Optic flow helps explain gulls’ altitude control over seas

For studies of how flying animals control their flight, seabirds are of particular interest to track with a biologger because they forage offshore where the visual environment can be simply modeled by a flat world textured by waves. This study suggests that optic flow can explain gulls altitude control over seas. In particular, a new flight model that includes both energy and optical invariants (called the ventral optic flow regulation) explain the dynamics of gulls altitude control during offshore takeoff and cruising flight. A linear statistical model applied to 352 flights from 16 individual lesser black backed gulls (Larus fuscus) gave a strong correlation between wind assistance and gulls altitude. Thereafter, an optic flow-based flight model was applied to 18 offshore takeoff flights from 1. individual gulls. By introducing an upper limit in climb rate in a non-linear first order parametric model on the gulls elevation dynamics, coupled with an optic-flow set-point, the predicted altitude gives an optimized fit factor value of 63% on average (min value: 30%, max value: 83%) with respect to GPS data. We conclude that the optic-flow regulation principle (here running close to 25{degrees}/s) allows gulls to adjust their altitude over sea without having to directly measure their current altitude.