First records of lesser black-backed gulls Larus fuscus crossing the Sahara non-stop

摘要

Lesser black-backed gulls Larus fuscus migrate regularly along coastlines or the Nile to their wintering grounds in sub-Saharan Africa. They usually avoid the interior of the Sahara, but occur occasionally far inland when moving upstream along rivers. We discovered several flocks of lesser black-backed gulls crossing the western Sahara about 500 km east of the Atlantic coast. Their concentrated flight directions along a northeast-southwest axis indicate that the gulls were on an intended migratory flight and not incidentally drifted inland. Ground speed of 8 flocks of gulls averaged 20 m/s and was considerably larger than the mean air speed (12 m/s) demonstrating that the gulls were strongly assisted by tailwinds. Highest ground speed was 26 m/s. Warmest temperature encountered when crossing the study site was 13p C due to relative high flight altitudes of 3,500 m above sea level. Thus, lesser black-backed gulls migrated under favourable conditions with respect to energy and water consumption. Our observations strongly suggest that the gulls performed long non-stop flights between the eastern Atlantic coast of sub-Saharan Africa and the Mediterranean Sea.