Nest switching and alloparental care in colonial white storks

White stork, Ciconia ciconia, chicks were observed to abandon their natal nests prior to independence and to be adopted by neighbouring foster parents in approximately 40% of broods at three breeding colonies. Nest switching coincided with a decrease in feeding rates by parents and an increase in aggression by siblings triggered by the flight exercises of nestmates, and mainly affected the eldest chicks in larger broods. Chicks that abandoned their natal nests moved into broods containing both fewer and younger chicks, and thus experienced a decrease in aggression rate compared with their natal broods. Adopted chicks may also have increased their food intake as a consequence of nest switching. Resident chicks suffered from lower feeding rates during adoption and had lower mass increments than chicks in non-fostering broods of comparable age and size. Both adults and chicks were highly aggressive against trespassing chicks. The probability of aggression was a good predictor of the extent of fostering across different brood sizes, suggesting that defence by residents was effective at preventing adoption. Residents appeared not to recognize kin but were capable of aggressive discrimination against non-familiar chicks. Fostering behaviour in this species can be understood as the equilibrial outcome of an intraspecific ‘co-evolutionary’ arms race between kleptoparasitic chicks and fostering host adults with weak selection pressures for both parties. Alloparental care (i.e. care of young by a conspecific other than its genetic parent) is widespread in social birds and mammals and includes a range of behaviour from brood amalgamation in precocial birds to babysitting in primates (Riedman 1982; Packer et al. 1992). This phenomenon has attracted much attention because its occurrence apparently violates the Darwinian principle by which animals are selected not to expend resources in the propagation of competing genotypes (Holley 1981; Riedman 1982; Carter & Spear 1986). One interesting form of alloparental care is nest switching by young birds which actively abandon their natal nests to seek temporary or full adoption by foster parents. Nest switching has been mainly documented in semi-precocial species of colonial seabirds such as gulls and terns, but evidence is now accumulating that it also occurs in fledglings of altricial species such as raptors, egrets and herons (see references below). Chicks seem to benefit from switching to nests where they receive better care (e.g. food) than at their natal nests (Hebert 1988). It is less clear, however, why foster parents should tolerate the presence of an unrelated chick in their brood. Some instances of alloparental care may be the result of reproductive errors or maladaptation arising from human interference (Plissner & Gowaty 1988; Bustamante & Hiraldo 1990). This explanation, however, fails to account for the high frequency of fostering observed in some species and also for the active role played by young seeking adoption (Holley 1981; Pierotti 1988), especially when they risk being attacked and killed by unrelated adults (Pierotti 1988; Morris et al. 1991). At least in some species (e.g. fully precocial birds), adoption may entail negligible costs, exerting a neutral effect on adult reproductive success (Patterson et al. 1982; Eadie et al. 1988; Bustnes & Erikstad 1990). Alternatively, even if adoption were costly to foster parents, they could obtain some compensating direct benefits, such as gaining reproductive experience or exploiting foster young (e.g. by diluting the risk of predation; Riedman 1982; Birkhead & Nettleship 1984; Eadie et al. 1988). In addition, alloparents may obtain indirect benefits through kin selection (if adopted young are close 0003–3472/95/041097+14 $08.00/0 ? 1995 The Association for the Study of Animal Behaviour