as the measure of fitness. When number of offspring recruiting to the breeding population was used as the measure of male fitness, selection on these traits appeared to be directional and stabilizing, respectively. Pairwise comparisons of cuckolded and cuckolding males revealed that males that sired young through extrapair copulations had wider forehead patches, and were paired to females that bred earlier, than the males that they cuckolded. Path analysis was used to partition selection on these traits into pathways via mate fecundity and sperm competition, and suggested that the sperm competition pathway accounted for between 64 and 90% of the total sexual selection via the two paths. The selection revealed in these analyses is relatively weak in comparison with many other measures of selection in natural populations. We offer some explanations for the relatively weak selection detected. Copyright 1999 The Association for the Study of Animal Behaviour.     

Mate Choice and Colored Beak Spots of King Penguins

The theory of sexual selection explains sexual dimorphisms in ornaments used in mate choice. Mutual mate choice is a form of sexual selection that might explain sexually monomorphic ornamental traits. Under mutual mate choice, both sexes select partners based on the same ornament. We tested the mutual mate choice hypothesis in a mutually ornamented seabird, the king penguin (Aptenodytes patagonicus), through observations of the pair formation process in the field. Penguins that were ready to mate formed displaying pairs at the edge of the colony. Some of these pairs moved into the breeding colony and produced an egg (definitive pairs), while other pairs separated and often switched to another potential partner (temporary pairs). Colored ornaments were quantified using color vision modeling. We predicted that birds would mate assortatively by their elaborate ornamental traits (specifically, colors of beak spots, auricular patches of feathers, and breast patch of feathers). We also predicted that definitive pairs would exhibit more elaborate ornaments than temporary pairs. The mutual mate choice hypothesis was supported by assortative pairing for color of the beak spots, but not for color or size of the auricular patches or for the color of the breast patch. An alternative hypothesis was also consistent with our results, that female choice for a male ornamental trait and superior female condition associated with the same trait produced assortative pairing patterns. More UV‐ and yellow‐colored beak spots for females in definitive than temporary pairs supported the female choice hypothesis over the mutual mate choice hypothesis, but previous experimental results from altered beak spot colors supported the latter. Evidence to date thus supports both the mutual mate choice and female choice hypotheses.     

LOCAL ADAPTATION IN TWO SUBSPECIES OF AN ANNUAL PLANT: IMPLICATIONS FOR MIGRATION AND GENE FLOW

Plant populations often adapt to local environmental conditions. Here we demonstrate local adaptation in two subspecies of the California native annual Gilia capitata using standard reciprocal transplant techniques in two sites (coastal and inland) over three consecutive years. Subspecies performance in each site was measured in four ways: probability of seedling emergence, early vegetative size (length of longest leaf), probability of flowering, and total number of inflorescences produced per plant. Analysis of three of the four variables demonstrated local adaptation through site-by-subspecies interactions in which natives outperformed immigrants. The disparity between natives and immigrants in their probability of emergence and probability of flowering was greater at the coastal site than at the inland site. Treated in isolation, these two fitness components suggest that migration from the coast to the inland site may be less restricted by selection than migration in the opposite direction. Two measurements of individual size (leaf length and number of inflorescences), suggest (though not strongly) that immigrants may be subject to weaker selection at the coastal site than at the inland site. A standard cohort life table is used to compare replacement rates (R₀) for each subspecies at each site. Comparisons of R₀s suggest that immigrants are under a severe demographic disadvantage at the coastal site, but only a small disadvantage at the inland site. The results point out the importance of integrating over several fitness components when documenting the magnitude of local adaptation.     

Information‐gathering as a response to manipulated signals in the eastern fence lizard,Sceloporus undulatus

Colorful visual signals are used across taxa to convey information during agonistic male‐male encounters, which are important for sexual selection. Although much is known about the information content of color signals, less is known about how receivers interpret this information. Here, using territorial Sceloporus undulatus lizards in a natural setting, we examined receiver response to intruders with different color patch sizes to determine (a) if patch size conveys information assessed during male‐male interactions and (b) if/how receivers modulate their behavioral responses to different types of behavioral signals. We found that larger lizards had longer and wider patches, indicating that the size of the patches may be one of the many characteristics of these patches that is used by males to convey information. Free‐ranging subject males also produced more headbob displays in response to intruders with small patches and took marginally longer to react to intruders with large patches. However, we found no differences in the aggressiveness of the response (i.e., fullshows). This indicates that patch size conveys information that is employed during territorial disputes, but that the response is primarily in terms of timing, allowing lizards to gather more information about intruders, instead of aggressive behavior (i.e., fullshows).     

Sexual selection in the barn swallow (Hirundo rustica). V. Geographic variation in ornament size

Models of sexual selection in a cline predict the patterns of clinal variation in female mate preference and male secondary sexual characters. These predictions were tested for the nominate subspecies of the barn swallow Hirundo rustica which demonstrates clinal variation in morphology, with several characters in both sexes showing increasing size at higher latitudes. Sexual size dimorphism in the length of the tail ornament and the short, central tail feathers increase with increasing latitude while size dimorphism in other morphological characters is independent of latitude. The main reason for the two divergent patterns of sexual size dimorphism appears to be the higher foraging cost of having a long tail ornamental at low latitudes. The control of development decreases with increasing latitude as demonstrated by an increasing latitudinal cline in fluctuating asymmetry of tail length. Phenotypic variance in tail length increases with latitude in males, but not in females, as shown by the coefficients of variation. Clinal variation in morphology is not due to natural selection associated with a latitudinal increase in the distance between breeding and wintering areas. The geographic patterns of morphological variation suggest that the tail character has diverged geographically as a result of a sexual process of reliable signalling.