Avian torpor or alternative thermoregulatory strategies for overwintering?

Abstract

It is unclear whether torpor really is uncommon amongst passerine birds. We therefore examined body temperature and thermoregulatory strategies of an Austral passerine, the white-browed babbler (Pomatostomus superciliosus), which has characteristics related to a high probability of torpor use; it is a sedentary, insectivorous, cooperative breeding species, which we studied during winter in a temperate habitat. Wild, free-living babblers maintained normothermy overnight, even at sub-zero ambient temperatures, with a mean minimum body temperature of 38.5±0.04°C that was independent of minimum black bulb temperature. Physiological variables measured in the laboratory revealed that babblers had a low basal metabolic rate and evaporative water loss, but their body temperature and thermal conductance were typical of those of other birds and they had a typical endothermic response to low ambient temperature. Huddling yielded significant energy savings at low temperatures and a roost nest created a microclimate that buffered against low temperatures. Low basal energy requirements, communal roosting and the insulation of a roost nest confer sufficient energetic benefits, allowing babblers to meet energy requirements without resorting to heterothermia, even in their depauperate, low-productivity landscape, suggesting that passerine birds use alternatives to torpor to balance their energy budgets when possible.

A long winter for the Red Queen: rethinking the evolution of seasonal migration

“…Other avian species (albeit from only a handful of lineages) undergo daily torpor (Geiser, ). Many more species have shallower facultative hypothermic responses for energy conservation (McKechnie & Lovegrove, ; Ruf & Geiser, ; Douglas, Cooper & Withers, ) or adjust metabolic rate to survive cold temperatures (Swanson & Garland, ; Swanson & Vézina, ; Stager et al ., ). Beyond migration and adjustments to metabolic rate, avian adaptations to withstand cold temperatures are diverse.…”

Associations between changing climate and body condition over decades in two southern hemisphere passerine birds

“…Nevertheless, differences between species in their response to winter extremes might be explained by differences in behavior: red-winged fairywrens huddle together at night, while scrubwrens apparently roost individually. Huddling at night has been shown to be highly effective in reducing energy expenditure, allowing birds to maintain body mass in cold conditions, which could be particularly important for small-bodied species because they carry limited fat reserves [53,55,56]. For example, compared with solitary roosting individuals, Blackcaps, Sylvia atricapilla (mean body mass 18 g) that huddled at night spent ca.…”

“…Birds use a suite of behaviours and physiological mechanisms to control heat exchange and thus reduce the energy required for thermoregulation. This can include use of micro refugia, postural changes including ptiloerection of feathers, and modification of peripheral blood flow [29,30,48,52,55,56,63,64]. Such strategies are used in both winter and summer and can be effective at maintaining body mass in the face of challenging conditions [65].…”

Body temperature maintenance acclimates in a winter-tenacious songbird

“…Unlike mammals that hibernate, a wide variety of birds remain active in temperate biomes all winter (Swanson, 2010). Some birds make use of heat-conservation mechanisms to cope with these conditions, such as huddling and utilizing microclimatic refugia, or employ facultative heterothermia, thereby decreasing their temperature differential with the environment and reducing energy consumption (Douglas et al, 2017;Korhonen, 1981;Mckechnie and Lovegrove, 2002). In spite of the benefits of these mechanisms, birds still need to eat, and they can frequently be seen foraging on even the most blustery days.…”