Male parental care in the orange-tufted sunbird: behavioural adjustments in provisioning and nest guarding effort

Monogamy is thought to be prevalent in bird species because of the role of male parental care. Evolutionarily stable strategies for biparental cooperation provide a behavioural mechanism for the maintenance of monogamous relationships, under which any shortfall in one parent's work rate is predicted to be partially compensated for by its partner. Both parents should invest more effort when there are more young. These predictions were tested simultaneously using tail-weights to reduce the visit rates of female members of pairs of orange-tufted sunbirds, Nectarinia osea, feeding broods of two or three chicks. Prior to the manipulation and in unmanipulated control pairs, females provisioned at a greater rate, removed proportionally more faecal sacs and spent longer at the nest, whilst males showed greater nest guarding effort. As predicted by game theory, males with tail-weighted females compensated incompletely for their partners' lower provisioning rate. Males failed to nest guard when provisioning at high rates, implying an important trade-off for biparental cooperation. Larger broods were fed at a greater rate by both parents. Visit rates per chick decreased in experimental nests and in larger broods, and this was reflected in lower chick growth rates. Greater chick growth rates per visit in control nests and larger broods suggest that they received superior load sizes and/or quality of prey. These results concur with previous studies of parental responses to partner effort, and further reveal the behavioural flexibility in provisioning and nest guarding strategies in a species with predominantly female care.