Figure 2.

Interaction variables calculated from the pigeon tracks and from the simulation. For the focal individual (grey), we calculated the distance (r) and angle (θ) to its neighbour (black), as well as the difference in flight direction (φ). We estimated the instantaneous direction of travel (α(t)) from the vector to the next GPS fix. The angles θ and φ were measured relative to α(t). (α(t + Δt) − α(t))/Δt gave the focal bird's turn rate. We calculated all variables in the horizontal plane, because the horizontal dimension contains most of the variation between homing routes and is therefore more relevant to route choice.