Global warming modifies the seasonal distribution of clutches on a Mediterranean great tit population

Global warming has multiple effects on phenological events on a wide range of plants and animals. Specifically, many bird species have advanced the start of their breeding season, which could also imply an extension in its duration and also a change in the distribution of clutches throughout the breeding season. We have tested whether this occurred in a population of Great Tits (Parus major), in Sagunto (eastern Spain). The increase of March temperatures between 1986 and 2019 was related to an advancement of the breeding season. Although the breeding season was longer in years with higher June temperatures, the length did not show a temporal trend throughout the study period. The clutches were more concentrated at the beginning of the season (increase in the skewness), while the kurtosis ( tailedness of the distribution) or the modality did not change significantly. Finally, the number of equally good months for breeding (a combined measure of length and distribution) has not changed throughout the years. Overall, all these phenological changes probably caused the observed increased proportion of pairs laying two clutches per year. It remains to be studied whether this increase in reproductive effort has positive or negative impact on fitness.

Automated summary

Global warming has various effects on the timing of breeding seasons for plants and animals. In this study, the researchers examined the breeding season of Great Tits in Sagunto, Spain, and found that higher March temperatures were associated with an earlier start of the breeding season. The length of the breeding season did not show a clear trend over the study period, but the distribution of clutches shifted towards the beginning of the season. The researchers also observed an increase in the proportion of pairs laying two clutches per year, which may be a result of these phenological changes. The impact of this increase in reproductive effort on fitness needs further investigation.