Removal of older males increases extra-pair siring success of yearling males.

In animals, reproductive performance typically improves over time early in life. Several ultimate and proximate mechanisms may contribute to such an age-related improvement and these mechanisms can act in a relative or in an absolute sense. Low performance of young individuals may be the consequence of a comparison or competition with older individuals (relative), or it may be due to specific traits of young individuals and be unrelated to the presence of older competitors (absolute). Here, we perform a test to disentangle whether the effect of age class (yearling or older) on male extra-pair siring success is relative or absolute. Male age is the most consistent predictor of male extra-pair siring success across bird species, yet the mechanisms underlying this pattern are not well understood. Low extra-pair siring success of yearling males may be a consequence of the presence of older ("adult") males (hypothesis 1), because adult males are more successful in intra- and intersexual interactions or because females prefer to copulate with adult males when available (relative preference). Alternatively, low extra-pair siring success of yearlings may be independent of the presence of adult males (hypothesis 2), for example, if yearling males on average invest less in extra-pair behavior or if females avoid them as extra-pair mates, independent of the availability of older males (absolute preference). To distinguish between these 2 hypotheses, we experimentally manipulated the age structure of a nest-box-breeding population of blue tits (Cyanistes caeruleus) by removing almost all adult males, and compared patterns of extra-pair paternity in the experimental year with those from the preceding 15 "control" years. Removal of adult males resulted in a substantial increase in the extra-pair siring success of yearling males compared to the "control" years, but did not affect the population-level frequency of extra-pair paternity or its spatial patterns. Our results provide clear evidence that extra-pair siring success of yearlings can increase and that it depends on the presence of older males in the population, indicating a relative effect of age on reproductive performance. These results suggest that older males outcompete yearling males in direct or indirect interactions, in sperm competition or as a result of differences in attractiveness to females.

Local selection signals in the genome of blue tits emphasize regulatory and neuronal evolution.

Understanding the genomic landscape of adaptation is central to understanding microevolution in wild populations. Genomic targets of selection and the underlying genomic mechanisms of adaptation can be elucidated by genome-wide scans for past selective sweeps or by scans for direct fitness associations. We sequenced and assembled 150 haplotypes of 75 blue tits (Cyanistes caeruleus) of a single Central European population by a linked-read technology. We used these genome data in combination with coalescent simulations (i) to estimate an historical effective population size of ~250,000, which recently declined to ~10,000, and (ii) to identify genome-wide distributed selective sweeps of beneficial variants probably originating from standing genetic variation (soft sweeps). The genes linked to these soft sweeps, but also those linked to hard sweeps based on new beneficial mutants, showed a significant enrichment for functions associated with gene expression and transcription regulation. This emphasizes the importance of regulatory evolution in the population's adaptive history. Soft sweeps were further enriched for genes related to axon and synapse development, indicating the significance of neuronal connectivity changes in the brain potentially linked to behavioural adaptations. A previous scan of heterozygosity-fitness correlations revealed a consistent negative effect on arrival date at the breeding site for a single microsatellite in the MDGA2 gene. Here, we used the haplotype structure around this microsatellite to explain the effect as a local and direct outbreeding effect of a gene involved in synapse development.

Negative effects of individual heterozygosity on reproductive success in a wild bird population.

The evolutionary consequences of individual genetic diversity are frequently studied by assessing heterozygosity-fitness correlations (HFCs). The prevalence of positive and negative HFCs and the predominance of general versus local effects in wild populations are far from understood, partly because comprehensive studies testing for both inbreeding and outbreeding depression are lacking. We studied a genetically diverse population of blue tits in southern Germany using a genome-wide set of 87 microsatellites to investigate the relationship between proxies of reproductive success and measures of multilocus and single-locus individual heterozygosity (MLH and SLH). We used complimentary measures of MLH and partitioned markers into functional categories according to their position in the blue tit genome. HFCs based on MLH were consistently negative for functional loci, whereas correlations were rather inconsistent for loci found in nonfunctional areas of the genome. Clutch size was the only reproductive variable showing a general effect. We found evidence for local effects for three measures of reproductive success: arrival date at the breeding site, the probability of breeding at the study site and male reproductive success. For these, we observed consistent, and relatively strong, negative effects at one functional locus. Remarkably, this marker had a similar effect in another blue tit population from Austria (~400 km to the east). We suggest that a genetic local effect on timing of arrival might be responsible for most negative HFCs detected, with carry-over effects on other reproductive traits. This effect could reflect individual differences in the distance between overwintering areas and breeding sites.

Timing of arrival in the breeding area is repeatable and affects reproductive success in a non-migratory population of blue tits.

Events in one part of the annual cycle often affect the performance (and subsequently fitness) of individuals later in the season (carry-over effects). An important aspect of this relates to the timing of activities. For example, many studies on migratory birds have shown that relatively late-spring arrival in the breeding area reduces both the likelihood of getting a mate or territory and reproductive success. In contrast, relatively little is known about the movements of individuals in non-migratory populations during the non-breeding season. Few studies have investigated the timing of arrival at the breeding area in such species, possibly due to the assumption that most individuals remain in the area during the non-breeding season. In this study, we used 4 years of data from a transponder-based automated recording system set up in a non-migratory population of blue tits Cyanistes caeruleus to describe individual variation in arrival at the breeding site. We investigated whether this variation can be explained by individual characteristics (sex, body size or status), and we assessed its effect on aspects of reproductive success in the subsequent breeding season. We found substantial variation in arrival date and demonstrate that this trait is individual-specific (repeatable). Females arrived later than males, but the arrival dates of social pair members were more similar than expected by chance, which suggests that individuals may mate assortatively depending on their arrival in the breeding area. Arrival predicted both whether an individual would end up breeding that season and several aspects of its breeding success. Our study suggests that individuals of non-migratory species leave the breeding area during the non-breeding season. Hence, it may be useful to consider variation in the scale of movements between breeding and non-breeding sites, rather than using a simple dichotomy between 'resident' and 'migratory' species. We conclude that the timing of pre-breeding events, in particular arrival date, may be an overlooked, but important, fitness-relevant trait in non-migratory species.