Bird migration on Helgoland: the yield from 100 years of research.

The island of Helgoland (or Heligoland; in the North Sea) was the very first place on Earth to be the focus of systematic studies on bird migration (since the mid-nineteenth century). The 'Vogelwarte Helgoland' was founded in 1910, and since 1909 more than 800,000 birds have been ringed. Most of these are nocturnally migrating songbirds that cross the German Bight in a broad front on their way to or from their Scandinavian breeding grounds. In good weather conditions (tail winds, clear sky), the majority of the migrants does not land on Helgoland, but deteriorating weather strongly increases their attraction to this island. Wind can cause rare birds such as raptors and continental landbirds to be blown offshore or pelagic seabirds to be blown into the German Bight, where they can drift to Helgoland. Migrants stay for hours to weeks, depending on species, age, sex, body condition, season, weather, food availability, intra- and interspecific competition, and predation pressure. The direction of departure also appears to be influenced by body and weather conditions. The reported circumstances for the almost 7,000 recoveries have changed substantially over the last 100 years, and show definite regional differences. Indices from constant-effort trapping reveal decreased numbers of long-distance migrants, while numbers of short/medium-distance migrants have not changed much. In all 'true migrants,' the mean spring passage time has become earlier since 1960 (by up to 18 days). Whereas changes in autumn are less uniform, the time between both periods has increased in most species. This earliness in spring is best explained by local temperatures in short/medium-distance migrants, and by the winter NAO index in long-distance migrants. [ABSTRACT FROM AUTHOR]