In White-backed Mousebirds ('Colius colius'), clustering is an important component of rest- phase thermoregulation, particularly at low air temperature. We tested predictions concerning thermoregulation under natural conditions by recording rest-phase body temperature in a flock of free-ranging White-backed Mousebirds in an arid habitat during winter in the Karoo, South Africa. We found that the mouse- birds' restphase body temperature was relatively labile, but was generally maintained above 32°C, despite air temperatures as low as -3.4°C. Patterns of rest-phase body temperature were highly synchronized among individuals within the cluster. The mousebirds showed no evidence of torpor under natural conditions; a shallow departure from normothermy, during which body temperature was reduced to 29-33°C, was observed on only one occasion. The observed patterns of thermoregulation supported our predictions, and confirm that thermoregulation in clustering White-backed Mousebirds in the wild is significantly different