VIGILANCE DOES NOT COVARY WITH GROUP-SIZE IN AN ISLAND POPULATION OF SILVEREYES (ZOSTEROPS-LATERALIS)

Numerous studies on different taxonomic groups have found that vigilance behavior is negatively correlated with group size, as predicted by several theoretical models. This follows the logical argument that each individual in a larger group can spend less time scanning for predators (and more time feeding), while the group as a whole maintains a high probability of detecting an approaching predator. We investigated the relationship between these variables in the population of silvereyes (Zosterops lateralis chlorocephala) on Heron Island, Great Barrier Reef. The study used procedures of field observation that controlled or measured other variables with which vigilance is often correlated: food density and quality, temperature, time of day, distance from cover, habitat obstructions, observer proximity, breeding status, age, sex, and dominance. There was no relationship between scanning behavior and group size under these conditions. There are several possible explanations, related to the silvereyes' biology, for this lack of covariation. An additional explanation is that Heron Island is a predator-free environment for this species, and covariation between group size and vigilance is not expected in the absence of predators.