Table 1.

Measures of spatial position used in our analysis. The distance from front measure was only applied to prey in moving groups.

measures of local crowding
Voronoi polygon area (i.e. domain of danger) [6]	Area around an individual prey that is closest to that individual and not another individual. The Deldir function in the deldir R package was used. The four corners of the projection area (figure 1) were used as the boundaries.
limited domain of danger [27]	As above, although the domains of danger are limited to a maximum distance from each individual prey.
nearest neighbour distance	The distance between the focal prey and the closest other prey in the group.
number of near neighbours	The number of other prey within a predefined distance. This distance was the radius of a circle centred around each prey.
measures of prey position defined by reference to the group centre or periphery
distance to the group centroid	Distance from the mean x and y coordinate of all prey in the group.
on the convex hull?	The convex hull is the polygon with minimum area that encloses all prey positions (thus also known as the minimum convex polygon). Calculated using the chull function in R. Each prey was classified as being either on this convex hull perimeter or not.
angle of vulnerability [21]	Determined by calculating the angles from the prey as the vertex to all possible pairs of other prey (creating a triad); the largest of these angles where no other prey were located within it was the angle of vulnerability.
distance from front	The distance of each prey from the front of the group, where the leading individual is given a distance of zero.