Modelling optimal heading and airspeed of migrating birds in relation to energy expenditure and wind influence

A goal oriented migrant optimizing energy expenditure during flight should coordinate the adaptation of heading and airspeed with respect to wind. The model introduced in this study combines for the first time the simultaneous adaptation of heading and airspeed in an optimal strategy. Compared to other models (constant track or constant airspeed), the largest benefit from using the optimal strategy results with lateral and opposing winds. With decreasing distance to the goal, compensation for wind drift should be extended by adjusting heading and increasing airspeed. Regardless of their flight mechanics, birds completing their migratory journey in a few long flights should fly faster and compensate more for lateral wind drift, than birds travelling in numerous short hops. An analysis of flight paths of small passerines gathered with a tracking radar at night showed good correlations between measured headings and airspeeds and those predicted by the optimal strategy.