11Department of Biology, Faculty of Science, Kyushu University, Fukuoka 812–8581, Japan
22Department of Zoology, Faculty of Science, Kyoto University, Kyoto 605–8502, Japan
3*Present address: Yamashina Institute for Ornithology, Abiko, Chiba 270-1145, Japan
*Corresponding author, E-mail: kegucscb@kyushu-u.org
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The Rufous Vanga breeds cooperatively, with pairs accompanied by one or more helper males. Although the contribution of helpers may be considerable, helping does not enhance reproductive success. In this study, we investigated individual differences in helping behavior and discuss the adaptive significance of helping. Contributions by helpers were very low during the nest building and incubation stages, but were greater during the nestling stage. In the first half of the nestling stage, both one-year-old and older helpers contributed less than breeders, with one-year-old helpers feeding nestlings less frequently than older helpers. However, the contribution of helpers increased during the second half of the nestling stage, while that of breeding females decreased. Both provisioning frequency and the size of food items carried by one-year-old helpers reached the same level of older helpers during the second half of the nestling stage. One-year-old sons of breeding females contributed more than unrelated helpers, however relatedness did not influence significantly the contribution of older helpers. Contribution to provisioning by one-year-old helpers did not influence their fate in subsequent years. Our data did not reveal any direct fitness benefits gained by helping at the nest for one-year-old helpers, but suggested that they helped their mothers in order to gain indirect fitness benefits through lightening their mothers labor. Older helpers may have gained direct fitness benefits through advertising to an unrelated breeding female or by feeding their extra-pair fertilized off-spring.
