Ecology: Tree health linked to variation in timing of egg laying

The well-documented shift in the timing of egg laying by great tits due to climate change can show marked, small-scale spatial variation within a population and may be linked to the health of nearby oak trees, according to a study published in Nature Climate Change.

Warmer temperatures and the earlier occurrence of spring resulting from climate change has required plant and animal populations to shift the timing of life events—such as breeding— to keep pace with more suitable conditions. Great tits (Parus major) belong to a well-studied food-web, in which these songbirds feed on caterpillars, which in turn feed on newly emerged oak (Quercus robur) leaves. Warmer springs are associated with the timing of life events for all three organisms occurring earlier. However, for birds, this advance is the slowest, placing them at risk of becoming out of sync with their food source.

Charlotte Regan and colleagues used breeding data for more than 13,000 great tits as part of a 60-year study (1961–2020) conducted at a 385-hectare woodland in Oxfordshire, UK. The authors found that there was small-scale spatial variation in the timing of egg laying by great tits that cannot be revealed by population-scale analysis alone. Although the laying date advanced by an average of 16.2 days in the population as a whole over the course of the study, at the level of individual nest boxes (964 in total), laying dates for breeding females varied significantly, advancing between 7.5 and 25.6 days. Moreover, a significant predictor of the variability in laying date was the health of oak trees located within 75 metres of each nest box. For example, birds breeding in boxes surrounded by healthy oaks advanced their laying by 0.34 days per year, while those nesting in areas with oaks in poor health advanced by only 0.25 days per year.

This research has implications for understanding the ability of organisms to survive under climate change at the smaller scale. The authors argue that there is a need to assess climate change responses in the context of complex and local selective factors.