Sex differences in parental response to begging nestlings in superb fairy-wrens

Descriptive studies of provisioning in cooperatively breeding superb fairy-wrens, Malurus cyaneus, show that females are responsive to the needs of the brood, increasing nest visits with brood size and age. Both dominant and helper males provision at a constant rate regardless of brood size and age, but dominant males reduce provisioning as the number of helpers increases. This reduction could reflect load lightening or reduced relatedness, as dominants are more likely to be cuckolded by extragroup males when they have helpers. To distinguish between these alternatives we conducted playback experiments that augmented the natural begging of 6-day-old nestlings with the calls of 4-day-old nestlings (control) or the much louder calls of 10-day-old nestlings (experimental treatment). We presented playbacks to the female alone, all birds, or males alone. We predicted that females would always respond to increased begging, helper males would not respond, and dominant males would always respond (load-lightening hypothesis) or respond when they had paternity in the brood (relatedness hypothesis). Contrary to these predictions, males always responded and females did not. Females may require additional stimulation or alternative cues in order to respond. Dominant males spend less time at the nest, and may judge nestling need on the basis of simpler cues. They may always feed as little as possible to minimize their commitment to nestlings to which they are unlikely to be related. However, they may increase their response when they lack helpers, or females are unable to meet the demands of the brood.