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Abstract

Social units of the cooperatively breeding superb fairy-wren Malurus cyaneus consist of either a simple male-female pair or a pair plus one or more additional birds, usually males. Early work on this species led to the conclusion that females were less numerous than males in the population and that this sex ratio imbalance prevented some males from dispersing and breeding. Such males then obtained indirect fitness benefits by helping to rear close relatives. Recent studies have cast doubt on these interpretations. An alternative perspective of cooperative breeding suggests that natal philopatry does not necessarily reflect a response to a constraint on independent breeding; rather, under certain ecological conditions it is an optimal strategy. This paper describes the results of experimental manipulations designed to test between the constraints and benefits views of natal philopatry in M. cyaneus. (1) Females were removed from territories containing two or more males. All were soon replaced, indicating that females available for breeding were present in the vicinity of multimale territories. (2) Suitable critical habitat (piles of thorny brush) were placed near territories containing both male and female nonbreeding helpers. None was occupied. This, together with several lines of observational evidence, suggests that adequate breeding habitat was available. (3) All helpers (1—4) were removed from 14 experimental territories and reproductive success, persistence and survival on these territories were compared with 20 control territories originally holding only a simple pair. Significantly more young were produced on the experimental territories and significantly more control territories were vacated during the breeding season. The greater production of young and persistence of breeders on the experimental territories could not be attributed to helpers, because they had been removed, and probably was due to greater predation in control territories. These data support the idea that presence of helpers, which indicates breeding success in previous years, and current production of young both reflect territory quality. Specifically, our results suggest that in the population of M. cyaneus that we studied, some territories provide more security than others, and that this may in large part account for the natal philopatry of males.