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Wind as Selective Agent in Bird Migration

Thomas Alerstam
Ornis Scandinavica (Scandinavian Journal of Ornithology)
Vol. 10, No. 1 (1979), pp. 76-93
Published by: Wiley on behalf of Nordic Society Oikos
DOI: 10.2307/3676347
Stable URL: http://www.jstor.org/stable/3676347
Page Count: 18
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Wind as Selective Agent in Bird Migration
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Abstract

Wind is of foremost importance to affect migrating birds' optimal flying speed and altitude, and their optimal migratory routes. This paper contains two main sections, wherein (1) the selective influence of wind on the compensatory behaviour of migrating birds is evaluated, and (2) a first attempt is made to examine the complex interplay between different selection factors governing the relationship between winds and bird migratory activity. Wind conditions determine the advantages and disadvantages of wind compensation and drift, respectively, in migrating birds. Complete compensation for wind drift is the optimal behaviour under constant winds, but with varying winds the optimal behaviour is partial and flexible compensation, i.e. the migrants should allow extensive drift far away from the goal and gradually compensate to a higher and higher degree on approaching the goal, finally compensating completely during the last flight to the goal. Different species of migrating birds are expected to be selective of favourable winds for their migratory flights to different degrees. The relationship between costs for fuel transportation and benefit by being highly selective of winds will determine optimal wind selectivity and fuel ratio in migrating birds. Optimal wind selectivity will be expected to be high with, i.a., widely chinging winds along the migratory route, a short distance and long duration of migration, and long durations of migratory flights. It is furthermore expected to be higher in adult than in juvenile birds, and in migrants recently having experienced successful flights as compared to migrants being delayed.

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