Thermoregulatory patterns, the use of rest-phase heterothermy, the energetic significance of clustering behaviour, and the role of sunning behaviour in thermoregulation were assessed in the white-backed mousebird Colius colius, an southern African arid zone species. I hypothesised that C. colius makes significant rest-phase energy savings by means of clustering behaviour, and has the ability to reduce diurnal energy costs by utilising solar radiation. I tested these hypotheses using indirect calorimetry and surgically implanted temperature-sensitive telemeters, and by simulating solar radiation in a constant-environment chamber. Rest-phase body temperature (T[b]s) was highly labile, with rest-phase T[b]s of down to 26 °C being recorded. During the rest-phase, T[b] was not regulated with respect to a setpoint temperature, as typically occurs in endotherms. Rather, rest-phase T[b] patterns were characterised by periods of linear decreases (passive cooling) at a rate dependent on ambient temperature and the number of individuals in a group. I did not observe any instances of torpor, as described in the literature. When T[b] depression did occur, it appeared to be the result of passive heat loss, and not the metabolic down-regulation which typically precedes a torpor bout. These findings, together with the phylogenetic position of the Coliidae, raise questions regarding the evolution of torpor. The inability of individual mousebirds to maintain a rest-phase setpoint T[b] suggests that in C. colius the physiological mechanism responsible for the defence of a rest-phase setpoint T[b] is replaced by a behavioural mechanism, namely clustering behaviour. The birds in this study showed a basal metabolic rate 40% below the predicted allometric values, and a circadian amplitude of body temperature (T[b]) of 5.2°C, 195% of the predicted value. The use of linear independent contrasts revealed that these deviations from the expected allometric values are not due to the phylogenetic relationships of the Coliiformes, and hence are likely to represent adaptation. These conservative metabolic traits offer opportunities for significant energy savings and are presumed to be adaptive in the unpredictable habitat of this species. The birds were able to make significant energy savings by means of huddling behaviour. At an ambient temperature of 15°C, the energy expenditure of birds in a group of six was 50% of that of single birds. The energy savings the birds were able to make were dependent on ambient temperature and the number of birds in the group. The ability to lower energy requirements by means of huddling behaviour is likely to be highly adaptive when dealing with low nocturnal temperatures in an environment where food supplies are spatially and temporally unpredictable. When allowed access to simulated solar radiation, individual mousebirds showed 15 - 30% reductions in their energy expenditure, while maintaining a constant body temperature. These reductions in energy expenditure hence represent thermoregulatory savings. My results support the hypothesis that solar radiation may be important in the energy budgets of some bird species. The ability to reduce food requirements by means of absorbing solar radiation is likely to be strongly selected for in the arid areas inhabited by C. colius, which are characterised by large circadian fluctuations in ambient temperature, in addition to resource unpredictability. Thermoregulation in C. colius appears to involve an interaction of behavioural patterns and physiological traits. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1998.