Foraging Behavior, Reproductive Success, and Colonial Nesting in Ospreys

Abstract

-We tested several hypotheses about the evolution of colonial nesting in Ospreys. We examined foraging behavior and reproductive success of members of a dense colony in coastal North Carolina to test (1) if the colony functioned as a central place for exchanging information about the location of food patches (Information Center Hypothesis), (2) if the colony was located centrally relative to foraging sites, which would minimize foraging travel costs, (3) if the colony functioned to promote synchronized breeding, which would swamp predators with more young than they could consume, (4) if nesting in the center of a group decreased predation pressure, and (5) if safe nest-site distribution controls actual breeding distribution. The hypothesis that the colony functioned as an information center was supported by synchrony in departure, but contradicted by evidence of fidelity to foraging areas. The colony did not function as the geometric center of an individual's foraging locations. Colony members traveled 1.5-7 times as far as they would have traveled had they nested at their geometric centers. There was a negative correlation between the proportion of eggs lost in the colony and the number of eggs available to predators, suggesting some benefits to synchronous nesting in a close spatial group. However, there was no numerical response in predation relative to the total number of eggs available in the colony. Birds nesting toward the center of the colony produced significantly more fledglings than peripheral nesters, but strong nest-site fidelity prevented shifting to more central locations by peripheral nesters. Spatial distribution of nests reflected the distribution of potential nest trees, both within the lake and within the region. This and the plasticity of nesting density shown in Ospreys support Lack's (1968) Nest-site Hypothesis. We conclude that the spatial distribution of safe nest sites and advantages related to predation maintain coloniality in Ospreys. Received 22 May 1989, accepted 15 January 1990. COLONIAL nesting in birds is a common yet poorly understood phenomenon. A colony may be defined as a group of animals that nest at a centralized location, from which they recurrently depart in search of food (Wittenberger and Hunt 1985). Members of a nesting colony of birds are constrained by the immobility of their clutch or brood. The costs of traveling to and from such a central location are obvious, but the benefits are not. Explanations of colonial breeding fall into three categories (Alexander 1974). Colonial breeding may enhance foraging (Crook 1965, Emlen 1971, Fisher 1954, Lack 1968, Ward and Zahavi 1973), reduce the probability of predation (Burton and Thurston 1959, Kruuk 1964, Horn 1968, Burger 1974, Hoogland and Sherman 1976), or result from resource constraints (e.g. nest sites, food distribution) (Horn 1 Present address: Manomet Bird Observatory, P.O. Box 936, Manomet, Massachusetts 02345 USA. 1968, Lack 1968). The first two explanations postulate that individuals nesting near others derive a higher fitness as a result of interaction. The third explanation postulates no such advantage. For most species it is not clear whether nesting near others is beneficial or is a by-product of a habitat constraint. The Information Center Hypothesis (Ward and Zahavi 1973) relates colonial nesting to foraging success. Individuals may learn about the location of patchily distributed, ephemeral foods from other colony members. Information need not be exchanged actively. Simply following or cuing in on the return direction of an individual known to have been a successful forager qualifies as exchange. Ward and Zahavi's Information Center Hypothesis has become widely cited (ISI 1983), but clear evidence that information transfer induces colonial nesting is lacking. It is a difficult hypothesis to test and involves distinguishing between successful and unsuccessful foragers, and then demonstrating 506 The Auk 107: 506-521. July 1990 This content downloaded from 157.55.39.177 on Sat, 19 Nov 2016 04:23:45 UTC All use subject to http://about.jstor.org/terms July 1990] Colonial Nesting in Ospreys 507 that unsuccessful foragers improve their success by obtaining information on food location from successful birds (Mock et al. 1988). Nevertheless, evidence that information exchange seems to occur in colonies (or roosts) has recently been obtained for three avian species (Brown 1986, Rabenold 1987, Greene 1987). The role of the information exchange in the evolution of colonies remains debatable, however. Information centers could be widespread phenomena, critical to the evolution of coloniality, or they could simply be a secondary adaptation that sometimes occurs as a result of group living (coloniality having evolved for other reasons). To address this question, we examined another colony of a species that has already been shown to exchange information. Demonstration of information exchange at an independent site would imply that it can be a widespread phenomenon, and therefore may be an important factor leading to, or at least maintaining, group living. Its relative importance to colony maintenance should be correlated to its common-