Mousebirds (Coliiformes) exhibit well-developed communal roosting behaviour as well as a pronounced capacity for facultative hypothermic responses. We recorded body temperature (T_b) in speckled mousebirds (Colius striatus) under semi-natural conditions in outdoor aviaries, and examined interactions between behavioural and metabolic thermoregulation by experimentally manipulating food availability and communal roosting behaviour. When food was available ad libitum, mousebirds roosting in a cluster maintained approximately constant rest-phase T_b, with 32°C < T_b < 42°C. By contrast, rest-phase T_b in single mousebirds decreased at 0.5°C/hr and minimum rest-phase T_b was significantly lower than when clustering. When food availability was restricted, the mousebirds exhibited facultative hypothermic responses that were less pronounced in clustering groups (minimum rest-phase T_b = 33.3°C, circadian amplitude of T_b = 9.5°C) compared to single birds (minimum rest-phase T_b = 30.7°C, circadian amplitude of T_b = 11.8°C). When clustering, rest-phase T_b was highly synchronized among individuals. Our data reveal that communal roosting has profound consequences for rest-phase thermoregulation in C. striatus, and provide further insights into the potential role of physiological constraints in the evolution of avian sociality.