Energetics of communal roosting in chestnut-crowned babblers: implications for group dynamics and breeding phenology

Abstract

For many endotherms, communal roosting saves energy in cold conditions, but how this might affect social dynamics or breeding phenology is not well understood. Using chestnut-crowned babblers (Pomatostomus ruficeps), we studied the effects of nest use and group size on roosting energy costs. These 50 g cooperatively breeding passerine birds of outback Australia breed from late winter to early summer and roost in huddles of up to 20 in single-chambered nests. We measured babbler metabolism at three ecologically relevant temperatures: 5°C (similar to minimum nighttime temperatures during early breeding), 15°C (similar to nighttime temperatures during late breeding) and 28°C (thermal neutrality). Nest use alone had modest effects: even for solitary babblers at 5°C, it reduced nighttime energy expenditures by <15%. However, groupsize effects were substantial, with savings of up to 60% in large groups at low temperatures. Babblers roosting in groups of seven or more at 5°C, and five or more at 15°C, did not need to elevate metabolic rates above basal levels. Furthermore, even at 28°C (thermoneutral for solitary babblers), individuals in groups of four or more had 15% lower basal metabolic rate than single birds, hinting that roosting in small groups is stressful. We suggest that the substantial energy savings of communal roosting at low temperatures help explain why early breeding is initiated in large groups and why breeding females, which roost alone and consequently expend 120% more energy overnight than other group members, suffer relatively higher mortality than communally roosting group mates.

Avian torpor or alternative thermoregulatory strategies for overwintering?

“…The reduction in energy expenditure of huddling babblers, with a huddling MR 65-74% of individual MR at an equivalent T a , is as expected for other huddling endotherms (see Gilbert et al, 2010, for review) and similar to that measured for two to three huddling chestnut-crowned babblers, and is likely to be even greater for more huddling individuals (Chappell et al, 2016). Presumably, the substantial reduction in MR of huddling babblers has a significant role in balancing their energy budget and partially negates the need for heterothermia.…”

“…7). Our value of BMR for solitary-roosting white-browed babblers (M b 46 g) of 1.24 ml O 2 g −1 h −1 was similar to, but lower than, the 1.51 ml O 2 g −1 h −1 measured by Chappell et al (2016) for the chestnut-crowned babbler (Pomatostomus ruficeps; M b 50 g), despite our birds having a less arid habitat and slightly lower M b , suggesting no stress response for solitary-roosting white-browed babblers. Whitebrowed babblers had a BMR that was only 64.0% of that Table S6).…”

“…White-browed babblers always roosted communally in the field during the study, but even when held individually in the laboratory at T a as low at 10°C, all babblers maintained T b within 1.15°C of thermoneutral values. Therefore, babblers are facultative social thermoregulators and although they can survive roosting individually overnight in the wild (Chappell et al, 2016), they gain substantial energetic benefits by huddling.…”

“…Extrapolating the linear relationship between T a and VȮ 2 below thermoneutrality for solitary birds outside roost nests to these T a , huddling babblers in roost nests would have an energy expenditure only 55-65% of that of single, exposed birds under the same environmental conditions, and this would be even lower for larger groups (e.g. Chappell et al, 2016). In other words, small groups of communal roost-nesting babblers at a T a of −3 to +15°C and a T roost of 6.8 to 15.4°C would have a MR equivalent to that of a single, exposed bird at a T a of 17.6 to 24.3°C.…”

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.…”