We used thermal imaging to show that two environmental factors—acute stress and diet—influence thermoregulatory performance of a known thermal window, the avian bill. The bill plays important roles in thermoregulation and water balance. Given that heat loss through the bill is adjustable through vasoconstriction and vasodilation, and acute stress can cause vasoconstriction in peripheral body surfaces, we hypothesized that stress may influence the bill’s role as a thermal window. We further hypothesized that diet influences heat dissipation from the bill given that body condition influences the surface temperature of another body region (the eye region). We measured the surface temperature of the bills of song sparrows (Melospiza melodia) before, during, and after handling by an observer at 37°C ambient temperature. We fed five birds a restricted diet intended to maintain bodyweights typical of wild birds, and we fed six birds an unrestricted diet for five months prior to experiments. Acute stress caused a decrease in the surface temperature of the bill, resulting in a 32.4% decrease in heat dissipation immediately following acute stress, before recovering over approximately 2.3 minutes. The initial reduction and subsequent recovery provide partial support for the haemoprotective and thermoprotective hypotheses, which predict a reduction or increase in peripheral blood flow, respectively. Birds with unrestricted diets had larger bills and dissipated more heat, indicating that diet and body condition influence bill-mediated heat dissipation and thermoregulation. These results indicate that stress-induced vascular changes and diet can influence mechanisms of heat loss and potentially inhibit optimal thermoregulation.

These data were extracted from thermal images collected during experiments on Song Sparrows (Melospiza melodia) during the summer of 2013. In summary, the experiments were conducted at 37°C (above the thermoneutral zone) and involved the use of a thermal imaging camera to measure the surface temperatures of birds during a stress response (elicited by handling the bird in photographer's grip). The data are measurements of the surface temperature of the bill, eyeball, and eye region, which are regions of interest for thermoregulation and/or stress monitoring via thermography.

R is necessary for the analysis, and we recommend using R Studio for convenience and ease of installation for the necessary R packages associated with the analysis.