Dawn ascents to astonishing altitudes during migration of Eleonora's Falcons (Falco eleonorae)

By Prof.dr.ir. W. Bouten, UvA

Eleonora's Falcons, a small raptor breeding on the Canary Islands and wintering on Madagascar revealed astonishing dawn ascents (short-lasting vertical spikes, occasionally up to 5500 m above ground level) at about half of the migration days. Steep vertical movements in the atmosphere or sea have been reported for some species but were never described for raptors. In general, these movements are thought to be related to foraging, probing the atmosphere for following winds or acquiring orientation cues. We evaluated these options for Eleonora's Falcons using high resolution GPS tracking (on average 22000 measurements per bird per season) while flight modes were monitored using a tri-axial accelerometer. Birds used flapping flight to climb with vertical speeds of 0.8-1.2 m/s, starting at 0.5-1 hour before sunrise although these ascents require two to three times as much power as horizontal flapping flight. Dawn ascents mainly occurred along the magnetic equator (where inclination is zero) but not in mountainous areas where landmarks are clearly visible above the landscape. The altitude of ascents is related to winds and we still try to understand the influence of clouds. We preliminary concluded that this mainly diurnal migrant seeks a wide overview of the landscape and acquires polarized light cues during dawn to orient or recalibrate its compass, especially along the magnetic equator.

Prof. dr. ir. Willem Bouten, studied Soil Chemistry at Wageningen University, did his PhD on forest hydrology at the University of Amsterdam and is now leading a research group on Computational Geo-Ecology. His current research focuses on the influence of the (atmospheric) environment on the behaviour of birds.