A Model for the Evolution of Mixed-Species Colonies of Ciconiiformes

The main factors contributing to coloniality in birds relate to predation and resource utilization. Nesting in inaccessible sites reduces predation, as does anti-predator behavior such as early warning and mobbing. Breeding synchrony (presumably caused by social stimulation) leads to decreased predation by producing superabundant food (young and eggs) at one peak time. Since predators can eat only so much at one time, a lower proportion of young are eaten if predators are not recruited into the area. Colonies are usually visible to predators, and cryptic coloration of eggs and young plays a role in the protection of eggs and young, but not of adults. Resource competition includes competition for space, nest materials, mates, and food. Competition for food is offset by the colony acting as an information center whereby unsuccessful foragers follow successful ones to patchy, ephemeral food sources. In herons, all these factors apply, and the advantages of mixed-species heronries to individuals include minimizing nest competition and food competition, while maximizing anti-predator behavior and increasing social interactions and information transfer. A model is presented that uses these selection factors to predict the optimal colony size for Ciconiiformes. Using the model, predictions are made concerning heronries, and several are tested from published data.