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

Most Australian birds do not migrate over long distances and therefore have to cope with seasonal changes in weather and food availability. We investi gated whether the small (11 g) silvereye Zosterops lateralis changes its th ermal tolerance from winter to summer. Body mass and body temperature of si lvereyes exhibited little seasonal variability. However, metabolic rates (M R) and thermal conductance showed significant changes. Below the thermoneut ral zone (TNZ), winter-acclimatized birds had significantly lower resting M R and thermal conductance than summer-acclimatized birds. Within the TNZ (s imilar to 27.0-33.6 degreesC winter; similar to 25.4-33.5 degreesC summer) basal MR of winter-acclimatized birds (2.30 +/- 0.29 mL O-2 g(-1) h(-1)) wa s significantly lower than that of summer-acclimatized birds (2.88 +/- 0.43 mt O-2 g(-1) h(-1)). The average daily MR also differed significantly betw een 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 s ilvereyes exhibit seasonal variability in physiology and thermal energetics , even when they live in areas with a relatively mild climate, to help over come seasonal changes in weather conditions and food availability.