To understand the consequences of ever-changing environment on the dynamics of phenotypic traits, distinguishing between selection processes and individual plasticity is crucial. We examined individual consistency/plasticity in several male secondary sexual traits expressed during the breeding season (white wing and forehead patch size, UV reflectance of white wing patch and dorsal melanin coloration) in a migratory pied flycatcher (Ficedula hypoleuca) population over an 11-year period. Furthermore, we studied carry-over effects of three environmental variables (NAO, a climatic index; NDVI, a vegetation index; and rainfall) at the wintering grounds (during prebreeding moult) on the expression of these breeding plumage traits of pied flycatcher males at individual and population levels. Whereas NAO correlates negatively with moisture in West Africa, NDVI correlates positively with primary production. Forehead patch size and melanin coloration were highly consistent within individuals among years, whereas the consistency of the other two traits was moderate. Wing patch size decreased with higher NAO and increased with higher rainfall and NDVI at the individual level. Interestingly, small-patched males suffered lower survival during high NAO winters than large-patched males, and vice versa during low NAO winters. These counteracting processes meant that the individual-level change was masked at the population level where no relationship was found. Our results provide a good example of how variation in the phenotypic composition of a natural population can be a result of both environment-dependent individual plasticity and short-term microevolution. Moreover, when plasticity and viability selection operate simultaneously, their impacts on population composition may not be evident.

Life is uncertain. To reduce uncertainty and make adaptive decisions, individuals need to collect information. Individuals often visit the breeding sites of their conspecifics (i.e., "prospect"), likely to assess conspecifics' reproductive success and to use such information to identify high-quality spots for future breeding. We investigated whether visitation rate by prospectors and success of visited sites are causally linked. We manipulated the reproductive success (enlarged, reduced, and control broods) in a nest-box population of migratory pied flycatchers, Ficedula hypoleuca, in Finland. We measured the visitation rates of prospectors at 87 nest-boxes continuously from manipulation (day 3 after hatching) to fledging. 302 adult pied flycatchers prospected 9194 times on these manipulated nests (at least 78% of detected prospectors were successful breeders). While the number of visitors and visits was not influenced by the relative change in brood size we induced, the resulting absolute brood size predicted the prospecting behaviour: the larger the brood size after manipulation, the more visitors and visits a nest had. The parental provisioning rate at a nest and brood size pre-manipulation did not predict the number of visitors or visits post-manipulation. More visitors, however, inspected early than late nests and broods in good condition. Our study suggests that individuals collect social information when visiting conspecific nests during breeding and provides evidence that large broods attract more visitors than small broods. We discuss the results in light of individual decision-making by animals in their natural environments.

Nesting holes are a scarce resource for obligated cavity-nesting birds and an important selective force for the evolution of aggressive female behaviours, which may be mediated by testosterone (T) levels. It is known that during periods of intense intrasexual competition such as initial breeding stages, females are highly aggressive towards intruding females. Here, we studied the implications of T levels for female-female competition by comparing levels of aggressiveness towards simulated female intruders (decoys) in two populations of the pied flycatcher (Ficedula hypoleuca) with a marked difference in breeding density. To this end, we exposed free-living females to simulated territorial intrusions during 30 min when nest construction was almost complete. T levels of females were measured at the beginning of incubation under the assumption that they are positively associated with T levels during nest building. We also related aggressiveness to T levels in both populations. Furthermore, we aimed at detecting whether variation of T levels may explain female incubation attendance. Females showed higher T levels in the populations where pied flycatchers were exposed to a higher likelihood of conspecific interactions (high breeding density) than in the population with low breeding density. Female territorial presence, vigilance at the nest box and proximity to decoys were negatively related to circulating T levels in the high-density population, but not in the low-density population. Differences in T levels between populations did not result in differences in female incubation attendance, but T levels were negatively related to the incubation attendance in females from the population showing high T levels. In our populations, T levels in females prior to laying reflect the need to defend nesting cavities which is higher at high breeding density and in subdominant females. High T levels are costly in terms of incubation attendance.Ethology 121 (2015) 946-957

No abstract

In a substantial number of species, females show some development of secondary sexual characters. These traits can function as signals of individual phenotypic or genetic qualities and status to conspecifics. Individuals may benefit potentially from expressing signals or badges of status if they are reliable and honest signals of individual quality. In many species, badge sizes have been shown to correlate with dominance rank, which may be mediated by testosterone (T) levels. Here, we explored geographic variation in the size and properties of the white wing patch of female pied flycatchers Ficedula hypoleuca and its relation to circulating T levels in three populations (two southern populations in central Spain and a northern population in Finland). Furthermore, we aimed at detecting if the size of the white wing patch and its ultraviolet (UV) reflectance indicate individual quality. We found that females in Spain had larger, brighter and more UV reflecting wing patches than those in Finland. Females with higher UV reflectance and larger primary white patches bred earlier. Younger females and females with larger primary white wing patches showed higher T levels. In contrast, higher values of UV reflectance in feathers from these patches were associated with low T levels. Despite genetic differentiation and differences in trait expression between populations, female pied flycatchers from different populations may converge and use the size of white wing patches to signal their T levels and thereby their social dominance.