How many diurnal migrants cross the Baltic Sea at night?

摘要

Whereas nocturnal migration across the Baltic Sea is generally thought to occur in a broad front, different spatial migration patterns occur in diurnal migrants. However, besides the obvious migration of soaring raptors along the "Vogelfluglinie" the crossing behaviour of other landbirds remains largely unexplored. Observations on visible bird migration at various offshore sites between the islands Falster and Bornholm carried out between 2002 and 2004 led us to conclude that only very few Passerine species cross the Baltic Sea during daytime in relatively small numbers. Low migration intensities in altitudes of up to 1,000m during daytime recorded by simultaneously operated vertical radars confirm the visual observations. Migration strategies and routes of several species can be mapped and quantified now by a combination of 1) data on breeding populations in Sweden, 2) simultaneous data on visible bird migration at coastal sites (e.g. Falsterbo, Darsssser Ort), and 3) simultaneous data on bird migration offshore.