Resource Partitioning: Nest Site Selection in Mixed Species Colonies of Herons, Egrets and Ibises

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Abstract
Factors determining nest site selection in ardeids were examined in 15 heronries of 11 spp. [Ardea herodias, Casmerodius albus, Ajaia ajaia, Florida caerula, Bubulcus ibis, Hydranassa tricolor, Nyctanassa violacea, Egretta thula, Nycticorax nycticorax, Plegadis chihi and P. falcinellus] in Argentina, Mexico and Texas, New Jersey and New York [USA]. The nesting pattern varied as a function of physiognomy. Heronries can be classified with respect to vegetation structure into homogeneous and heterogeneous types. Homogeneous heronries, usually containing a pure stand of 1 plant species, looked similar to the observer from area to area. Heterogeneous heronries contained dissimilar subareas as a result of differences in plant species, height, structure and open areas. In heterogeneous colonies, ardeid species selected nest sites as a function of these physiognomic variations. In homogeneous heronries, the species vertically stratify their mean nest heights, the larger nesting higher. The cattle egret is the only exception as its mean nest height is higher than predicted by its length. Some areas of heterogeneous heronries are in themselves homogeneous, and within such areas the species stratify themselves vertically with respect to body length. Mean nest height for a given species varies, among other things, as a function of: vegetation height, number of species present, the size of species and the size of other species present. Overlap in nesting heights correlated with the number of species present in the heronry and not the total vertical nesting space. The species of nearest neighbor seems to be a function of vegetation preferences in heterogeneous heronries, but in homogeneous heronries nearest neighbors tend to be conspecifics or species close in size to the species in question. Among heronries, nearest neighbor distances are not constant for a given species but are positively correlated with nest height and negatively correlated with colony size. Within a heronry, nearest neighbor distance is directly related to body size. Within a species, nearest neighbor distance is directly related to the size of the neighbor. The relationship between nearest neighbor distance when it is a conspecific vs. when it is not depends upon the size of the species in question and the sizes of its nearest neighbors. A species having all smaller-sized nearest neighbors has a larger conspecific than interspecific distance. A species having all larger-bodied neighbors has a smaller conspecific than interspecific distance. The empty space around nests is directly related to the size of the bird independently of plant shape; it is greater above the nest than below the nest, and is greater horizontally than below the nest. Visibility is less in the direction of the closest nest for most species. For some species, visibility in the direction of the closest neighbor is positively correlated with internest distance. Visibility is directly related to nest height. The visibility index is not constant for a given species between colonies. Even when a species mean visibility index and mean nearest neighbor distance were computed, they were correlated and still formed a regression line.