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Young, relatively undeveloped sugar gliders Petaurus breviceps have been observed to be left alone in the nest while the mother forages and may be subject to considerable thermal or energetic stress. As no information is available on the development of thermoregulation in this species, which begins reproduction in winter, we measured resting metabolic rate (RMR), body temperature (Tb) and thermal conductance over a range of ambient temperatures (Ta) in 10 sugar gliders from c. 55 days of age until they had grown to adult size. Sugar gliders were unable to maintain a stable Tb over a Ta range of 30–15 °C until the age of 95–100 days, although they raised RMR somewhat as Ta decreased. Further growth resulted in a steady decrease in mass-specific RMR, an increase in Tb and a substantial decrease in thermal conductance. Our study shows that young gliders below the age of 100 days rely largely on heat produced by adults to maintain a high Tb, but are well able to cope with regular falls in Tb of > 10 °C and a concomitant decrease of RMR. This thermal tolerance and reduction in energy expenditure should allow the mother to forage and replenish her own body fuels while her offspring are left alone in the nest.
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.
We characterized the day roost sites of four pairs and one solitary tawny frogmouth Podargus strigoides in a woodland in south-eastern Australia. The birds were equipped with radio transmitters which enabled us to locate them daily from autumn 1997 to late summer 1998. Tree species, tree size, roost height and orientation of the roosting bird were recorded. Over the study period tawny frogmouths frequented a large number of day roosts (up to 71 per pair). Birds rarely used the same roost over extended time periods and most roosts were used for less than 3 days. Mature trees with a girth of more than 0.5 m were preferred as day roosts. Tawny frogmouths exhibited a significant preference for the coarse and dark-barked stringybark trees, but other tree species such as the smooth-barked, light-coloured gums were also frequented. However, when roosting in gum trees, dead branches were preferred, presumably as these have a coarser appearance than living branches and therefore provided better camouflaging. Especially during winter, the birds showed a significant selection of branches on the northern side of roost trees presumably to maximize sun exposure. During summer, two pairs maintained a significant northerly preference, whereas the others used roosts with random orientations. Small-scale seasonal movements in the area used for day roosting were also observed, with two pairs selecting a distinct area with a south-westerly aspect during summer which appeared to have less sun exposure. Our study suggests that tawny frogmouths select roosts to (1) minimize visibility from day predators and (2) to facilitate passive thermoregulation by sun-basking.
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