Determinants of great blue heron colony distribution in coastal Maine

-The distribution of nesting Great Blue Herons (Ardea herodias) in coastal Maine was studied by examining the relationship between colony size and availability of food supplies near colonies, the selection of nesting habitat, and the spacing of colonies. Nineteen colonies (size range 4-252 nests) were located, all on marine islands. The number of nests in a colony was correlated positively (r = 0.82) with the area of tidal and inland wetlands within a 20-km radius of a colony. Nest and colony characteristics were highly variable, and suitable nesting habitat did not seem to limit colony size or distribution. The degree of forestation, presence of hardwoods, and distance of an island from towns and other islands with colonies were apparently important factors in selection of nesting islands. Colonies were uniformly distributed along the coast at intervals of about 16 km. We present a model for the observed dispersion of heron nests based on the conclusion that food competition between members of the same colony probably limits the size of colonies, whereas food competition between members of adjacent colonies may determine colony distribution. Received 7 February 1986, accepted 16 June 1986. THE value of nesting in colonies has been examined by a number of workers (Lack 1968, Ward and Zahavi 1973, Hoogland and Sherman 1976, Krebs 1978, Burger 1981, Gotmark and Andersson 1984), but less is understood of what determines where a group of birds will form a colony or how large that group will be. Determinants of colony location tend to complement factors that contribute to colonial nesting behavior. Two examples are increased protection from predators through the selection of inaccessible nesting grounds (e.g. Cullen 1957, Burger and Lesser 1978, Nelson 1978) and enhanced utilization of resources by nesting close to food resources (Ward and Zahavi 1973; Kushlan 1976a, 1978). Determinants of the number of individuals in a colony are less well understood. Among other factors, competition for sites may limit colony size, i.e. the number of individuals breeding in a colony, in some groups, e.g. some sulids (Nelson 1966), while food availability may limit colony size in ardeids (Werschkul et al. 1977, McCrimmon 1978, Beaver et al. 1980), phalacrocoracids, alcids, sulids, and larids (Fur3Present address: Division of Biological Sciences, 105 Tucker Hall, University of Missouri-Columbia, Columbia, Missouri 65211 USA. ness and Birkhead 1984). Measuring a relationship between food availability and colony size is difficult because food resources used by colonial birds are typically patchy and temporary and are spread over vast areas around colonies. Our purpose was to examine possible determinants of the distribution of nesting Great Blue Herons (Ardea herodias) in the coastal region of Maine. Nesting Great Blue Herons are large and conspicuous, and thus colony locations and sizes can be readily determined over a large area. Great Blue Herons also feed primarily in local, relatively permanent, wetland habitats. Relationships between colony size and an index of food resource availability, i.e. wetland availability, can be determined easily. We describe the pattern of nest dispersion for Great Blue Herons in the region. We then examine the relationship between the number of herons breeding in a colony and availability of foraging habitat; the selection of nesting habitat by herons by comparing the characteristics of nesting areas with those of similar, but unused, areas; and the spacing of colonies.