Evidence of Small Changes in Daytime Body Temperature in Active Black-Capped Chickadees in Response to Supplemental...

In winter, small birds may be able to reduce their energy expenditure by decreasing daytime body temperature (T _b). While the use of reduced T _b during the active-phase is not well understood, one factor predicted to shape T _b and heterothermy is food availability. We experimentally manipulated the presence of supplemental food at a bird feeder and monitored subcutaneous temperature (T _sub) in 15 overwintering black-capped chickadees (Poecile atricapillus) during feeder visits to investigate whether food availability shapes daytime T _sub. Chickadees had lower T _sub when the ambient temperature was lower, and this effect was strongest when the feeder was empty. Diurnal patterns in T _sub also varied as a function of food treatment; on days when the feeder was full, chickadees exhibited a mid-afternoon peak in T _sub, while on days when the feeder was empty, chickadees exhibited a mid-morning minimum in T _sub. Treatment-related differences in diurnal T _sub patterns and ambient temperature effects align with effects reported in earlier studies, though they are notably smaller in magnitude. We attribute the differences in effect sizes to the fact that our manipulations involved removal of supplemental food, rather than complete food deprivation, and that we measured T _sub in active individuals. Although the observed effect sizes for T _sub were small, this study is an important step toward understanding how access to a reliable food source during winter may shape energy management in small wintering birds. Future studies that obtain measures of T _b outside of when individuals are actively foraging are needed to elucidate how food availability shapes the overall use of T _b adjustments as an energy management strategy.