Species commonly respond to climate change by advancing the timing of springtime life-history events. In birds, this response is generally understood as a mechanism for tracking temporal peaks in biotic resources, yet spring and summer phenological advancement can also expose nests and nestlings to colder temperatures. We propose that phenological shifts represent an overlooked mechanism for thermal niche tracking. We apply novel formulations of community occupancy models to estimate the magnitude of breeding season phenological shift in the Californian avifauna over the last century. We then compute the mean temperature change resulting from this observed phenological shift during the breeding season – when phenological advancement could expose ectothermic nestlings to colder temperatures. To understand whether this temperature difference is biologically relevant to avian nesting success, we analyze a large database of bird nests across North America to examine the relationship between June temperature anomalies and avian nesting success.
Across the Californian avifauna, the data support an average phenological advancement of 5–12 days over the 20th century. In June, when nestlings are largely ectothermic, this shift lowers average temperatures by over 1°C, a difference similar to the total warming experienced over the past century. Nest monitoring data suggest that nesting biology plays a role in setting birds’ thermal niches: warm June temperature anomalies are associated with high nest success in the cold parts of a bird’s range but low success in the warm parts of the range. We propose phenological shifts as an overlooked mechanism for thermal niche tracking, with the potential to reshape both the need and opportunity to shift geographically.