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Abstract

The amount of aid a helper gave a single brood varied significantly according to the helper's age, sex and breeding status. The rate at which non-breeding helpers fed a brood increased as the helpers aged. Male helpers and unmated breeding-age helpers gave aid at higher rates and to a greater number of different breeding pairs than did female helpers and mated helpers.

Breeding pairs which were assisted by six or more helpers during the fledgling period produced more independent offspring per attempt than did pairs assisted by fewer helpers. It is suggested that the helpers reduced the provisioning burden of the breeding pair.

Individual helpers fed nondescendant kin at significantly higher rates than they fed recipients of no known genetic relationship. However, most helpers also aided broods to which they had no known genetic relationship. Their pattern of aid distribution amongst such recipients was neither random nor related to the proximity of the recipients. Naturally ‘widowed’ female breeders paired preferentially with the unrelated, unmated, breeding-age male which had provided the most aid to their previous broods.

The pattern of aid distribution by bell miners was most accurately predicted by the hypotheses that helpers gain indirect fitness benefits by aiding kin (whenever appropriate kin were present, which was predominantly when an individual was young), and direct fitness benefits through aiding breeders as the helpers approached sexual maturity, thereby increasing the likelihood of being accepted to fill a breeding vacancy.