Summary Population development of Common Tern Sterna hirundo in Denmark, 1970-2012 The Common Tern is an Annex I species on the EU Birds Directive. It is designated as a breeding species for 30 of the Danish SPAs. Knowledge about breeding occurrence of terns has been collected mainly in projects and surveys initiated and carried out by volunteers, in some cases in collaboration with the Danish Ornithological Society/BirdLife Denmark. The most substantial countrywide information about breeding occurrence of Common Terns has been provided by the Danish Tern Group, the Gull and Tern Group and the locality and atlas projects organized by the Danish Ornithological Society. Some localities have been followed on an almost annual basis by volunteers and/or by national or regional authorities. The Ministry of Environment initiated a national monitoring programme that ensured visits to a large number of potential breeding sites of Common Terns in 2006 and 2012. Furthermore, the Ministry hired Aarhus University to organize a national survey of breeding colonial coastal birds in 2010 and to build a database of existing data on breeding numbers of colonial coastal birds at all known breeding sites. To identify the most reliable data for each breeding site, we went through all data collated for Common Terns and evaluated the quality of the count data. The development in breeding numbers is given in Appendix 1 for 15 of the most important breeding areas; the location of these areas is given in Fig. 1. For many years these areas housed 75-80% of the Danish breeding population. Historical records suggest that the Common Tern possibly bred with several thousand pairs in Denmark in the first half of the 20th century, but numbers declined and the breeding population has not exceeded 1500 pairs since 1970. Nonetheless, the Common Tern has been widely distributed after 1970 with breeding colonies located along mainland coasts, on islets in fjords and offshore as well as on small islets in freshwater lakes (Fig. 3). We estimate that app. 1000 pairs were breeding in Denmark during 1975-1980 (850-900 pairs counted; Tab. 1). Breeding numbers subsequently increased, reaching 1470 pairs in 1988. But a decline began again already in the early 1990s (Fig. 2), reaching 1090 pairs in 1995, 855 pairs in 2000 and 530-670 pairs in 2006-2010 (Tab. 1). A sudden settlement of 150 breeding pairs on the island of Anholt in 2010 was surprising and led to the increase from 2006 to 2010. The long-term decrease was recorded in all parts of the country that had hosted large numbers of breeding pairs up until the first half of the 1990s (Tab. 1). Consequently, by 2006-2012 the species had more or less disappeared from larger parts of Jutland and Funen (compare maps in Fig. 3). Although the number of breeding colonies remained fairly stable (around 50 colonies, Tab. 5), the number of colonies along coasts decreased (Tab. 2), and the proportion of breeders nesting in coastal colonies declined from 85-90 % during 1980-1995 to 50-52 % in 2006-2010 (Tab. 3). The region Roskilde Fjord was the most important breeding area in Denmark during several periods between 1970 and 2007, but numbers decreased markedly after 1995 (Fig. 4). The decline in at least this fjord may have been related to a decrease in the production of fledglings. Thus a survey of breeding success during 1997-2002 indicated that only < 0.1-0.2 young fledged per pair despite high hatching success in most years (Andersen-Harild 1997, 1998, 1999, 2003). The mean annual frequency with which parents fed their young was < 0.60 times per hour during the three years with records, and this suggest that parents experienced low food availability (Andersen-Harild 1997, 1998, 1999). The other most important regions for breeding Common Terns have been the Danish part of the Wadden Sea, West Jutland, Limfjorden, Northern Kattegat and Lolland (Tab. 1). The causes of declines in these areas are generally unknown. However, there are indications that for some of the former key localities, declining numbers were related to increasing occurrence of red fox Vulpes vulpes. In recent years, Common Terns have bred in fairly high numbers and with high success on small islets (of which some are artificial) in freshwater lakes. We suggest that the conditions for breeding Common Terns can be improved in several parts of the country by controlling mammalian predators and managing the vegetation on key breeding sites, and/or by creating artificial breeding islets.
Dansk Ornitologisk Forenings Tidsskrift, 2013, Vol 107, p. 261-280