Adaptive flight speeds in the Common Redshank Tringa totanus

Birds fly with different objectives at different times of the annual cycle. For example, when migrating the objective may be to minimize energy cost per unit distance covered or increase the maximum overall speed of migration. A foraging bird flying between food patches may choose to fly at a speed that maximizes the overall food intake rate, while during a display flight the objective is likely to maximize the flight duration (signal) per unit of energy invested. Flight mechanical models combined with relevant currency assumptions provide a tool for deriving likely adaptive flight speeds in different ecological situations and predicting how speeds should be adjusted in relation to external factors such as winds. I measured flight speeds of Redshanks Tringa totanus to test if they adjust airspeed as predicted from optimality theory. During migratory flight, Redshanks adjusted airspeed in relation to vertical speed, flock size, and tail and sidewind components. Airspeed also differed between display flight, migratory flight, and flight between feeding sites in broad agreement with theoretical predictions. That airspeed increases with increasing flock size has also been observed in other shorebirds, but there is no theoretical underpinning for why this happens. This study shows that a relatively straightforward behavioural trait such as airspeed in birds has a complex background.