Orientation scatter of free-flying nocturnal passerine migrants: components and causes
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We investigated the variation in concentration of orientation among nocturnally migrating passerine birds. Using tracking radar, we recorded flight tracks of birds during spring and autumn migration and, with the aid of concurrent wind recordings, we calculated heading directions. The concentrations of track and heading directions were compared between seasons and also between different categories of migrants that were defined by flight speed and wing beat frequency. Wind drift was a dominant cause of the large scatter of track directions, especially for autumn migration. When wind effects were compensated for, we found only small differences in the concentration of heading directions between different categories of migrants. This shows that between-group variation is not a major source of the overall variation in orientation when groups are distinguished on the basis of airspeed and wing beat frequency. Although the total concentration of heading directions was almost exactly the same for spring and autumn migrants, there was an element of partial compensation for wind drift in spring but not in autumn. When we removed the effect of this compensatory behaviour by considering situations with low wind speeds, the concentration of headings during spring tended to exceed that during the autumn. This suggests a more accurate orientation of the birds during spring than in autumn, when a large proportion consists of naive migrants on their first migratory journeys. The high concentration of heading directions of free-flying migrants are in clear contrast to the widely scattered distributions generally observed in orientation experiments with caged birds.