Soaring across continents: decision-making of a soaring migrant under changing atmospheric conditions along an entire flyway

(1) Thermal soaring birds reduce flight-energy costs by alternatingly gaining altitude in thermals and gliding across the earth’s surface. To find out how soaring migrants adjust their flight behaviour to dynamic atmospheric conditions across entire migration routes, we combined optimal soaring migration theory with high-resolution GPS tracking data of migrating Honey Buzzards Pernis apivorus and wind data from a global numerical atmospheric model. (2) We compared measurements of gliding air speeds to predictions based on two distinct behavioural benchmarks for thermal soaring flight. The first being a time-optimal strategy whereby birds alter their gliding air speeds as a function of climb rates to maximize cross-country air speed over a full climb-glide cycle (Vopt). The second a risk-averse energy-efficient strategy at which birds alter their gliding air speed in response to tailwinds/headwinds to maximize the distance travelled in the intended direction during each glide phase (Vbgw...