Population trend over 100 years and conservation needs of breeding sandwich terns (Sterna sandvicensis) on the German North Sea coast

摘要

The breeding population of the sandwich tern (Sterna sandvicensis) on the German North Sea coast has undergone substantial fluctuations throughout the last 100 years. Numbers of breeding birds were fluctuating quite strongly in the first 30 years of the twentieth century. From 1930 to the mid-1950s, a relatively steady decrease occurred. A minimum was reached in 1965 with 2,243 pairs. Around 1970, numbers increased quite markedly up to the mid-1990s and reached a centurial maximum with 10,138 pairs in 1996. Most recently, the numbers have dropped to only 5,681 pairs in 2005, the lowest number over the last 30 years. Some colonies have existed over long periods, others only for short periods, with often substantial and sudden changes. The distribution at sea was studied by transect counts from ships. During the reproductive period, high totals were found between the mainland coast and the islands, up to 30 km from the outer coast/island line. The seaward extent of the sandwich tern distribution coincided quite well with the 20-m depth line. Maximum foraging ranges for single colonies were estimated to be ca. 45 km for Trischen, ca. 35 km for Norderoog and ca. 30 km for both Scharh?rn/Nigeh?rn and Juist. Overall flight ranges for all colonies were estimated at 33.8 km for 95% of the birds. Germany has a high international responsibility for the protection of this species. Only a few colonies exist every year, making this species very vulnerable to anthropogenic disturbance, pollution events and fishing activities.