Colonial breeding imposes increased predation: experimantal studies with herons

Experiments were carried out over 2 consecutive years to test whether heron nest predation differs between colonial and non-colonial nests. Dummy nests containing chicken eggs dyed light blue were placed in a mixed Night Heron and Little Egret heronry and in a control copse without nesting herons; in both areas Hooded Crows were the main egg predators. Experiments were performed both when the true nests had eggs (synchronized nests) and when the true nests contained chicks (delayed nests): Dummy nests within the heronry were more preyed upon than isolated dummy nests. Delayed dummy nests suffered higher predation than synchronized ones. A predictive logistic regression model for predation probability was developed for the dummy nests placed in the centre of the colony; for the synchronized dummy nests the distance from the nearest occupied real nest entered the model and dummy nests close to real nests were less likely to be preyed upon. Nesting in a colony is costly in terms of predation probability, probably because the great concentration of nesting herons attracts many predators. However, synchronization of nesting phases among pairs seems to reduce the rate of predation because of diluted risk and predator appetite satiation.