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Breeder-Helper-Interactions in the Pied Kingfisher Reflect the Costs and Benefits of Cooperative Breeding

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In most social species there is not only cooperation but also conflict between group members. Although various theoretical models have specified the conditions for, and the extent and direction of conflict and cooperation, there are few empirical data to test their predictions. This paper reports such a test for the pied kingfisher (Ceryle rudis), a cooperatively breeding bird species with two types of male helpers: primary (= related) and secondary (= unrelated). In a breeding colony at Lake Victoria (Kenya), the birds were studied with regard to: (a) aggressive and non-aggressive interactions between breeders and helpers before and after chicks hatched; (b) frequency and effect of prey transfer from helpers to breeders; and (c) sizes and types of prey brought to females and nestlings. The six major results of the study and their explanations are as follows: Result 1. Male breeders attack secondary male helpers more often than primary helpers (Table 1). Explanation: Because of a high male surplus, all helpers are also potential rivals, competing with male breeders for sexual access to the scarce females. Secondary helpers, however, impose higher costs and lower benefits on the male breeders' fitness than do primary helpers. This is because they provide less help, seem to be more capable of fertilizing eggs in the year of help, are more likely to displace breeders in subsequent years, and are more distantly related to breeders than are primary helpers. Result 2. Primary helpers treat secondary helpers in the same way as male breeders do (Table 1). Explanation: Primary helpers are closely related to the young they raise. Therefore, any competition from secondary helpers that affects the breeder will decrease the primary helpers inclusive fitness. Result 3. Female breeders tolerate secondary helpers more readily than do male breeders and primary helpers (Table 1). Explanation: Females do not incur the costs of male-male competition; indeed they even benefit from it, because (a) with additional males (= secondary helpers) they get a better food supply during egg formation than without them, and (b) secondary helpers lower the females' food contributions to nestlings more than those of male breeders and primary helpers. Result 4. By offering fish, secondary helpers reduce the probability that they will be attacked by the breeding pair and any primary helpers (Table I). Explanation: The helpers' food transfer is interpreted as a "payment" for being accepted as a group member. Receiving fish improves the breeders' and primary helpers' energy-budgets and their chances of fledging young and thus reduces the cost/benefit ratio of tolerating helpers. For females this holds already during egg formation, for males only after hatching, when the risk of kleptogamy is low, and when helpers can improve survival of the young. Result 5. After the young have hatched, primary helpers carry more food to nestlings (= usually their sibs) than to females, whereas secondary helpers carry more to females than to unrelated nestlings (Table 2 and Fig. 1). Explanation: Primary helpers increase their inclusive fitness mainly through raising close kin, secondary helpers mainly through improving their chances of finding a mate and reproducing themselves. Result 6. In groups with two or three secondary helpers, each helper tends to provide the nestlings with more food than in groups with only one helper (Fig. 2). Explanation: Groups with two or three secondary helpers differ from groups with only one helper (a) in the breeders' average gain from each helper, and (b) in the extent of male-male competition for females as prospective mates. The helpers' higher food contribution to nestlings in bigger groups is interpreted (a) as a higher payment for being tolerated, and (b) as increased effort to signal their parental qualities to the females. It is concluded that all behavioral interactions and food contributions closely reflect the costs and benefits of giving and receiving help, which vary with the sex of the breeder, the relatedness between the group members, and the period of the reproductive cycle.

Affiliations: 1: Max-Planck-Institut für Verhaltensphysiologie, D-8131 Seewiesen, West Germany


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