AGGRESSIVE INTERACTIONS IN MIXED-SPECIES FLOCKS OF MIGRATING SHOREBIRDS

We studied aggressive interactions in mixed-species assemblages of shorebirds in three tidal habitats in New Jersey. Aggression was higher on the inner beach than on the mudflat or outer beach. For all species on the inner beach, and for black-bellied plovers, Pluvialis squatarola, on the mudflat, aggression increased as density increased; whereas for sanderlings, Calidris alba, on the outer beach, aggression decreased as numbers increased. Interspecific aggression occurred only on the inner beach. Although the total amount of aggression did not correlate with body size, several parameters of aggression did: interspecific aggression index, percentage of encounters that were interspecific, and percentage of interspecific encounters won. These factors are discussed in terms of competition. © 1979.