Evidence for multiple factors influencing vigilance.

Vigilance behavior is thought to be largely controlled by the threat of predation on foragers. In addition, an inverse relationship between group size and vigilance has been documented repeatedly and is known as the group size effect. We suggest that beach-foraging northwestern crows (Corvus caurinus) foraging in groups are vigilant for both predators and for opportunities to steal (“scrounge”) from conspecifics. Data were collected using five-minute focal samples that recorded search time, scanning frequency and bout lengths, predator presence, prey type, and theft. Group size, time of day, temperature, and tide height and direction were also recorded, resulting in 2950 foraging trials. Results indicate that increased scanning during a trial predicts trials that end in theft. Group size does not significantly influence the proportion of scanning in a trial, but scan bout length increases with increasing group size. This is in the opposite direction that the group size effect would predict, thus suggesting that more birds means more opportunities to scrounge or be scrounged upon. This rejection of the group size effect is most likely due to the trade-offs between group size and scrounging opportunities.