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| The requirements of the orientation system of naišve long-distance night migrants
were analysed by comparing data on Barred Warbler Syl6ia nisoria, Marsh Warbler
Acrocephalus palustris and Spotted Flycatcher Muscicapa striata with data from a
computer model of a clock-and-compass system. These species show, respectively, a
rather restricted winter distribution in East Africa, migration through a very narrow
corridor in East Africa, and rather widely distributed recoveries in the Mediterranean
with more concentrated recoveries south of the Sahara. For all three species, to
obtain the observed concentrations either a very high directional migratory concentration
was needed in computer simulations to bring the birds successfully to their
wintering areas or misorientating individuals would be subjected to a very high
mortality. Neither the very high directional concentration nor the high mortality
amongst misorientating individuals fit the empirical data sets. On the basis of the
present study, the observed patterns seem difficult to explain by a simple clock-andcompass
system only, and to account for the exceptionally precise migratory routes
shown in this study it is proposed that first-time migrants might be able to use
landscape topography on a regional scale in combination with corrections of directional
mistakes:wind displacements. | |
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