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.