Acute stress and restricted diet reduce bill-mediated heat dissipation in the song sparrow (Melospiza melodia): Implications for optimal thermoregulation

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.