While it is well known that the overall timing of avian breeding in northern latitudes has generally advanced due to climate change, it is still unclear how climate warming has affected the beginning, end, and duration of the breeding period and reproductive success of birds. This is because changes in the phenological breeding metrics have often been studied using ringing data that are based on successful nests only and impacts 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 timing and duration of breeding in common goldeneyes (Bucephala clangula). There was strong evidence that the beginning of breeding has advanced during 1995‒2022 but only weak evidence that the end of breeding has advanced. Consequently, the duration of the breeding period lengthened, although statistical evidence for the trend was only weak. The relative importance of climate change and nest predation in affecting the timing 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 ice breakup, an indicator of climate change, whereas nest predation rate in the previous year was the main driver of the end of breeding, the duration of breeding being affected by both the timing of ice breakup and nest predation rate. Annual nest-stage success was best explained by nest predation rate. However, final reproductive success (proportion of nest-left ducklings that survived until independence) decreased with advancing timing of ice breakup, suggesting that climate change has negatively affected the production of independent offspring in the study population. The findings of this study underline the importance of considering also local ecological factors when analysing climate change impacts on phenological breeding metrics and breeding success of birds.

I used long-term (1995-2022) population level data of both successful and failed nesting attempts and studied shifts in the timing and duration of breeding, and associated changes in breeding success, of the common goldeneye (Bucephala clangula) in relation to climate change-induced shifts in spring phenology (measured in terms of the timing of ice breakup in the breeding lakes) and among-year variation in nest predation rate.