Circadian rhythms of metabolic rate and body temperature were measured in groups of huddling and solitary red-billed woodhoopoes (Phoeniculus purpureus) as a function of ambient temperature. Basal metabolic rates were 30-60% lower than those of similar-sized birds of other species. As a consequence, circadian amplitudes of Tb and VO2 fluctuation between the rest and activity phases of non-huddling birds were also larger than expected. Circadian rhythms of metabolism save huddling and non-huddling woodhoopoes 19-34% of their potential daily energy expenditure. Birds huddling in groups conserved 12-29% of the daily energy expenditure of non-huddling birds at ambient temperatures of 20°C and below. However, at 30°C huddling birds used c. 12% more energy than non-huddling birds. The various avenues of energy conservation employed by woodhoopoes are suggested to be beneficial in: a) augmenting the proximate benefits of cooperative breeding; and b) optimizing fitness in the drought-prone sub-Saharan savannahs which are heavily influenced by the El Niño Southern Oscillations.
