Colony and Nest Site Selection in White-Faced and Glossy Ibises

Abstract

The factors determining colony and nest site selection in the White-faced and Glossy Ibises (Plegadis) were examined from 1972 through 1975. White-faced Ibises were studied in Texas and Argentina, Glossy Ibises in New Jersey and New York. White-faced Ibises generally nest in tule marshes, infrequently on dry land. In selecting a dry land site, White-faced Ibises preferred to nest on the ground among low shrubs and in mixed forbs rather than in habitats containing grass and cactus. Glossy Ibises' habitat varies from Phragmites, Iva, and Smilax to other low shrubs. They did not always nest on the ground. White-faced Ibises nested next to conspecifics, whereas in Glossy Ibises the choice of nearest neighbor was random with respect to the species present. The mean distance to the nearest neighbor was generally less for Glossy Ibis when compared to White-faced Ibis. The space around nests that was devoid of any other nest was examined graphically. Nearest neighbors were farther away when they were on the same level than when the nearest neighbor was above the nest being examined for both species. The visibility index from the nests of Glossy and White-faced Ibises was low, and was less than the mean visibility index for heron and egret species nesting in the same colonies. Visibility was less in the direction of the closest nest compared to that in other directions for all colonies examined. In some colonies, the visibility index in the direction of the closest nest was directly related to the distance to the closest nest. Glossy Ibises nested in denser habitats than did White-faced Ibis, and this was reflected in lower visibility indices for Glossy Ibis. Possible reasons for the recent range changes in the two Plegadis species are discussed.