Determinants of false alarms in staging flocks of semipalmated sandpipers

False alarms occur when animals flee abruptly upon detection of a threat that subsequently proved harmless. False alarms are common in many species of birds and mammals and account for a surprisingly high proportion of all alarms. False alarms are expected to be more frequent in larger groups, where the odds of misclassifying threats are higher, and under environmental conditions where detection of threats is compromised, such as low light levels. In addition, false alarms should be less frequent when the energetic cost of fleeing increases. I examined these hypotheses in roosting flocks of staging semipalmated sandpipers (Calidris pusilla) over 2 years. False alarms increased with group size but the effect of group size was confounded by the fact that more attacks by falcons (Falco spp.) were directed at larger roosts. False alarms were more frequent at low light levels and later during staging. As individuals double their body mass during staging, the energetic cost of fleeing must greatly increase thus contributing to decreased responsiveness. A simple reduction in responsiveness caused by repeated exposures to harmless signals would also produce a temporal decrease in responsiveness but this hypothesis cannot account for the effect of group size and light level. Study of the determinants of false alarms provides an opportunity to examine adjustments in behavior in relation to changes in perceived predation risk. Copyright 2010, Oxford University Press.