What is the magnitude of the group-size effect on vigilance?

Vigilance has been predicted to decrease with group size due to increased predator detection and dilution of predation risk in larger groups. Although earlier literature reviews have provided ample support for this prediction, an increasing number of studies have failed to document a decline in vigilance with group size. In addition, support for this prediction has been based thus far on the P value of the relationship between vigilance and group size rather than on a quantitative assessment of effect magnitude. Here, I use a meta-analysis of empirical relationships between vigilance and group size in birds published in the last 35 years to provide a reassessment of the group-size effect on vigilance. Nearly one-third of all published relationships between vigilance and group size were not significant (n = 172). Results from the meta-analysis indicate weak to moderate negative correlations between group size and time spent vigilant (n = 43), scan frequency (n = 29), or scan duration (n = 20). The magnitude of the relationship was stronger in studies that controlled the amount of food available to birds. A funnel plot of the relationship between correlation coefficients and sample size failed to reveal an obvious publication bias. Although the meta-analysis results generally support the prediction that vigilance should decline with group size, a large amount of variation in vigilance remains unexplained in avian studies.