The orientation system and migration pattern of long-distance migrants: conflict between model predictions and observed patterns

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.