Autopilot helps birds to land
The nerve cells in birds’ eyes and brain identify and interpret the optical flow. In this way they experience how different objects below move in relation to them and can determine the distance to the ground when they fly at relatively low altitude.
The researchers describe this as the bird’s autopilot, which automatically adjusts flying altitude in relation to speed.
“There have been different theories, but this study shows for the first time that gulls use vision to determine the distance to the ground”, says Susanne Åkesson, professor at Lund University.
The researchers have used GPS data from lesser black-backed gulls, Larus fuscus, collected at Stora Karlsö. Using this data, they created a mathematical model that describes the gull’s autopilot. The autopilot registers the optical flow, i.e. the speed that the ocean waves move across the bird’s retina.
The birds’ aim is to keep the optical flow constant, which means that they fly higher in a following wind and lower in a headwind, something that saves energy.
“It’s with the help of the autopilot that the bird determines the altitude above the surface of the water and can land smoothly”, says Anders Hedenström, professor at Lund University.
The investigation covers birds that fly at relatively low altitude, a maximum of approximately 100 metres.