Nest Site Characteristics among Five Species of Herons on the North Carolina Coast

A study of nest site characteristics of five species of herons and egrets on a North Carolina coastal heronry using principal component analysis revealed four components describing sampled nest sites: vegetation structure, accessibility, protection, and shrub/tree-center distance.Comparison of nest sites on the basis of component scores revealed that nest sites of Great Egrets were significantly different from those of the other species.Comparison of mean vectors revealed nest site differences among most of the smaller species.The study suggests that the smaller species may be reducing competition for nest sites through differential use of environmental variables associated with the site.Confirmation of the biological significance of these differences will require additional studies.Great Egrets began nesting the earliest, starting in late March.Cattle Egrets began nesting somewhat later than most other species and were more temporally dispersed in nesting, continuing into June.Horizontal partitioning of nest sites occurred, Great Egrets, Snowy Egrets and Cattle Egrets nesting in significantly different proportions in different areas of the heronry.Great Egrets nested significantly closer to other Great Egrets than to individuals of other species.The smaller herons generally nested closer to contraspecifics than to conspecifics.There was no tendency for the smaller species to nest in exclusively conspecific groups.Conspecific nesting in Great Egrets is most likely related to the unique nest site of the species.Received 16 July 1976, accepted 17 January 1977.WHERE herons and egrets are abundant, species commonly nest in colonies containing hundreds to thousands of pairs in relatively limited areas and in high density.The nest sites of many species appear similar and species may compete interspecifically for the available resources.Differential division of the nest site resources may reduce competition among species.Several studies have suggested this (Meanley 1955, Ralph and Ralph 1958, Lowe-McConnell 1967, Dusi 1968), but most rely on qualitative assessments of nest site differences.Jenni (1969) reported more quantitative information on vertical stratification and some horizontal zonation, as well as partitioning of food resources, among several species nesting in a central Florida heronry.Burger (in press) has noted vertical nest site stratification in colonies characterized by vegetation of uniform appearance.In one such colony in New Jersey, she suggests that nest height and inter-nest distances may be predicted from aggressive interactions among species.This study attempted to measure various nest site characteristics to determine the extent of resource division among Great Egrets (Casmerodius albus), Snowy Egrets (Egretta thula), Cattle Egrets (Bubulcus ibis), Little Blue Herons (Florida caerulea), and Louisiana Herons (Hydranassa tricolor) nesting in a colony on the mid-coast of North Carolina during 1974.I described each nest site by physical variables judged potentially important to birds selecting nesting sites.I applied several multivariate statistical techniques to characterize the major independent groups of habitat variables associated with nest sites and to assess the separation of species' nest site