How does a first year passerine migrant find its way? Simulating migration mechanisms and behavioural adaptations

In the Palaearctic-African migration system, birds face several trade-offs on their first autumn migration. The shortest route, minimising travelling time, would lead them directly south across the Mediterranean Sea and the Sahara Desert, involving long stretches of no refuelling possibilities. This route is risky because of flight range constraints. Besides taking longer, a detour along the Iberian peninsula may require a more complex orientation mechanism. We simulated migrants with stopover and orientation behaviour and investigated the effect of flight costs and behavioural rules (e.g. crossing or flying along coast lines, a shift in migratory direction) on the resulting flight path and especially on the evolution of endogenous directions. The simulation of autumn migration from southern Scandinavia to south of the Sahara showed that it would be possible to reach the winter quarters by vector summation with a constant endogenous direction, but then either orientation must be very accurate or flight costs must be small. For small passerines both requirements are so far not corroborated by empirical studies. Alternatively, flying along coast lines or shifting direction in northern Africa from south-westerly to southerly, resulted in similar survival rates as with a constant south-westerly endogenous direction, but with a larger range of feasible values. Although weather factors were not included, our results suggest that the Mediterranean Sea and the Sahara desert had a dominating influence on the evolution of endogenous directions. This influence is probably acting through flight range constraints.