Elsevier

Animal Behaviour

Volume 127, May 2017, Pages 117-123
Animal Behaviour

Adaptive airspeed adjustment and compensation for wind drift in the common swift: differences between day and night

https://doi.org/10.1016/j.anbehav.2017.03.010Get rights and content

Highlights

  • Diurnally migrating common swifts adjust airspeed in relation to winds.

  • High-altitude night-migrating swifts did not adjust airspeed to winds.

  • A sensory threshold to detect small changes in optic flow may explain the results.

Migratory birds are known to be capable of adjusting their heading direction to compensate for wind drift and their airspeed adaptively with respect to head and tail winds. High-flying nocturnally migrating common swifts, Apus apus, have been shown to compensate for wind drift, but they failed to adjust airspeed as expected (increase in head wind and decrease in tail wind in relation to neutral wind). We report on new measurements of diurnally migrating common swifts at a coastal site in the Baltic, where the birds did adjust airspeed adaptively during spring and autumn migration. During autumn migration, they compensated for lateral wind drift by adjusting heading direction similarly to high-altitude migrants in autumn. We also recorded flight speed and wind compensation during a summer weather-related exodus, when the birds behaved similarly to those during autumn migration, although they showed a small degree of wind drift. Why birds failed to adjust airspeed adaptively at high altitude is discussed, and we argue there is a threshold in the sensory system to detect small changes in optic flow based on visual landmarks.

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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