Surviving in a changing world: weather and juvenile condition matter for a long‐lived avian predator, but blood parasites do not appear to

Survival is commonly the most critical factor influencing population growth in long‐lived species. Hence, identifying factors shaping variation of survival rates is crucial to understand and predict population trajectories in a changing world. We investigated the factors influencing survival in a long‐lived diurnal bird of prey, analysing the effectsof body condition, sex, climate variation (North Atlantic Oscillation [NAO] index) and blood parasite infection ( Leucocytozoon spp.), on age‐ and sex‐specific survival rates of common buzzards Buteo buteo . A total of 2723 individuals were wing‐tagged as nestlings between 2007 and 2020 in eastern Westphalia, Germany as part of an ongoing long‐term study. Thanks to continuous resightings and a citizen‐science approach, we followed the fates of thousands of individuals over space and time, encompassing the entire dispersal range of the population. Annual survival, estimated with capture−mark−resighting models, increased with age, but did not differ between sexes. Long‐term averages were estimated at 0.46 ± 0.04 (mean ± SE) for juveniles, 0.51 ± 0.05 for subadults, and 0.75 ± 0.03 for adults. The best models of survival included age‐dependent effects of body condition, which were strongest for first year survival, and NAO, which was strongest for the subadult age class. By contrast, models including Leucocytozoon haemosporidian infection status received little support, thus delivering no evidence for parasite‐mediated effects on survival. These results were further supported by comparing individuals that died as nestlings or as juveniles (n = 212) to individuals that survived at least to subadult age (n = 534). Individual survival of the most successful European bird of prey depends on early body condition and climate conditions during winter. As winter severity has declined over the last decades, this may explain the significant population growth observed in many populations of this species.