Abstract: Herons and Egrets that are seen feeding in the NJ Meadowlands mainly breed on islands in NY Habor and Long Island Sound. Their long-term population dynamics are complex, with dramatic fluctuations from year to year that imply large-scale ‘switching cascades’ — mass movement of individuals between locations from one breeding season to the next. We present a stochastic metapopulation model for a seasonal, colonially breeding species. A key component of the model is the development, at the beginning of each breeding season, of a set of breeding colonies, based on sequential choices by individuals about where to nest. Individuals favor the colony they nested in previously, but are also attracted to colonies that are rapidly establishing, and may switch locations. This positive feedback process destabilises the population dynamics at each location, producing exactly the kind of dynamics observed in the real data. We discuss how to further test and refine the model by tracking the movements of the birds.