The effect of male removal on female parental care in the orange-tufted sunbird

The importance of male care and the nature of facultative adjustments in female care were assessed by experimentally removing males in pairs of orange-tufted sunbirds, Nectarinia osea, feeding broods of two or three chicks. The predation of all three-chick broods with female-only care suggested that male nest guarding is critical, especially because males also deter infanticidal floater males. Comparison of provisioning and nest guarding behaviour in male-removed and control nests was therefore limited to two-chick broods. As expected, lone females provisioned at a greater rate, but compensated incompletely for the lack of male provisioning effort. Surprisingly, even when provisioning at high rates, lone females still performed the usual levels of nest guarding and nest sanitation. Lower rates of chick mass gain in the nest, and subsequent premature disappearance of fledglings from male-removed broods, confirmed that reduced provisioning to broods of lone females had detrimental effects upon chick fitness. Sunbirds exhibit sex-specific roles in parental care. Female provisioning appears flexible, ensuring adequate care for young in the nest. Nest guarding is incompatible with high foraging effort, but it may be compatible with male non-parental activities such as territorial defence and extra-pair mating. These results concur with recent data concerning male responses to experimentally reduced female care in this species, and are discussed in terms of evolutionary strategies for stable biparental cooperation.