P.M. Sirkiä

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 QST (PST)–FST 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 (PST) and neutral genetic distances (FST) 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.

Distinguishing between male and territory quality: females choose multiple traits in the pied flycatcher

To understand how sexual selection works, it is important to distinguish between the mechanisms underlying mating success. One of the basic questions is to differentiate between the importance of territory-related resources (gained through male–male competition) and male quality in female choice. Few studies, however, have tried to separate these two often intercorrelated variables. We randomized territory quality among male pied flycatchers, Ficedula hypoleuca , and studied which male traits were associated with their success in female mate choice. Females appeared to choose multiple male traits in the wild when the males were not allowed to choose territories before female choice for males. Males with a long tarsus, large white wing patch and versatile song were favoured by females. In addition, the contrast of ultraviolet (UV) reflectance in the white wing patch relative to its background, that is, the dorsal brown-black coloration of the males, appeared to be an important mate choice cue. The UV reflectance had a stronger effect on pairing success in dark males than in brown ones. An experimental manipulation of the UV reflectance revealed that the UV coloration of the males appeared to be used in female choice early but not late in the pairing season, which suggests time-dependent plasticity in female choice. Our study thus exemplifies the complexity of mate choice; it is both temporally constrained and dependent on multiple ornaments.

Melanin coloration has temperature‐dependent effects on breeding performance that may maintain phenotypic variation in a passerine bird

Fluctuating selection pressure may maintain phenotypic variation because of different types of individuals being adapted to different environmental conditions. We show that the extensive variation in the coloration of male pied flycatchers (Ficedula hypoleuca) can be maintained through differences in the reproductive success of male phenotypes under different conditions. The effects of weather conditions on the relative success of different male phenotypes varied between different phases of breeding. The reproductive output of black males was the highest when it was cold during egg‐laying but warm during the nestling period, whereas the fledgling production of brown males was highest when it was continuously warm. In addition, male forehead and wing patch sizes had context‐dependent effects on timing of breeding and nestling mortality, respectively. These results indicate that environmental heterogeneity plays a role in maintaining phenotypic variation. As melanin‐based coloration is heritable, climate change may alter phenotype frequencies depending on the patterns of warming.

Candidate genes for colour and vision exhibit signals of selection across the pied flycatcher (Ficedula hypoleuca) breeding range

The role of natural selection in shaping adaptive trait differentiation in natural populations has long been recognized. Determining its molecular basis, however, remains a challenge. Here, we search for signals of selection in candidate genes for colour and its perception in a passerine bird. Pied flycatcher plumage varies geographically in both its structural and pigment-based properties. Both characteristics appear to be shaped by selection. A single-locus outlier test revealed 2 of 14 loci to show significantly elevated signals of divergence. The first of these, the follistatin gene, is expressed in the developing feather bud and is found in pathways with genes that determine the structure of feathers and may thus be important in generating variation in structural colouration. The second is a gene potentially underlying the ability to detect this variation: SWS1 opsin. These two loci were most differentiated in two Spanish pied flycatcher populations, which are also among the populations that have the highest UV reflectance. The follistatin and SWS1 opsin genes thus provide strong candidates for future investigations on the molecular basis of adaptively significant traits and their co-evolution.

Temporal peaks in social information: prospectors investigate conspecific nests after a simulated predator visit

Individuals of many taxa gather social information in order to make informed decisions with lowered unpredictability. Social information may show temporal periods of higher information value during certain events. However, the value of information is expected to decrease with time since the event and individuals accessing this information are expected to do so quickly. In birds, prospectors visit the nests of other individuals to gather information about reproductive decisions and breeding success. Individuals are known to prospect even during their own breeding to access this information. We investigated prospecting behavior of pied flycatchers (Ficedula hypoleuca) at conspecific nests within a short time period following a simulated predator visit. We performed predator and control presentations at flycatcher nests, and recorded all prospectors (foreign conspecific individuals) visiting these nests within 45-min periods before and after the presentations. We found a significant increase in conspecific prospectors visiting focal nests after the presentation of a predator near the nest. The prospectors in our data comprised mainly pied flycatchers actively breeding in the neighborhood of the focal nest. We also found that the increase in prospecting at these nests is of short-duration, and that prospecting rates decreased back to background prospecting levels relatively quickly. Our results find support for the prediction that individuals may respond to temporally peaking value of information after certain observable events. This result suggests that prospecting may be especially aimed at specific periods of high information value.

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 >50xa0years 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.