When is a Heronry Crowded: A Case Study of Huckleberry Island, New York, U.S.A.

Abstract

Biologists working with colonial species often record basic data on how herons use space in a nesting colony. Such data are often too complex to analyze succinctly, yet understanding how species pack in colonies is critical to determining whether a colony is full, whether expansion is possible, and whether new recruits to the colony will have to seek space elsewhere. Herein we present data on tree use on Huckleberry Island, New York, hope to stimulate discussion concerning when and how a heronry is packed, and suggest a methodology for evaluating crowding in mixed species colonies. At Huckleberry Island the dominant nesting species are double-crested cormorant (Phalacrocorax auritus), great egret (Egretta alba), snowy egret (E. thulai), and black-crowned night-heron (Nycticorax nycticorax). Of the 331 trees censused, 52% were occupied by at least one nest. All cherry, locust and hickory trees had at least one nest, whereas only 30 % of privet clumps were occupied. Taller trees had more nests than shorter trees, and most trees had 2 to 4 available but unused, sites suitable for supporting nests. Cormorant nests mostly in dead trees whereas the other species usually nested in live trees. Cormorant nests were more clustered than nests of other species. Using the number of unused trees within and adjacent to the heronry, the number of available but unused nest sites, and nest height characteristics, we develop a method of determining the degree of crowding and whether a heronry has room for new pairs. We conclude that the heronry at Huckleberry Island is not crowded at present.