Phenological plasticity enables thermal homeostasis in a wild bird population

Shifts in the timing of seasonal events are widely documented biological responses to climate change, but measuring responses on human calendars reveals little about the underlying biological causes of such changes. Here, using six decades of individual-based data from wild great tits Parus major , we show that plasticity in reproductive phenology has enabled stable long-term thermal homeostasis despite marked local warming. This homeostasis has matched average temperatures at which reproductive success is maximized, enabling synchronization with a key invertebrate food source. Shifting our perspective from analyzing the phenological timing of life history events to analyzing changes relative to environmental gradients has the potential to shed light on the causes, mechanisms, and consequences of these responses by establishing direct links with biologically relevant variables.