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Individual Contributions To Cooperative Behaviour in the Acorn Woodpecker: Effects of Reproductive Status, Sex, and Group Size

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Acorn woodpeckers (Melanerpes formicivorus) are communal breeders that live in permanently territorial family groups of 2-15 individuals. All group members participate in several forms of cooperative behaviour, which we define as the investment of resources in a common interest shared by other group members (CHASE, 1980). Acorn woodpecker group interests include the caching of acorns in a specialized storage tree (the granary), maintenance of the granary, and defence of the granary against intra- and interspecific intruders. Because of a strong positive relationship between the availability of stored acorns and group reproductive success, investments in acorn storage, granary maintenance, and granary defence have a direct positive effect on the individual fitness of breeders. Non-breeding helpers, however, do not realize a direct reproductive reward, and therefore profit less directly from investment in these forms of cooperative behaviour. Thus, we predicted that breeders should invest more heavily in cooperative behaviour than should helpers. The prediction was upheld: for nearly every form of cooperative behaviour examined, breeders contribute significantly more than do helpers. On the other hand, while established breeders only rarely intrude on neighbouring territories, nonreproductive helpers frequently do so. These dispersal "forays" constitute attempts by helpers to locate ecologically restricted breeding vacancies. The effect that group size has on individual contributions to cooperative behaviour is unclear. Individual rates of acorn storage decrease significantly with increasing group size, but granary attendance and granary maintenance do not. Male and female acorn woodpeckers do not differ in their contributions to acorn storage, granary maintenance, or interspecific defence. Females, however, play a significantly greater role in intraspecific defence because of a female-biased intruder sex ratio and sex-specific defensive behaviour.

Affiliations: 1: (Hastings Reservation and Museum of Vertebrate Zoology, University of California, Berkeley, California 94720, U.S.A.)


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