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Nocturnal passerine migration without tailwind assistance

In English

Författare

Summary, in English

Nocturnal passerine migrants could substantially reduce the amount of energy spent per distance covered if they fly with tailwind assistance and thus achieve ground speeds that exceed their airspeeds (the birds' speed in relation to the surrounding air). We analysed tracking radar data from two study sites in southern and northern Scandinavia and show that nocturnally migrating passerines, during both spring and autumn migration, regularly travelled without tailwind assistance. Average ground and airspeeds of the birds were strikingly similar for all seasonal and site-specific samples, demonstrating that winds had little overall influence on the birds' resulting travel speeds. Distributions of wind effects, measured as (1) the difference between ground and airspeed and (2) the tail/headwind component along the birds' direction of travel, showed peaks close to a zero wind effect, indicating that the migratory flights often occurred irrespective of wind direction. An assessment of prevailing wind speeds at the birds' mean altitude indicated a preference for lower wind speeds, with flights often taking place in moderate winds of 3-10 m/s. The limited frequency of wind-assisted flights among the nocturnal passerine migrants studied is surprising and in clear contrast to the strong selectivity of tailwinds exhibited by some other bird groups. Relatively high costs of waiting for favourable winds, rather low probabilities of occurrence of tailwind conditions and a need to use a large proportion of nights for flying are probably among the factors that explain the lack of a distinct preference for wind-assisted flights among nocturnal passerine migrants.

Publiceringsår

2011

Språk

Engelska

Sidor

485-493

Publikation/Tidskrift/Serie

Ibis

Volym

153

Issue

3

Dokumenttyp

Artikel i tidskrift

Förlag

Wiley-Blackwell

Ämne

  • Biological Sciences

Nyckelord

  • airspeed
  • flight speed
  • Passeriformes
  • tracking radar
  • wind

Status

Published

ISBN/ISSN/Övrigt

  • ISSN: 0019-1019