A meta-analysis of the relationship between flock size and flight initiation distance in birds

When encountering an approaching predator, prey often must decide on an optimal distance for escape, measured as flight initiation distance (FID). As a fundamental behavioural indicator, FID has been widely measured in many species, and many biological or environmental factors have been found to be associated with FID. However, the effect of flock size on FID remains unclear. In this study, we conducted a phylogenetic meta-analysis using 142 effect sizes (based on 7666 measurements) from 99 avian species to explore the global trend of the effect of flock size on FID in birds, as well as the role of several biological and environmental factors in shaping the flock size effect. Generally, the overall effect size (weighted average) did not significantly differ from zero, with significant heterogeneity existing among effect sizes. Birds of larger body size, with a larger maximum flock size or living in nontropical regions tended to exhibit a more positive relationship between flock size and FID. Compared to passerines, waders exhibited a more positive relationship between flock size and FID. We detected significant between-taxon differences in ecological correlates of the flock size effect. For waders, body size, maximum flock size and hand–wing index were positively associated with the flock size effect on FID. For passerines, however, none of the explanatory factors played a significant role in shaping the relationship between flock size and FID. Our results suggest that the effect of flock size on FID is largely context dependent and may be influenced or even overridden by effects of other confounding factors. We also detected a publication bias, with studies reporting nonsignificant effects and those having smaller sample sizes less likely to be published.