Fitness is at the core of evolutionary theory, but it is difficult to measure accurately. One way to measure long-term fitness is by calculating the individual’s reproductive value, which represents the expected number of allele copies an individual passes on to distant future generations. However, this metric of fitness is scarcely used because the estimation of individual’s reproductive value requires long-term pedigree data, which is rarely available in wild populations where following individuals from birth to death is often impossible. Wild study systems therefore use short-term fitness metrics as proxies, such as the number of offspring produced. This study compared two frequently used short-term metrics for fitness obtained at different offspring life stages (eggs, hatchlings, fledglings and recruits), and compared their ability to predict reproductive values derived from the genetic pedigree of a wild passerine bird population. We used twenty years of precise field observations and a near-complete genetic pedigree to calculate reproductive success, individual growth rate and de-lifed fitness as lifetime fitness measures, and as annual de-lifed fitness. We compared the power of these metrics to predict reproductive values and lineage survival to the end of the study period. The three short-term fitness proxies predict the reproductive values and lineage survival only when measured at the recruit stage. There were no significant differences between the different fitness proxies at the same offspring stages in predicting the reproductive values and lineage survival. Annual fitness at one year old predicted reproductive values equally well as lifetime de-lifed fitness. However, none of the short-term fitness proxies were strongly associated with the reproductive values. The commonly used short-term fitness proxies best predict long-term fitness when measured at recruitment stage. Thus, because lifetime fitness measured at recruit stage and annual fitness in the first year of life were the best proxies of long-term fitness in short-lived birds, we encourage their future use.

To maximize the effectiveness of conservation interventions, it is crucial to have an understanding of how intraspecific variation determines the relative importance of potential limiting factors. For bird populations, limiting factors include nest-site availability and foraging resources, with the former often addressed through the provision of artificial nestboxes. However, the effectiveness of artificial nestboxes depends on the relative importance of nest-site vs. foraging resource limitations. Here, we investigate factors driving variation in breeding density, nestbox occupation and productivity in two contrasting study populations of the European Roller Coracias garrulus, an obligate cavity-nesting insectivorous bird. Breeding density was more than four times higher at the French study site than at the Latvian site, and there was a positive correlation between breeding density (at the 1-km 2 scale) and nest-site availability in France, whereas there was a positive correlation between breeding density and foraging resource availability in Latvia. Similarly, the probability of a nestbox being occupied increased with predicted foraging resource availability in Latvia but not in France. We detected no positive effect of foraging resource availability on productivity at either site, with most variation in breeding success driven by temporal effects: a seasonal decline in France and strong interannual fluctuations in Latvia. Our results indicate that the factors limiting local breeding density can vary across a species' range, resulting in different conservation priorities. Nestbox provisioning is a sufficient short-term conservation solution at our French study site, where foraging resources are typically abundant, but in Latvia the restoration of foraging habitat may be more important.Conservation actions tend to focus on manipulating breeding season processes because these are generally the easiest to control, especially for migratory populations which often disperse over distant and disparate non-breeding sites (Finch et al. 2017). Understanding the factors which limit the breeding density or productivity of threatened species is therefore often crucial for their †

No abstract

Animal sociality, an individual’s propensity to associate with others, has fitness consequences through mate choice, for example, directly, by increasing the pool of prospective partners, and indirectly through increased survival, and individuals benefit from both. Annually, fitness consequences are realized through increased mating success and subsequent fecundity. However, it remains unknown whether these consequences translate to lifetime fitness. Here, we quantified social associations and their link to fitness annually and over lifetime, using a multi-generational, genetic pedigree. We used social network analysis to calculate variables representing different aspects of an individual’s sociality. Sociality showed high within-individual repeatability. We found that birds with more opposite-sex associates had higher annual fitness than those with fewer, but this did not translate to lifetime fitness. Instead, for lifetime fitness, we found evidence for stabilizing selection on opposite-sex sociality, and sociality in general, suggesting that reported benefits are only short-lived in a wild population, and that selection favors an average sociality.

Until recently, and when compared with diurnal birds that use contrasting plumage patches and complex feather structures to convey visual information, communication in nocturnal species was considered to follow acoustic and chemical channels. However, many nocturnal birds have evolved intensely white plumage patches within otherwise inconspicuous plumages. We used spectrophotometry, electron microscopy, and optical modelling to explain the mechanisms producing bright white tail feather tips of the Eurasian woodcock Scolopax rusticola. Their diffuse reflectance was ~30% higher than the highest previously measured feather. This intense reflectance is the result of incoherent light scattering from a disordered nanostructure composed of keratin and air within the barb rami. In addition, the flattening, thickening, and arrangement of those barbs creates a Venetian-blind-like macrostructure that enhances the surface area for light reflection. We suggest that the woodcocks have evolved these bright white feather patches for long-range visual communication in dimly lit environments.

Animal sociality, an individuals propensity to association with others, has consequences for fitness, and particularly mate choice. For example, directly, by increasing the pool of prospective partners, and indirectly through increased survival. Individuals benefit from both over the short-term as these benefits are associated with mating status and subsequent fecundity, but whether animal sociality also translates into fitness is unknown. Here, we quantified social associations and their link with annual and lifetime fitness, measured as the number of recruits and in de-lifed fitness. We measured this in birds visiting a feeding station over two non-breeding periods, using social network analysis and a multi-generational genetic pedigree. We find high individual repeatability in sociality. We found that individuals with an average sociality had the highest fitness, and that birds with more opposite-sex associates had higher fitness, but this did not translate to improved lifetime fitness. For lifetime fitness, we found evidence for stabilizing selection on between sex sociality measures, suggesting that such benefits are only short-lived in a wild population.

The question of why socially monogamous females engage in extra-pair behaviour is long-standing in evolutionary biology. Recent theoretical work has moved away from the indirect-benefits hypothesis to explain female extra-pair behaviours, instead favouring suggestions that they are the result of pleiotropic effects. That is, a trait under strong positive selection in either or both sexes are genetically linked to another, often unrelated, trait. For example, where genes beneficial to female fecundity (contributing to within-pair solicitation of her social partner) are linked with extra-pair behaviour (soliciting copulations from extra-pair males). Here, we test two predictions from this hypothesis: We test the prediction that female divorce, measured from the number of social mates within a given year, is linked with (1) the number of extra-pair males and (2) the proportion of her offspring that are extra-pair. Our results suggest that females who frequently divorce social partners are more likely to produce extra-pair offspring than those who maintain social monogamy. However, by contrast, those females do not also have a higher proportion of extra-pair offspring. The number of broods initiated was also positively correlated with extra-pair males, probably through increased opportunity for extra-pair males to sire offspring over a longer breeding season. Our results provide an empirical example of a behavioural trait, beneficial to female fecundity, that is linked with extra-pair behaviour. These empirical results support the intrasexual pleiotropy hypothesis as a driver of female extra-pair behaviour.

Migratory connectivity describes the extent to which different individuals from specific breeding populations use similar non‐breeding sites, with important consequences for the demography and population structure of birds. We used mark‐recapture data from the EUring data bank to describe the non‐breeding range of Twite Linaria flavirostris within Western Europe. We quantified the strength of migratory connectivity across all European breeding populations, and for the British and continental breeding populations separately. Twite from different parts of the British breeding range used different non‐breeding areas from one another, whereas Twite from different parts of the continental breeding range used similar non‐breeding areas. There was essentially no overlap during the non‐breeding season between British and continental Twite.