Thermal Variation and Mean Temperature Independently Affect Breeding Behavior Across Temporal Scales in Two Cavity-nesting Birds

Avian incubation behaviors respond to the thermal environment, and this is best studied in relation to average thermal conditions. Although climate change models predict increases in environmental thermal variation, how thermal variability shapes incubation behaviors of birds is not as well explored as responses to average conditions. We examined the relationship between nesting behaviors and temperature in Eastern Bluebirds (Sialia sialis) and Tree Swallows (Tachycineta bicolor) and found that they were time dependent. We also found that incubation behaviors respond independently to average temperature and thermal variation. The rate of temperature change and average temperature experienced just prior to an off-bout reduced individual off-bout length in the mornings but not the afternoons. Patterns of incubation at the scale of the incubation period reflected these decision-level patterns, as incubation constancy, or percentage of time spent on the nest, increased with increasing mean temperature and thermal variation. On the daily scale, increasing thermal variation led to increased daily investment in Eastern Bluebirds. Our findings suggest a complex dynamic between behavior and environmental temperature that is shaped by thermal characteristics present as incubation decisions are made and by general thermal patterns experienced over the entire incubation period.