No abstract

The rise of temperatures due to global warming is related to a lengthening of the breeding season in many bird species. This allows more pairs to attempt two clutches within the breeding season, thus finishing their breeding activity later in the season and therefore potentially overlapping these with post–breeding moult. We tested whether this occurred in two Spanish great tit Parus major populations. The proportion of pairs laying second clutches increased from 1 % to 32 % over the study period in one of the populations (Sagunto, 1995–2019), while it did not change in the other (Quintos, 2006–2019; mean 5 %). We did not find any temporal trend for moult start date of late–breeding birds in any population. The proportion of individuals of both sexes that overlapped moult and breeding increased in Sagunto. For this latter population, sex and age, but not clutch type, contributed to the variability in the probability of overlapping in late–breeding individuals, this being higher for first–year males and lower for older females.

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.