Obligatory barrier crossing and adaptive fuel management in migratory birds: the case of the Atlantic crossing in Northern Wheatears (<Emphasis Type="Italic">Oenanthe oenanthe</Emphasis>) |
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Authors: | Julia Delingat Franz Bairlein Anders Hedenström |
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Institution: | (1) Institute of Avian Research, An der Vogelwarte 21, 26386 Wilhelmshaven, Germany;(2) Department of Theoretical Ecology, Ecology Building, 223 62 Lund, Sweden |
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Abstract: | Behaviour on migration was often suggested to be selected for time-minimising strategies. Current optimality models predict
that optimal fuel loads at departure from stopover sites should increase with increasing fuel deposition rates. We modified
such models for the special case of the east Atlantic crossing of the Northern Wheatear (Oenanthe oenanthe). From optimality theory, we predict that optimal time-minimising behaviour in front of such a barrier should result in a
positive correlation between fuel deposition rates and departure fuel loads only above a certain threshold, which is the minimum
fuel load (f
min) required for the barrier crossing. Using a robust range equation, we calculated the minimum fuel loads for different barrier
crossings and predict that time-minimising wheatears should deposit a minimum of 24% fuel in relation to lean body mass (m
0
) for the sea crossing between Iceland and Scotland. Fuel loads of departing birds in autumn in Iceland reached this value
only marginally but showed positive correlation between fuel deposition rate (FDR) and departure fuel load (DFL). Birds at
Fair Isle (Scotland) in spring, which were heading towards Iceland or Greenland, were significantly heavier and even showed
signs of overloading with fuel loads up to 50% of lean body mass. Departure decisions of Icelandic birds correlated significantly
with favourable wind situations when assuming a migration direction towards Spain; however, the low departure fuel loads contradict
a direct non-stop flight. |
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Keywords: | Barrier crossing Flight costs Optimal migration Oenanthe oenanthe Fuel loads |
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