When gulls are flying forward, the image of the sea sweeps backward across their ventral viewfield and forms an “optic flow”, which depends on both the groundspeed and the groundheight. The ‘‘ventral optic flow’’ perceived by the gull, Gibson (1966) i.e., the apparent angular velocity created by a point directly below on the flight track, is simply equal to the ratio Vx/h, between groundspeed, Vx and the groundheight, h. By introducing a ceiling in the climbing rate according to Hedenström et al., 2003, In: Avian Migration (prediction 10), the non-linear parametric model on 18 trajectories gives a fit factor value of 57.1% on average (range 15 – 80%). For GPS-tracked offshore takeoffs by gulls, the normalized altitude computed with the mathematical model using GPS derived horizontal groundspeed data predicts altitudes close to actual GPS recorded altitude, thus suggesting gulls use an optic flow-based system for control of takeoff flight.