Climate change and nest predation affect shifts in timing and duration of breeding as well as reproductive success in a migratory species

While it is well known that the overall timing of avian breeding in northern latitudeshas generally advanced due to climate change, it is still unclear how climate warminghas affected the beginning, end, and duration of the breeding period and reproduc-tive success of birds. This is because changes in the phenological breeding metricshave often been studied using ringing data that are based on successful nests only andimpacts of local factors such as nest predation have not been analysed simultaneously.This study used both successful and failed nesting attempts to estimate the annual tim-ing and duration of breeding in common goldeneyes Bucephala clangula. There wasstrong evidence that the beginning of breeding has advanced during 1995‒2022 butonly weak evidence that the end of breeding has advanced. Consequently, the durationof the breeding period lengthened, although statistical evidence for the trend was onlyweak. The relative importance of climate change and nest predation in affecting thetiming and duration of breeding as well as breeding success was also studied. Among-year variation in the beginning of breeding was mainly governed by the timing of icebreakup, an indicator of climate change, whereas nest predation rate in the previ-ous year was the main driver of the end of breeding, the duration of breeding beingaffected by both the timing of ice breakup and nest predation rate. Annual nest-stagesuccess was best explained by nest predation rate. However, final reproductive success(proportion of nest-left ducklings that survived until independence) decreased withadvancing timing of ice breakup, suggesting that climate change has negatively affectedthe production of independent offspring in the study population. The findings of thisstudy underline the importance of also considering local ecological factors when ana-lysing climate change impacts on phenological breeding metrics and breeding successof birds.