Variation in the nocturnal flight behaviour of migratory birds along the northwest coast of the Mediterranean Sea

The Mediterranean Sea is one of the largest obstacles that has to be crosse d by Palearctic birds migrating from Europe to Africa; it thus offers a goo d opportunity to study variations in migratory behaviour of birds facing a major ecological barrier. Using a passive infrared device, the flight direc tions of nocturnal migrants were determined and flight altitudes estimated at ten sites along the French and Spanish coast of the Mediterranean Sea in September and October 1995. The variation of migratory intensity, flight d irection and altitude in the course of the night was examined. The highest density of migration was recorded within the first hour after sunset, follo wed by relatively high densities over the next several hours, and a progres sive decrease in the last third of the night. In spite of broad variation i n the course of the coastline relative to the basic directions of migration and specific reactions of the migrants to the local conditions, a decrease in seaward migration corresponding to an increase in landward migration fr om the first to the second half of the night was a general feature at nearl y all sites. The results suggest a shift in the motivation of the birds dep ending on the time of arrival in a coastal area, leading to an adjustment i n the flight behaviour of nocturnal migrants.