Seasonal variations in thermal energetics of Australian silvereyes (Zosterops lateralis)

Abstract

Most Australian birds do not migrate over long distances and therefore have to cope with seasonal changes in weather and food availability. We investigated whether the small (11 g) silvereye Zosterops lateralis changes its thermal tolerance from winter to summer. Body mass and body temperature of silvereyes exhibited little seasonal variability. However, metabolic rates (MR) and thermal conductance showed significant changes. Below the thermoneutral zone (TNZ), winter-acclimatized birds had significantly lower resting MR and thermal conductance than summer-acclimatized birds. Within the TNZ (˜27.0–33.6 °C winter; ˜25.4–33.5 °C summer) basal MR of winter-acclimatized birds (2.30 ± 0.29 mL O₂ g^-1 h^-1) was significantly lower than that of summer-acclimatized birds (2.88 ± 0.43 mL O₂ g^-1 h^-1). The average daily MR also differed significantly between summer and winter largely due to a greater reduction of MR at night and the decreased conductance. Our study shows that small passerines such as silvereyes exhibit seasonal variability in physiology and thermal energetics, even when they live in areas with a relatively mild climate, to help overcome seasonal changes in weather conditions and food availability.