Antipredator vigilance decreases with food density in staging flocks of Semipalmated Sandpipers (Calidris pusilla)

When animals face time constraints, antipredator vigilance is expected to decrease in patches with higher food density. Indeed, sacrifices in safety are worthwhile in rich food patches that allow substantial foraging gains in response to a decrease in vigilance. This prediction has received little empirical attention. I tested this prediction in fall-staging Semipalmated Sandpipers (Calidris pusilla (L., 1766)) using the frequency of looks while foraging as a proxy for vigilance. Fall-staging sandpipers face time constraints, as individuals must accumulate fat rapidly before undertaking their long migration south. Controlling for known correlates of vigilance, such as distance to obstructive cover, bird density, and position in the flock, the frequency of looks decreased as predicted when the density of food in a patch was higher. That vigilance can vary with food density is relevant for observational studies of vigilance. When food density is positively correlated with group size, food density can become a confounding factor in the well-known negative relationship between vigilance and group size.