Optic flow cues help explain altitude control over sea in freely flying gulls

Abstract : For studies of how birds control their altitude, seabirds are of particular interest because they forage offshore where the visual environment can be simply modeled by a flat world textured by waves then generating only ventral visual cues. This study suggests that optic flow, i.e., the rate at which the sea moves across the eye's retina, can explain gull's altitude control over seas. In particular, a new flight model that includes both energy and optical invariants help explain the gulls' trajectories during offshore takeoff and cruising flight. A linear mixed model applied to 352 flights from 16 individual lesser black backed gulls (Larus fuscus) revealed a statistically significant optic flow set-point of ca. 25°/s. Thereafter, an optic flow-based flight model was applied to 18 offshore takeoff flights from 9 individual gulls. By introducing an upper limit in climb rate on the elevation dynamics, coupled with an optic flow set-point, the predicted altitude gives an optimized fit factor value of 63% on average (30%-83% in range) with respect to the GPS data. We conclude that the optic flow regulation principle helps gulls to adjust their altitude over sea without having to directly measure their current altitude.
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Julien Serres, Thomas Evans, Susanne Akesson, Olivier Duriez, Judy Shamoun-Baranes, et al.. Optic flow cues help explain altitude control over sea in freely flying gulls. Journal of the Royal Society Interface, the Royal Society, 2019, 16 (159), pp.20190486. ⟨10.1098/rsif.2019.0486⟩. ⟨hal-02309725v2⟩

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