The timing of egg laying is a critical event for the fitness of birds because to a large extent it determines their reproductive success. The effects of weather conditions and climate on the onset of reproduction have been widely studied over recent decades and an advance in the timing of breeding phenology has been confirmed in many species. We explored the variation in the timing of the onset of breeding in a Cantabrian population of European shags over the past decade. The results demonstrate that the laying date of this population has advanced by almost 40 days in only 10 years. This advance in the timing of breeding coincides with the global warming trend detected in the last century. However, of the climatic variables studied, only the temperature anomaly index showed a positive correlation with laying date, whereas the effects of the NAO index and SST were negligible, suggesting that local conditions have a much more important effect on this population than large-scale climate change.
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