Abstract
Soaring raptors can fly at high altitudes of up to 9000 m. The behavioural adjustments to high-altitude flights are largely unknown. We studied thermalling flights of Himalayan vultures (Gyps himalayensis) from 50 to 6500 m above sea level, a twofold range of air densities. To create the necessary lift to support the same weight and maintain soaring flight in thin air birds might modify lift coefficient by biophysical changes, such as wing posture and increasing the power expenditure. Alternatively, they can change their flight characteristics. We show that vultures use the latter and increase circle radius by 35% and airspeed by 21% over their flight altitude range. These simple behavioural adjustments enable vultures to move seamlessly during their annual migrations over the Himalaya without increasing energy output for flight at high elevations.
Highlights
Migrant birds often spend part of their lives at low altitudes and ascend to altitudes of up to 9000 m [1]
Lift acceleration is proportional to air density, and when keeping all other parameters constant, will decrease with elevation (equation (2.1)), whereas the gravitational force is near constant
Realistic estimates of air density during the observed flight were obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim weather reanalysis dataset and annotated to the tracks using the Movebank-Env-DATA service [8,9]
Summary
Migrant birds often spend part of their lives at low altitudes and ascend to altitudes of up to 9000 m [1]. Lift acceleration is proportional to air density, and when keeping all other parameters constant (such as flight speed, lift coefficient, wing area), will decrease with elevation (equation (2.1)), whereas the gravitational force is near constant. Obligate soaring birds, such as vultures, are largely unable to employ a powered flight mode for long durations [6]. We assess the birds’ strategies of thermalling from 50 to 6500 m altitude, while crossing the Himalaya during their natural annual flights. A thermalling bird with non-powered flight has two contrasting options to create the lift necessary to maintain upward acceleration in a rising thermal in thinner air: (i) biophysical—change its wing or feather posture and inclination to increase the lift coefficient; (ii) behavioural—change its flight characteristics, i.e. increase its thermalling radius and/or flight speed. Observations of the strategy employed by free-flying wild vultures soaring over the Himalaya
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