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, group-size 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.
The authors declare no competing or financial interests.
All authors planned the work and all contributed extensively to writing the paper. The field experiments and analyses were conducted by M.A.C. and A.F.R., with considerable assistance in equipment setup and logistics by W.A.B.
Funding was provided by the Australian Research Council [DP0879261 and LE0775666 to W.A.B., and DP0774080 to A.F.R.], the Royal Society Fellowship Scheme [to A.F.R.] and the Academic Senate, University of California, Riverside [to M.A.C.].
Supplementary information available online at http://jeb.biologists.org/lookup/doi/10.1242/jeb.144972.supplemental
- Received June 21, 2016.
- Accepted August 15, 2016.
- © 2016. Published by The Company of Biologists Ltd