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Trans-Sahara migrants select flight altitudes to minimize energy costs rather than water loss

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Abstract

Meteorological conditions influence strongly the energy and water budget of birds. By adjusting their flights spatially and temporally with respect to these conditions, birds can reduce their energy expenditure and water loss considerably. By radar, we quantified songbird migration across the western Sahara in spring and autumn. There autumn migrants face the trade-off between (a) favorable winds combined with hot and dry air at low altitudes and (b) unfavorable winds combined with humid and cold air higher up. Thus, it can be tested whether birds may chose altitudes to minimize water loss instead of energy expenditure. We predicted optimal flight altitudes with respect to water loss and energy expenditure based on a physiological flight model when crossing the western Sahara and compared these model predictions spatially and temporally with measured songbird densities. The model aiming for minimal water consumption predicted a mean flight altitude of 3,400 m under autumn conditions. However, 64% of the nocturnal songbird migration flew at altitudes below 1,000 m above ground level profiting from tailwind. This preference for tailwind in autumn, despite the hot and dry air, emphasizes the importance of energy savings and diminishes the significance of possible water stress for the selection of flight altitude. Nevertheless, during daytime, high energy expenditure due to air turbulences and water loss due to warmer air and direct solar radiation prevent songbirds from prolonging their nocturnal flights regularly into the day. Birds crossing the Sahara save water by nocturnal flights and diurnal rests.

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Acknowledgments

We thank all those who helped to gather the data in Mauritania. We are most grateful to Thomas Steuri who engineered the radar hardware and the software together with Erich Bächler whom we also thank for assistance in the field. We thank the main supporters of our project: The Swiss Army and Oerlikon-Contraves AG for the radar equipment; for important financial support, we thank the Swiss National Science Foundation (Project No. 31-65349), the Foundations Volkart, Vontobel, MAVA for Nature Protection, Ernst Göhner, Felis and Syngenta, the companies Bank Sarasin and Co, Helvetia Patria Insurances and F. Hoffmann-La Roche AG, the European Science Foundation as well as BirdLife Switzerland and International. We would like to thank the authorities of Mauritania among them especially “Ministère du Dévelopement Rural et de l'Environnment, Direction de l'Environnment et de l'Aménagement Rural” and “Centre de Lutte Antiacridienne” for their support and assistance. Further partners are found under “www.vogelwarte.ch/sahara”. The Swiss Ornithological Institute was the stronghold of the whole project throughout. Meteorological data were kindly provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, from their web site at http://www.cdc.noaa.gov/. Marcel Klaassen kindly provided his software for the model calculations. We thank Lukas Jenni, Franz Bairlein, and two anonymous referees for valuable comments on the manuscript. Our study complied with the animal care laws of Mauritania.

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  1. Heiko Schmaljohann

    Present address: Institute of Avian Research “Vogelwarte Helgoland”, An der Vogelwarte 21, 26386, Wilhelmshaven, Germany

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  1. Swiss Ornithological Institute, Luzernerstrasse 6, 6204, Sempach, Switzerland

    Heiko Schmaljohann, Felix Liechti & Bruno Bruderer

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  1. Heiko Schmaljohann
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  2. Felix Liechti
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Correspondence to Heiko Schmaljohann.

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Communicated by W. Wiltschko

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Schmaljohann, H., Liechti, F. & Bruderer, B. Trans-Sahara migrants select flight altitudes to minimize energy costs rather than water loss. Behav Ecol Sociobiol 63, 1609–1619 (2009). https://doi.org/10.1007/s00265-009-0758-x

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