How does food density influence vigilance in birds and mammals?

Vigilance in animals decreases with group size in many species of birds and mammals, but the effect of group size can be confounded by several ecological factors, most notably food density. Given that large groups often aggregate in areas of higher food density and that vigilance may be reduced when animals feed more, the group-size effect of vigilance may be confounded by the direct effect of food density on vigilance. In this paper, I examine the theoretical expectations regarding the effect of food density on vigilance and review the available empirical evidence. Two bodies of theory predict either no relationship or a negative relationship between vigilance and food density, but none has been tested explicitly. The empirical support for a negative relationship is mixed given that many studies actually document a positive correlation between vigilance and food density. I review the factors modulating the effect of food density on vigilance and show that there are several reasons to believe that the case against a confounding effect of food density is not as strong as expected. In fact, food density can actually mask the expected decrease in vigilance with group size. A statement about the magnitude of the group-size effect on vigilance will require an assessment of the role of food density as well as the many factors reviewed here that modulate the effect of food density on vigilance in birds and mammals.