With 1 figure in the text)Analyses of the contents of pellets of Grey herons from different breeding colonies and breeding seasons indicated that the mammal and invertebrate species included in the heron diet varied between colonies and between years. The large differences in mammal species found in neighbouring heronries suggested the existence of important small-scale variability in the exploitation of mammals. Almost no fish and bird remains were found in the pellets. We believe that most invertebrates, even the smallest ones, were actively taken by the herons. Identification of mammal species composition by means of skulls and hair remains gave different results. The shape of a pellet seemed to be dependent upon the (moisture?) conditions during its formation.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology. Summary 1. Foraging data for great crested grebes (Podiceps cristatus L.) feeding in shallow fish-ponds (0 8-1 m deep) were used to test whether pond characteristics, fish density, mean wet weight of the fish or fish species influenced foraging behaviour and success in this fish-eating grebe. 2. When fishing for roach/rudd (Rutilus rutilus L.IScardinius erythropthalmus L.), the number of prey eaten per unit time foraging was positively influenced by the proportion of successful dives, and negatively by the mean diving time. The proportion of successful dives increased as fish weight decreased. Mean diving time, however, was not related to any component of the food supply, nor to pond characteristics. 3. The overall mean diving time was low in comparison with other studies, probably because the study ponds were so shallow. Since the water depth was equal in each pond, mean diving time is not an accurate measure of fish densities in shallow waters. 4. When roach/rudd were the prey, handling time above the water did not constrain the number of prey eaten per unit time foraging at any fish density. With roach/rudd and carp (Cyprinus carpio L.) as prey, the above-water handling time increased with the size of the fish consumed. The total handling time under and above the water surface together constituted on average 4-7% of the time required to consume one fish. 5. When fishing for roach/rudd, successful dives lasted significantly longer than unsuccessful ones. As the mean duration of successful dives increased with fish weight, we suggest that this can be explained by handling prey under water. For carp, however, this could not be shown. Furthermore, unsuccessful dives lasted significantly longer when fishing for carp than for roach/rudd. This is probably related to behavioural differences between the fish species. In roach/rudd ponds, it seemed that grebes took on average the smaller fish.

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Wiley and Nordic Society Oikos are collaborating with JSTOR to digitize, preserve and extend access to Ornis Scandinavica. Vessem, J. van and Draulans, D. 1986. The adaptive significance of colonial breeding in the Grey Heron Ardea cinerea: interand intra-colony variability in breeding success. Ornis Scand. 17: 356-362.We studied variations in reproductive success within and between Belgian heronries to evaluate the hypotheses that colonial breeding in Grey Herons is an anti-predator adaptation or an adaptation to facilitate food-finding. Breeding success of Grey Herons did not increase with number of breeding pairs present, nor with increasing synchronization of breeding, and pairs nesting in the centre of the heronry did not fledge more chicks than pairs nesting at the edge. We found no consistent differences in the timing of onset of reproduction and in clutch size between and within heronries. There was a suggestion that first-year breeding birds tended to occupy nests at the edge of a heronry, but their proportion in the breeding population probably was too small to affect the results. We suggest that the predation pressure is too low to cause anti-predator responses, such as mobbing, to evolve among herons. There was no evidence for a higher chick survival as a consequence of increasing information transfer through increased number of breeding pairs. The data suggested that new heronries establish near suitable foraging sites, but that overall reproduction success decreases when more pairs settle.

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