The energetic significance of communal roosting and insulated roost nests in a small arid-zone passerine

Abstract

Small endotherms have evolved behavioural mechanisms for reducing rest-phase energy expenditure, which reduce the likelihood of mismatches between energy supply and demand during periods of cold weather and/or food scarcity. Although the energetic consequences of communal roosting and the use of insulated roosts have been the subject of numerous studies, less is known about the energy savings achieved by species that use these two behaviours simultaneously. We hypothesised that communal roosting in insulated roost nests by a small arid-zone passerine, the Scaly-feathered Finch Sporopipes squamifrons, results in additive energetic benefits that reduce nocturnal energy requirements far below those of individual birds roosting in the open. We measured metabolic rates in finches over air temperatures (T_a) between −5 and 20 °C using flow-through respirometry. Measurements were taken from single finches and groups varying in size from two to 12 individuals, with or without a roost nest. Consistent with our predictions, rest-phase resting metabolic rate (RMR) of finches decreased when the birds roosted communally and decreased further when groups were roosting in a nest. In the absence of a nest, groups of eight or 12 birds reduced RMR by >30% compared with single birds. These energy savings increased further when groups roosted in nests; at T_a = 0 °C, groups of eight or 12 finches approximately halved their RMR compared with that of groups without nests. Our data confirm that Scaly-feathered Finches save considerable energy by roosting communally in roost nests, and these behaviours likely are a key reason why this small species from subtropical latitudes can occur in areas with winter night-time temperatures as low as −10 °C.

Keywords: energetics, huddling, nest, Scaly-feathered Finch, Sporopipes squamifrons, thermoregulation