CONSERVATION OF MARINE BIRDS IN THE DANISH MONARCHY 



269 



Table 1. Estimated average number of breed- 

 ing pairs of seabirds in Denmark, based on a 

 census in 1970-72. (Data for terns from 

 Mardal 1974, and for other species from 

 Sten Asbirk and N. O. Preuss, personal 

 communications . ) 



the same habitat, usually mixed together, the 

 eider is probably dependent on herring gulls 

 for protection against predators. When the 

 ducklings are fledged, the herring gull acts as 

 a successful predator itself, but the eider 

 nevertheless maintains a close association 

 with herring gulls. 



More than 90% of the herring gull popula- 

 tion breeds on small islands, and a large pro- 

 portion occurs in a few large colonies. It never 

 breeds in freshwater localities, but is exclu- 

 sively found as a breeding bird in coastal habi- 

 tats. The population has particularly in- 

 creased in the last 5 decades, some colonies 

 reaching their maximum size in the 1960's. 

 Others are still expanding and occupying new 

 breeding grounds. Today the largest colonies 

 are found on the following islands: Saltholm, 

 20,000-40,000 pairs; Christians^ 9,000 pairs; 

 Hirsholmene, 2,500 pairs; Jordsand, 1,800 

 pairs; Samsb, 2,000 pairs; Hjelm, 1,500 pairs; 

 and the archipelago south of Funen, a total of 

 3,500 pairs in several colonies. 



Attempts have been made to reduce the 



breeding population of herring gulls at 

 Hirsholmene and Christians^ sanctuaries (in 

 1973 and 1974, respectively), to improve con- 

 ditions for other nesting seabirds. In 1969 the 

 Bird Strike Committee of the Royal Danish 

 Airforce also initiated a program to reduce 

 the number of herring gulls breeding on 

 Saltholm Island, which is near the Kastrup 

 airport in Copenhagen. Nests were sprayed 

 with a formaldehyde oil dye, which resulted in 

 a 33% reduction in population. In Christians^ 

 and Hirsholmene, where the adult breeding 

 birds were poisoned, the effect is not yet 

 known. 



The total number of seabirds occurring in 

 the Danish waters as passage migrants and 

 winter visitors is substantially larger than the 

 breeding population, because Denmark is 

 situated on a very important fall migration 

 route for seabirds from Scandinavia, the Bal- 

 tic countries, northern Russia, and northwest- 

 ern Siberia. Furthermore, the shallow waters 

 of the Danish seas (less than 10 m deep) that 

 occupy extensive regions bordering the coasts 

 are important feeding grounds for diving 

 ducks. Birds frequenting the seas outside the 

 breeding season include hundreds of thou- 

 sands, or probably millions, of gulls; 

 numerous ducks (especially diving ducks); 

 swans and brants, Branta bernicla; jaegers, 

 Stercorarius spp. (four species); loons, Gauia 

 spp. (four species); grebes, Podiceps spp. (four 

 or five species); gannet, Morus bassanus; 

 great cormorant; northern fulmar, Fulmarus 

 glacialis; common murre; razorbill; and other 

 species of alcids. To these should be added a 

 number of species of various seabirds, espe- 

 cially gulls, tubenoses, phalaropes, and others 

 which appear as casual or accidental visitors 

 and which are not further mentioned in this 

 paper. 



A comprehensive investigation of the non- 

 breeding waterfowl in Danish waters was re- 

 cently undertaken by the Game Biology Sta- 

 tion Kak> (Joensen 1974). Aerial surveys of 

 marine ducks indicate that a large percentage 

 of the ducks that winter in European waters 

 do so in the shallow areas of the Danish seas. 

 A census in January 1973 indicated a total of 

 more than 1.2 million birds (Table 2). In a 

 number of other countrywide surveys, under- 

 taken in all winters since 1967, usually 1.0-1.5 

 million birds have been recorded. Since such 



