Limited Lifetime Costs of Inbreeding in a Large Wild Population of Superb Fairy-Wrens.

Inbreeding depression can have major negative effects on both individual fitness and population performance. The impact of inbreeding depression in wild populations remains poorly understood, and our knowledge is primarily based on estimations in insular populations with limited dispersal, which has been predicted to enhance the capacity for outcrossing. Limited migration and small population sizes, which make insular and endangered populations tractable models for fieldwork, often experience low genetic variation and may be subject to stronger inbreeding effects. Estimating inbreeding effects is difficult in many populations because pedigrees of migrants are unknown. Genomic data offer methods to estimate realised inbreeding even in migrants. Here, we apply genomic methods to a superb fairy-wren (Malurus cyaneus) population that has been subject to detailed study of fitness variation over three decades. The population has sex-biased dispersal, resulting in many immigrant females. We use genomic inbreeding estimates to explicitly test for inbreeding depression across several key components of individual fitness, including nestling condition, juvenile survival and adult traits. Genomic methods were able to recover previous pedigree-based findings of inbreeding depression in early nestling condition and additionally identified some evidence for inbreeding depression in fledgling but not juvenile mortality. The only adult fitness trait affected, male extra-pair paternity, is highly variable and under strong sexual selection. Our findings suggest that natural populations experiencing high effective population sizes and regular dispersal might be less affected by inbreeding depression than previously assumed based on studies of often insular and smaller populations.