Nest attendance and thermoregulation behaviors correlate with nest site microclimate in breeding dark-capped bulbul, Pycnonotus tricolor

Temperatures within bird nests should not become too high to prevent adults and their offspring from overheating. Reproducing birds can influence nest temperature relative to ambient temperature through nest-site selection and control egg temperature through incubation. To what extent this thermal microclimate varies among nests and how it relates to parental behavior and heat-induced stress remains poorly understood. To help address this knowledge gap, we used dark-capped bulbuls ( Pycnonotus tricolor ) as a model system to (1) characterize nest site thermal microclimate as the extent to which nest site temperature buffers or amplifies ambient temperature variability, (2) test the relationship between nest site thermal microclimate and nest attendance, and (3) test the relation between nest site thermal microclimate and parental panting. We found no evidence that nest sites buffered ambient temperature variability. Instead, to varying degrees, all nest sites amplified ambient temperature variability. Bulbul parents whose nest site temperature closely mirrored the ambient temperature increased nest attendance duration at higher temperatures. Parents whose nest site temperature was more variable than ambient temperature (i.e. amplified ambient temperature variation, meaning higher temperatures than ambient temperature at warm moments and lower temperatures than ambient temperature at cool moments) decreased their nest attendance at higher temperatures. Birds at nest sites with greater thermal amplification spent more time panting, indicating an association with nest-site thermal microclimate. These findings suggest that parents at nest sites that exceed ambient temperature variability may face increased energetic demands, potentially affecting parental care and self-maintenance when temperatures are elevated. In contrast, when nest site temperatures closely match wider ambient conditions, remaining at the nest may be preferred to temporarily abandoning it, potentially explaining the longer duration in nest attendance. Importantly, if increased nest attendance at elevated ambient temperatures imposes costs on parents or offspring, reproductive success may be affected by nest site selection. Thus, our results raise new questions about the factors driving these choices, including potential thermal and non-thermal benefits.