Energetic influence on gull flight strategy selection.

During non-migratory flight, gulls (Larids) use a wide variety of flight strategies. We investigate the extent to which the energy balance of a bird explains flight strategy selection. We develop a model based on optimal foraging and aerodynamic theories, to calculate the ground speeds and airspeeds at which a gull is expected to flap or soar during foraging flight. The model results are compared with observed flight speeds, directions, and flight strategies of two species of gulls, the black-headed gull Larus ridibundus and the lesser black-backed gull Larus fuscus. The observations were made using a tracking radar over land in The Netherlands. The model suggests that, especially at combinations of low ground speed (approximately 5-10 m s(-1)), high air speed (approximately 20-25 m s(-1)) and low ground and air speed, gulls should favor soaring flight. At intermediate ground and air speeds the predicted net energy gain is similar for soaring and flapping. Hence the ratio of flapping to soaring may be higher than for other air and ground speed combinations. This range of speeds is broadest for black-headed gulls. The model results are supported by the observations. For example, flapping is more prevalent at speeds where the predicted net energy gain is similar for both strategies. Interestingly, combinations of air speed and flight speed that, according to the model, would result in a loss of net energy gain, were not observed. Additional factors that may influence flight strategy selection are also briefly discussed.