Optimal thermal niche-tracking buffers wild great tits against climate change

Animal and plant populations are responding to climate change by shifting the timing of key seasonal events. However, while measuring responses of natural systems on human calendars provides a standard scale of measurement it reveals little about the underlying biological causes and mechanisms of change. Here, using data from a six decade longitudinal, individual-based, study of great tits Parus major, we show that changing the timing of breeding has enabled birds to track specific temperatures at different stages in reproduction. We show, further, that multiple measures of reproductive success are maximised at intermediate, stage-specific, temperatures that correspond to the temperatures tracked while climate has changed in recent decades. Finally, we show that tracking temperatures has allowed great tits to match reproduction with a key food source for nestling development, the winter moth Operophtera brumata. Consequently, phenotypic plasticity in reproductive timing has enabled great tits to track temporal changes in the environmental conditions that maximise individual reproductive success. Shifting our perspective from analysing the phenological timing of life history events under climate change to analysing changes relative to environmental gradients has the potential to shed new light on the causes, mechanisms, and consequences of these shifts.