Definition of spatial positions within foraging Greater White-fronted Geese flocks from an individual's perspective: Cost-benefit dynamics change with the distance to flock edge

The costs and benefits of group living are not evenly distributed among individuals that occupy different spatial positions within a group (central or peripheral). However, the definitions of central and peripheral positions may artificially mask the true dynamics of cost and benefit of individuals. Using wintering foraging Greater White-fronted Geese (Anser albifrons) flocks as our study system, we measured individual spatial position within a flock as distance to the flock edge. First, we tested the effects of distance to edge on the antipredation cost and foraging benefit of individual goose using generalized linear mixed models (GLMMs). Flock size and number of neighbors and their interactions with distance to edge were included as fixed effects in GLMMs. Then we used piecewise regression to determine the threshold value of distance to edge to classify central and peripheral individuals. Our results indicated distance to edge significantly affected percent of time spent on vigilance (vigilance time), percent of time spent on foraging (foraging time) and intake rate (pecks/min), whereas other factors had no effects. We found that the critical distance that predicted changes in behavior was around 2.5 ​m. Birds 2.5 ​m or less from the edge of the group were more vigilant, and had less efficient foraging than more central individuals. Behavioral differences between central and peripheral individuals were more easily detected when the birds were classified by the method introduced in this study than by the traditional minimum convex polygon method. Our research provides researchers a dichotomous classification scheme that places individuals into peripheral and central positions from an individual's perspective.