Avian torpor or alternative thermoregulatory strategies for overwintering?

• Published in: Journal of experimental biology Vol. 220; no. Pt 7; pp. 1341 - 1349
• Main Authors: Douglas, Tegan K, Cooper, Christine E, Withers, Philip C
• Format: Journal Article
• Language: English
• Published: England The Company of Biologists Ltd 01.04.2017
• Subjects: Alternative energy sources; Ambient temperature; Animals; Basal Metabolism; Birds; Body Temperature; Body Temperature Regulation; Breeding; Communal breeding; Conductance; Energy budget; Energy conservation; Energy requirements; Huddling; Insulation; Low temperature; Metabolic rate; Microclimate; Overwintering; Passeriformes - physiology; Resistance; Seasons; Temperature effects; Thermal conductivity; Torpor; Water loss; Winter; Metabolic rate; Temperature telemetry; Evaporative water loss; Body temperature; Respirometry; Heterothermia
• ISSN: 0022-0949; 1477-9145
• DOI: 10.1242/jeb.154633

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 ( ), 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.