Effect of group size on field metabolic rate of Arabian Babblers provisioning nestlings

Arabian Babblers (Turdoides squamiceps; adult body mass 65-75 g) are territ orial, cooperatively breeding passerines that inhabit hot, dry deserts. Gro ups include breeding adults and helpers and generally consist of 3 to 5 ind ividuals (range 2 to 22). All group members provision nestlings at similar rates, and individual visitation rates decline with increasing group size. Consequently, we predicted that the field metabolic rate (FMR) of individua ls provisioning nestlings would decrease with increasing group size. To tes t this prediction, we determined FMR of primary female, primary male, femal e helper and male helper babblers in different sized groups provisioning ne stlings. Field metabolic rate of primary females, but not other classes, de creased linearly with group size. This energy savings could allow primary f emales in larger groups to start a new nest more quickly. FMR for all babbl ers was 61% to 66% of the value predicted for a passerine of its body mass provisioning nestlings and was 3.11 x BMR, similar to the mean value of 3.1 3 x BMR reported for a number of terrestrial species.