Social and ecological factors influencing vigilance by northwestern crows,Corvus caurinus

Vigilance behaviour is thought to be largely controlled by the threat of pr edation on foragers. In addition, an inverse relationship between group siz e and vigilance has been documented repeatedly and is known as the group si ze effect. We suggest that groups of beach-foraging northwestern crows are vigilant for predators and for opportunities to steal ('scrounge') from con specifics. We collected data using 5-min focal samples that recorded search time, scanning frequency and bout lengths, predator presence, prey type an d theft. We also recorded group size, time of day, temperature, and tide he ight and direction, resulting in 2950 foraging trials. Results indicated th at increased scanning during a trial predicted trials that would end in the ft. Group size did not significantly influence the proportion of scanning i n a trial, but scan bout length increased with increasing group size. This result is opposite to that predicted by the group size effect and suggests 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-off s between group size and scrounging opportunities. (C) 2001 The Association for the Study of Animal Behaviour.