Adaptive airspeed adjustment and compensation for wind drift in the common swift: differences between day and night
Section snippets
Methods
Observations of migrating common swifts were made at Ottenby on the island of Öland in the Baltic Sea, off the east coast of southern Sweden. The observation site (56°12′34.21″N, 16°24′21.27″E) was situated 1.5 km north of the southern point of Öland, with unobstructed views in all directions. Swifts were tracked as part of a general programme for tracking migratory birds during 2012–2016. The data were subdivided into spring migration (15–30 May), summer weather movement (8 July 2015) and
Characteristic Flight Behaviour
Swifts typically migrated singly or in small groups in spring (mean flock size = 1.8, maximum flock size = 6, N = 56) and autumn (mean flock size = 2.4, maximum flock size = 20, N = 47), while during the summer weather-related movement (SWM) there was a continuous stream of swifts. The counts of southward-passing common swifts during the SWM yielded on average 71 birds/min (SD = 15.8, total 1052 individuals), which taken over 6 h gives an estimated 25 500 swifts. This is likely to be an underestimate and yet
Discussion
When migrating in daylight and at low altitudes the common swifts adjusted their airspeed in relation to the tail wind component as predicted if minimizing the cost of transport (Hedenström et al., 2002, Pennycuick, 1978), which contrasts with high-altitude nocturnally migrating swifts (Henningsson et al., 2009, Karlsson et al., 2010). During nocturnal high-altitude migratory flight, common swifts instead adjusted airspeed with respect to the side wind component (Karlsson et al., 2010), which
Acknowledgments
We thank Professor C. J. Pennycuick for developing the Ornithodolite system used in this study and for developing software to analyse the data. The manuscript benefitted from the constructive comments of two anonymous referees. The research received support from the Swedish Research Council to A.H. (621-2012-3585), S.Å. (621-2013-4361) and the Centre for Animal Movement Research (CAnMove) financed by a Linnaeus grant (349-2007-8690) from the Swedish Research Council and Lund University. This is
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