Movements of radio‐tagged Grey Herons Ardea cinerea during the breeding season in a large pond area

This paper describes the strategies of resource utilization in the course of the breeding season by five radio‐tagged Grey Herons Ardea cinerea . The seasonal changes in exploitation of the environment by two breeding adults, one non‐breeding adult and two non‐breeding first‐year birds were studied from March to August 1982, near Zonhoven in Belgium. Two adult breeding birds could be followed continuously from the end of March until the middle of June. During the first month they explored an extended area all around the colony, but each concentrated its search in a specific direction. From the end of April until the beginning of June, most probably from egg‐hatching until the end of breeding activities, each bird spent a very large proportion of its time at a particular feeding site, from which other herons were actively excluded. In the first part of June they again visited different sites, each maintaining its preferred direction. From the middle of June onwards they seemed to have left the fish‐pond area. The pattern of movements of the first‐year birds differed markedly from that of the breeding adults. In April, although both non‐breeding and breeding birds explored large areas, only the areas used by non‐breeders were centred on the colony. From the end of April onwards, probably after general egg‐hatching in the colony, the non‐breeders very rarely revisited the colony, and from May till August their ranges became more and more restricted to very small areas at an increasing distance from the colony. They were never observed defending particular sites. The results are discussed with regard to recent speculations about the evolution of colonies as an adaptation for the exploitation of food resources. Breeding herons seem to explore a large part of the environment during incubation and defend a particular site while feeding their young. Choice of feeding site by non‐breeding birds may be influenced by the site defence of the breeding birds. Non‐breeding birds exploit a large area when breeding birds occupy feeding territories. Perhaps they are forced to forage in less suitable places at this time. Colonies might have evolved as a strategy to minimize effort in resource esploitation as, especially at the beginning of the breeding season, the colony could act as an assembly point in the exploration of the environment. However, its importance as an assembly point diminishes in the course of the season, as non‐breeding birds no longer visit the colony and adults defend territories.