Seasonal differences in migration patterns of a soaring bird in relation to environmental conditions: a multi-scale approach
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Many studies suggest that migratory birds are expected to travel more quickly during spring, when they are en route to the breeding grounds, in order to ensure a high-quality territory. Using data recorded by means of Global Positioning System satellite tags, we analysed at three temporal scales (hourly, daily and overall journey) seasonal differences in migratory performance of the booted eagle (Aquila pennata), a soaring raptor migrating between Europe and tropical Africa, taking into account environmental conditions such as wind, thermal uplift and day length. Unexpectedly, booted eagles showed higher travel rates (hourly speed, daily distance, overall migration speed and overall straightness) during autumn, even controlling for abiotic factors, probably thanks to higher hourly speeds, more straight routes and less non-travelling days during autumn. Tailwinds were the main environmental factor affecting daily distance. During spring, booted eagles migrated more quickly when flying over the Sahara desert. Our results raise new questions about which ecological and behavioural reasons promote such unexpected faster speeds in autumn and not during spring and how events occurring in very different regions can affect migratory performance, interacting with landscape characteristics, weather conditions and flight behaviour.
KeywordsRaptors Wind GPS satellite telemetry Sahara Flight Aquila pennata
All information about tagged Eagles has been obtained in the framework of the ‘Migra’ project (www.migraciondeaves.org/en/) developed by SEO/BirdLife and financed by Iberdrola Foundation. The Extremadura Regional Government provides information on two tagged booted eagles (special thanks to D. Rivera). Grefa and Generalitat Valenciana provided fieldwork material. Thomas Alerstam and an anonymous reviewer made useful comments on a first version of the manuscript. We are very grateful to Ana Mellado for the statistical pieces of advice. Different people from La Rioja Government Conservation and Biodiversity from Dirección General de Medio Natural, forest agents from Málaga and Madrid, SEO-Málaga Ornithological Group, Montes de Málaga Natural Park, FAB, GIA-León, research group in Mediterranean Ecosystems from Murcia University Murcia helped during fieldwork and location of pairs: A. Llopis, A. Leal, B. Fuertes, C. Pino, C. Ponce, D. Miguélez, E. Herrero, E. Miera, E. Navarro, F. Bustamante, F. de la Cruz, F. Ortega, G. Sierra, G. Negrete, H. Astiárraga, I. Gámez I. García, J. Bosch J.A. Cortés, J.E. Martínez, J.F. Calvo, J.F. Pedreño, J.C. del Moral, J. Robres, J.J. Jiménez, J.M. Abad, L.A. Ramos, M. Aguilera, M. Moreno, M.V. Jiménez, N. Rodríguez, P. Díaz, R.A. Haro, R. Ruiz, R. Prades and T. Álvarez. PL-L is supported by a ‘Juan de la Cierva’ postdoctoral grant of the Spanish Ministry of Science and Innovation (reference JCI-2011-09588).
This work complies with the current environmental laws in Spain, and all necessary permissions to catch and deploy transmitters were provided by the environmental departments of the regional governments of Andalucía, Castilla y León, Cataluña, Comunidad Valenciana, Extremadura, La Rioja, Madrid and Murcia.
Conflict of interest
Authors declare that no conflict of interest exists.
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