Increased sociality in herons, family Ardeidae, is often considered to be associated with the occurrence of white plumage. If white plumage increases sociality, we hypothesized that white-plumaged birds should forage closer to one another and dark birds should remain more solitary. We tested this hypothesis by investigating the inter-individual spacing (nearest neighbor distance) in two pairs of closely related species that differ in plumage coloration: (1) Great Blue Heron (Ardea herodias)/Great Egret (A. alba) and (2) Little Blue Heron (Egretta caerulea)/Snowy Egret (E. thula). All birds observed were in adult plumage. We recorded distances to nearest conspecific, nearest congeneric, nearest white (regardless of species) and nearest dark bird to each focal bird within a given (focal) pond. Because species occurred in varying numbers, we compared the observed mean distances with randomly generated distances based on the total number of birds in the pond. All observations were conducted under steady water levels, avoiding drying ponds as this might concentrate prey. We found that Snowy Egrets were significantly farther from conspecifics and dark congenerics (Little Blue Herons) than expected under a null model of random spacing within the focal pond. Little Blue Herons also were spaced farther apart from white congenerics (Snowy Egrets) than expected. Great Blue Herons and Great Egrets exhibited no distinct social tendencies and neither avoided nor preferred foraging near other herons. Our results do not support the hypothesis that white-plumaged birds exhibit increased social tendencies relative to dark-plumaged birds within a given pond.
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Vol. 156 • No. 2