Weather conditions determine migration speed of raptors
Researchers at the UvA’s Institute for Biodiversity and Ecosystem Dynamics (IBED) have determined the influence of wind and thermals on the migration speed of raptors. Based on their migratory journeys between Europe and Africa patterns in flight speed and annual timing were found, which are factors that determine their breeding success. The results of the research were published in an open access article in the Journal of Avian Biology on 1 August 2014.
Large birds, such as raptors, use thermals (vertical movements of air caused by temperature differences) to minimise their energy use during marathon journeys between Europe and Africa. But because the birds often soar passively on warm air, it is also easy for them to drift with the wind off course. IBED researcher Wouter Vansteelant studied the flight routes used by raptors (wearing GPS backpacks) combined with global weather models to calculate the relative influence of both wind and thermals on the flight speed and daily migratory distance of raptors. Besides thermals, wind speed along the bird’s travel direction was found to be the dominant factor determining flight speed, such that moderate differences in wind conditions en route may accumulate to several days of difference in the seasonal timing of migration. Weather influences along the way also accounted for differences in flight speed between seasons and various biogeographic regions, which are usually ascribed to changes in flight behaviour.
Researchers have been aware of these kinds of weather influences on migratory birds for some time thanks to visual observations of raptors around the Mediterranean Sea and in the US. However, in order to determine the consistency of these influences along entire migratory routes, individual birds need to be tracked very precisely for the whole year. This has been technically possible since 2008 thanks to the development of the UvA Bird Tracking System (www.uva-bits.nl). Between 2008 and 2011, researchers at the UvA, working with Stichting Boomtop (Treetop Foundation) and the Werkgroep Grauwe Kiekendief (Montagu’s Harrier working group), fixed UvA BiTS GPS backpacks onto 20 honey buzzards and Montagu’s Harriers, enabling them to monitor entire migratory routes at resolutions of up to one hour.
Vansteelant, W. M. G., Bouten, W., Klaassen, R. H. G., Koks, B. J., Schlaich, A. E., van Diermen, J., van Loon, E. E. and Shamoun-Baranes, J. (2014), Regional and seasonal flight speeds of soaring migrants and the role of weather conditions at hourly and daily scales. Journal of Avian Biology. doi: 10.1111/jav.00457