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Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Airspeed of the Song Thrush in relation to the wind during autumnal nocturnal migration (2019)
Auteurs :	Alexandra Sinelschikova, Auteur ; Martin Griffiths, Auteur ; Michael Vorotkov, Auteur ; Victor Bulvuk, Auteur ; Casimir Bolshakov, Auteur
Type de document :	Article
Dans :	Ornis Fennica (96(2) December, 2019)
Article en page(s) :	64-76
Résumé :	Birds possess behavioural and physiological adaptations which permit them to minimize time and energy expenditure during migration in a broad spectrum of winds, for instance, by varying their airspeed. Nocturnally migrating birds were recorded by an optical-electronic matrix system, which permitted recording their images and flight parameters in the dark. Among medium size birds, Song Thrushes (Turdus philomelos) were identified by their silhouette, linear size, wing-beat pattern, and phenology. The equivalent airspeed at sea level (V-Eq) of the observed thrushes without wind assistance (mean value 14.4 m/s) was close to the maximum range speed (Vmr) predicted from flight mechanical theory. This indicated an energy-selected migration strategy of the thrushes in autumn. The characteristic speed Vmr is wind-dependent: it increases with increasing velocity of head- and sidewinds. The airspeeds of the Song Thrushes showed a similar pattern of wind-dependence. The birds significantly increased their airspeed with increasing headwind component, relative to its value in still air, and decreased it with increasing tailwind component. We also found an effect of cross winds on airspeed: those birds that compensated for wind drift completely along the leading line of the Courish Spit, which coincided with general migratory direction, increased their airspeed with increasing sidewind component. The flying birds tended to maintain their airspeed close to the speed of minimal cost of transport under various winds. Nevertheless, this trend was more in qualitative than quantitative agreement with Pennycuick's flight model: migrating thrushes varied their airspeed in a smaller range than expected and their airspeed was significantly slower under calm and headwinds than the model predicted.

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