Winter-ground microhabitat use by differently coloured phenotypes affects return rate in a long-distance migratory bird

Migratory bird populations are declining globally at alarming rates. Non-breeding site conditions affect breeding populations, but generalising non-breeding habitat conditions over large spatial regions cannot address potential fine-scale differences across landscapes or local populations. Plumage characteristics can mediate the effects of environmental conditions on individual fitness. However, whether different phenotypes use distinctive non-breeding sites, and whether they respond to non-breeding site conditions differently remains largely unknown. Stable isotopes (δ13C, δ15N, δ2H) of inert tissues are useful to infer habitat characteristics and geographic origins where those tissues were grown. We collected winter-grown feathers from pied flycatchers (Ficedula hypoleuca) on their breeding grounds over several years from males whose dorsal plumage colouration ranged continuously from brown to black and assessed their stable isotope values as proxies of local habitat conditions. Based on feather δ2H profiles we found that browner males spent their non-breeding season in drier habitats than black males. Assignment to origin analysis shows potential regional non-breeding ground separation between differently coloured males. High within-individual repeatability of both δ13C and δ15N indicate the pied flycatcher males return yearly to similar areas. Blacker males were more likely to return to the breeding grounds after dry years compared with brown males. The opposite was found in wet years. Our study demonstrates that different phenotypes are exposed to different non-breeding site conditions which can differentially affect individual survivorship. This has important ramifications for population dynamics under predicted climate change scenarios where especially brown phenotype pied flycatcher males may be under a risk of decreasing.

Adaptive coloration in pied flycatchers (Ficedula hypoleuca)-The devil is in the detail

Understanding the origin and persistence of phenotypic variation within and among populations is a major goal in evolutionary biology. However, the eagerness to find unadulterated explanatory models in combination with difficulties in publishing replicated studies may lead to severe underestimations of the complexity of selection patterns acting in nature. One striking example is variation in plumage coloration in birds, where the default adaptive explanation often is that brightly colored individuals signal superior quality across environmental conditions and therefore always should be favored by directional mate choice. Here, we review studies on the proximate determination and adaptive function of coloration traits in male pied flycatchers (Ficedula hypoleuca). From numerous studies, we can conclude that the dark male color phenotype is adapted to a typical northern climate and functions as a dominance signal in male-male competition over nesting sites, and that the browner phenotypes are favored by relaxed intraspecific competition with more dominant male collared flycatchers (Ficedula albicollis) in areas where the two species co-occur. However, the role of avoidance of hybridization in driving character displacement in plumage between these two species may not be as important as initially thought. The direction of female choice on male coloration in pied flycatchers is not simply as opposite in direction in sympatry and allopatry as traditionally expected, but varies also in relation to additional contexts such as climate variation. While some of the heterogeneity in the observed relationships between coloration and fitness probably indicate type 1 errors, we strongly argue that environmental heterogeneity and context-dependent selection play important roles in explaining plumage color variation in this species, which probably also is the case in many other species studied in less detail.

EVOLUTION OF PLUMAGE COLOR IN MALE PIED FLYCATCHERS (FICEDULA HYPOLEUCA): EVIDENCE FOR FEMALE MIMICRY

We present the first evidence for sexual deception by female mimicry in birds. Using live, caged birds we show that territorial male pied flycatchers behave aggressively toward bright-colored males but display sexually toward female-like male intruders. We also show that the males that are fooled are those that lack recent sexual experience. All male pied flycatchers are dull-colored in winter. It is possible that young males are more constrained during the spring molt than older males since the former are more dull-colored in spring. According to the molt-constraints hypothesis a subadult plumage would be maladaptive in the breeding season. Analysis of male settling pattern at breeding sites in spring suggests that brownish males are allowed to settle closer to already-established males than dark-colored males. This result suggests an adaptive value of having a subadult plumage color, in particular for young males arriving late from spring migration. However, we also show that mimicry incurs a cost, that of increased aggression from females, which may explain why female-like males have reduced mating success.

Sympatric divergence and clinal variation in multiple coloration traits of Ficedula flycatchers

Geographic variation in phenotypes plays a key role in fundamental evolutionary processes such as local adaptation, population differentiation and speciation, but the selective forces behind it are rarely known. We found support for the hypothesis that geographic variation in plumage traits of the pied flycatcher Ficedula hypoleuca is explained by character displacement with the collared flycatcher Ficedula albicollis in the contact zone. The plumage traits of the pied flycatcher differed strongly from the more conspicuous collared flycatcher in a sympatric area but increased in conspicuousness with increasing distance to there. Phenotypic differentiation (PST ) was higher than that in neutral genetic markers (FST ), and the effect of geographic distance remained when statistically controlling for neutral genetic differentiation. This suggests that a cline created by character displacement and gene flow explains phenotypic variation across the distribution of this species. The different plumage traits of the pied flycatcher are strongly to moderately correlated, indicating that they evolve non-independently from each other. The flycatchers provide an example of plumage patterns diverging in two species that differ in several aspects of appearance. The divergence in sympatry and convergence in allopatry in these birds provide a possibility to study the evolutionary mechanisms behind the highly divergent avian plumage patterns.

Heterospecific female mimicry in Ficedula flycatchers

Mimicry is a widespread phenomenon. Vertebrate visual mimicry often operates in an intraspecific sexual context, with some males resembling conspecific females. Pied flycatcher (Ficedula hypoleuca) dorsal plumage varies from the ancestral black to female-like brown. Experimental studies have shown that conspecific and heterospecific (collared flycatcher, F. albicollis) individuals of both sexes respond, at least initially, to brown individuals as if they were female. We quantified the perceptual and biochemical differences between brown feathers and found that brown pied flycatcher males are indistinguishable from heterospecific, but not from conspecific, females in both aspects. To our knowledge, this is the first evidence of a visual mimetic signalling system in a sexual context where the model is heterospecific to the mimic. By only mimicking heterospecific females, brown pied flycatcher males can establish territories next to the more dominant collared flycatcher in sympatry, suffer less aggression by darker conspecifics in allopatry and preserve within-species sexual recognition throughout the breeding range. A closer look at the evolutionary history and ecology of these two species illustrates how such a mimetic system can evolve. Although likely rare, this phenomenon might not be unique to Ficedula flycatchers.

Fluctuating selection and immigration as determinants of the phenotypic composition of a population

It is important to identify the factors that affect the evolutionary potential of populations to respond to environmental changes. Such processes are for example the ones affecting the amount of heritable phenotypic variation in a population. We examined factors explaining the wide phenotypic variation in the genetically determined black-brown dorsal colouration of male pied flycatchers (Ficedula hypoleuca) during a period of >50 years in a northern European breeding population. We demonstrate that the temperature-dependent relative breeding success of brown males predicts the inter-annual change in the proportion of the brown male phenotype. The proportion of brown males also appears to reflect immigration from Central Europe, where the brown type prevails due to local selection pressure. Warm springs in northern Central Europe had a positive effect on the proportion of the brown phenotype in the north in the early part of the study period, which suggests prolonged migration in favourable conditions. However, the association between warm springs and a high proportion of brown males has weakened from the 1950s to the present, which may explain why the proportion of the brown males in our study area decreased by a third during the period 1954 to 2008. This is likely a result of decreasing population size in Central Europe. These results demonstrate that temporal variation in environmental conditions is maintaining variation in the pied flycatcher male phenotype. They also indicate that climate warming has the potential to change the population composition both through temperature-dependent selection and environmental factors affecting long-distance immigration.

Geographic patterns of genetic differentiation and plumage colour variation are different in the pied flycatcher (Ficedula hypoleuca)

The pied flycatcher is one of the most phenotypically variable bird species in Europe. The geographic variation in phenotypes has often been attributed to spatial variation in selection regimes that is associated with the presence or absence of the congeneric collared flycatcher. Spatial variation in phenotypes could however also be generated by spatially restricted gene flow and genetic drift. We examined the genetic population structure of pied flycatchers across the breeding range and applied the phenotypic Q(ST) (P(ST))-F(ST) approach to detect indirect signals of divergent selection on dorsal plumage colouration in pied flycatcher males. Allelic frequencies at neutral markers were found to significantly differ among populations breeding in central and southern Europe whereas northerly breeding pied flycatchers were found to be one apparently panmictic group of individuals. Pairwise differences between phenotypic (P(ST)) and neutral genetic distances (F(ST)) were positively correlated after removing the most differentiated Spanish and Swiss populations from the analysis, suggesting that genetic drift may have contributed to the observed phenotypic differentiation in some parts of the pied flycatcher breeding range. Differentiation in dorsal plumage colouration however greatly exceeded that observed at neutral genetic markers, which indicates that the observed pattern of phenotypic differentiation is unlikely to be solely maintained by restricted gene flow and genetic drift.

Latitudinal Variation in Female Local Return Rate in the Philopatric Pied Flycatcher (Ficedula hypoleuca)

Philopatry and dispersal distances of female Pied Flycatcher (Ficedula hypoleuca) are presented for European populations using data from 25 breeding areas from 40 to 70°N. Female annual survival probabilities according to capture–recapture models were similar in two study areas in central Spain (45 and 52%). The present study shows that survival is underestimated by using annual local return rate in one of the two breeding populations under study in central Spain. In southern and central Europe, females were found to return equally regularly to their breeding areas, whereas in northern Europe (latitude >60°N) females returned at lower rates. I did not find that median dispersal distance varied among sites, nor was breeding distance related to locate survival rate. Therefore, the present study suggests that the decline in between-year local return rate of female Pied Flycatchers with increasing latitude over Europe may be more probably caused by differences in mortality than by geographical differences in site fidelity.

The role of mating preferences in shaping interspecific divergence in mating signals in vertebrates

Vertebrates represent one of the best-studied groups in terms of the role that mating preferences have played in the evolution of exaggerated secondary sexual characters and mating behaviours within species. Vertebrate species however, also exhibit enormous interspecific diversity in features of mating signals that has potentially led to reproductive isolation and speciation in many groups. The role that sexual selection has played in interspecific divergence in mating signals has been less fully explored. This review summarizes our current knowledge of how mating preferences within species have shaped interspecific divergence in mate recognition signals among the major vertebrate groups. Certain signal modalities appear to characterize mating signal diversification among different vertebrate taxa. Acoustic signals play an important role in mating decisions in anuran amphibians and birds. Here, different properties of the signal may convey information regarding individual, neighbor and species recognition. Mating preferences for particular features of the acoustic signal have led to interspecific divergence in calls and songs. Divergence in morphological traits such as colouration or ornamentation appears to be important in interspecific diversity in certain groups of fishes and birds. Pheromonal signals serve as the primary basis for species-specific mating cues in many salamander species, most mammals and even some fishes. The evolution of interspecific divergence in elaborate courtship displays may have played an important role in speciation of lizards, and particular groups of fishes, salamanders, birds and mammals. While much research has focused on the importance of mating preferences in shaping the evolution of these types of mating signals within species, the link between intraspecific preferences and interspecific divergence and speciation remains to be more fully tested. Future studies should focus on identifying how variation in mating preferences within a species shapes interspecific diversity in features of mating signals in order to better understand how sexual selection may have led to speciation in vertebrates.

Discrepancies in population differentiation at microsatellites, mitochondrial DNA and plumage colour in the pied flycatcher--inferring evolutionary processes

Genetic differentiation between three populations of the pied flycatcher Ficedula hypoleuca (Norway, Czech Republic and Spain, respectively) was investigated at microsatellite loci and mitochondrial DNA (mtDNA) sequences and compared with the pattern of differentiation of male plumage colour. The Czech population lives sympatrically with the closely related collared flycatcher (F. albicollis) whereas the other two are allopatric. Allopatric populations are on average more conspicuously coloured than sympatric ones, a pattern that has been explained by sexual selection for conspicuous colour in allopatry and a character displacement on breeding plumage colour in sympatry that reduces the rate of hybridization with the collared flycatcher. The Czech population was genetically indistinguishable from the Norwegian population at microsatellite loci and mtDNA sequences. Recent isolation and/or gene flow may explain the lack of genetic differentiation. Accordingly, different selection on plumage colour in the two populations is either sufficiently strong so that gene flow has little impact on the pattern of colour variation, or differentiation of plumage colour occurred so recently that the (presumably) neutral, fast evolving markers employed here are unable to reflect the differentiation. Genetically, the Spanish population was significantly differentiated from the other populations, but the divergence was much more pronounced at mtDNA compared to microsatellites. This may reflect increased rate of differentiation by genetic drift at the mitochondrial, compared with the nuclear genome, caused by the smaller effective population size of the former genome. In accordance with this interpretation, a genetic pattern consistent with effects of small population size in the Spanish population (genetic drift and inbreeding) were also apparent at the microsatellites, namely reduced allelic diversity and heterozygous deficiency.