Preroosting aggregations in the American crow,Corvus brachyrhyncos

Abstract

Preroosting aggregations (PRAs), or site-specific gatherings of individuals made prior to flying into a night roost, are frequently associated with avian roosting behavior. We investigated PRAs in the American crow (Corvus brachyrhyncos) in order to describe preroosting behavior and test possible hypotheses concerning their function. Crows were aggregated among the available fields and within a field. Although crows in PRAs in fields commonly foraged, in previous studies crow PRAs have been found in the area at sites where little foraging probably took place, such as large rooftops. As the evening progressed, individuals shifted to PRA sites closer to the location of night roosts. Crows left PRAs for night roosts in a highly aggregated fashion, but prior to arriving at a night roost, individuals often separated from those with whom they left the PRA. Those individuals that were still together upon arriving at the night roost rarely settled next to each other. This pattern of behavior, along with characteristics of crow night roosts, is inconsistent with functional explanations based on crow PRAs being information centers for location of either daytime foraging areas or night roosts, nor does it support hypotheses solely related to foraging benefits. Rather, the results suggest that while crow PRAs are probably associated with multiple benefits, lowering predation risk may be a primary function.