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Titre :	The breeding population of sandwich tern in Denmark, 1993-2005
Auteurs :	Jens Gregersen, Auteur
Type de document :	Article
Dans :	Dansk Ornitologisk Forenings Tidsskrift (100(2) 2006)
Article en page(s) :	88-96
Langues:	Anglais
Résumé :	Since 1993, the number of breeding pairs has been estimated in Danish Sandwich Tern Sterna sandvicensis colonies by nest counts conducted in mid-late May by experienced counters. The results, reflecting minimum estimates of the actual breeding population, are presented in Table 1. In summary, 3700-4900 pairs distributed in 912 colonies have nested each year. As is the general rule for the species, the colonies are placed among breeding Black-headed Gulls Larus ridibundus. Several colonies have disappeared and others been established during the period, but some have been fairly stable and inhabitated during most or all of the study years.The most important of these stable colonies is the one situated on Hirsholmene i northern Kattegat, the history of which is known for more than 60 years. In the 1940s it numbered over 4000 nests on two islands (Graesholm and Kjolpen), but numbers decreased during the early 1950s because larger gulls (Herring Gull L. argentatus, Lesser Black-backed Gull L.fuscus, Great Black-backed Gull L. marinus) invaded the islands and started to breed in rapidly increasing numbers. Like the Black-headed Gulls, the terns abandoned the islands, disappearing entirely in the late 1950s except that a few pairs attempted breeding in 1960-61. But in 1967 the terns became established on another island in the group, Hirsholm, which because of its human population had no large gulls breeding. This allowed the settlement of thriving colonies, first of Black-headed Gulls and shortly thereafter of Sandwich Terns. The Black-headed Gulls numbered 32000 pairs in 1984 but have since declined to a few thousand.The problems facing the Sandwich Terns on Hirsholm are in part the same that threatens other colonies of the species in Denmark. Much fewer humans live on Hirsholm than in the past, and large gull species have immigrated and are becoming more numerous - a development partly caused by repeated, grave disturbances on Graesholm, where a Great Cormorant Phalacrocorax carbo colony is sought 'regulated', implying frequent and protracted visits during the breeding season. The numerous tourists visiting Hirsholm is another problem.A major threat to Sandwich Tern colonies elsewhere, but not on Hirsholm, is disturbance and predation from foxes. Denmark has a high population density of red fox Vulpes vulpes, and some will occasionally settle on islands close to the mainland, including islands with breeding terns. Feral mink Mustela vison, apparently becoming increasingly common in Denmark, may pose a similar problem for some colonies.The most efficient ways to safeguard the continued presence of breeding Sandwich Terns in Denmark appears to be control of predators (gulls, foxes) at the colonies, many of which are situated in protected reserves. Specifically for Hirsholm, regulation of visitors' access to certain parts of the island might be considered as a supplementary step.

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