Seabirds regularly breeding in Denmark include five species of terns (common tern, Sterna hirundo; arctic tern, S. paradisaea; least tern, S. albifrons; Sandwich tern, S. sandvicensis; and gull-billed tern, Gelochelidon nilota); seven species of gulls (black-headed gull, Larus ridibundus; herring gull, L. argentatus; lesser black-backed gull, L. fuscus; great black-backed gull, L. marinus; mew gull, L. canus; little gull, L. minutus; and black-legged kittiwake, Rissa tridactyla); four species of geese, swans, and ducks (mute swan, Cygnus olor; greylag goose, Anser anser; common eider, Somateria mollissima; common merganser, Mergus merganser; and red-breasted merganser, M. serrator); three species of auks (black guillemot, Cepphus grylle; common murre, Uria aalge; and razorbill, Alca torda); and one species of cormorant (great cormorant, Phalacrocorax carbo). Shorebirds have not been included in this review. Some of the species mentioned are partly freshwater birds—for example, the black-headed gull, little gull, mute swan, greylag goose, and the two species of mergansers. The gull-billed tern forages in terrestrial habitats, but nests along the coast with the other seabirds. It is often difficult, therefore, to make a clear-cut distinction between seabirds and freshwater birds.
Among the auks, the black guillemot breeds in the Cattegat area in the huge heaps of boulders on small raised islets, or in holes (mostly formed by starlings, Sturnus vulgaris) on steep clayey slopes or promontories. The common murre and razorbill are restricted to the islet Graesholm in the Christiansø Archipelago, about 24 km east of Bornholm Island in the Baltic, where they breed on small cliffs of Precambrian granite rock.
The estimated number of seabirds of different species that breed in Denmark is shown in Table 1. Species like the mergansers, mute swan, and greylag goose, which breed partly or mostly in freshwater localities, are not included. Overall, the number of breeding seabirds is slowly declining, probably due to many factors which are discussed below. There are two exceptions, however, to this general decrease—the herring gull (and to a lesser degree the other big gull species) and common eider. Both species have increased during the last 50 years. Since they breed in 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.
| Species | Number of breeding pairs |
|---|---|
| Sterna paradisaea | 5,750 |
| S. hirundo | 900 |
| S. sandvicensis | 4,000 |
| S. albifrons | 600 |
| Gelochelidon nilotica | 105 |
| Larus marinus | 300 |
| L. argentatus | 60,000 |
| L. fuscus | 2,000 |
| L. canus | 28,500 |
| L. ridibundus | 135,000 |
| L. minutus | 25 |
| Rissa tridactyla | 125 |
| Phalacrocorax carbo | 600 |
| Somateria mollissima | 3,800 |
| Cepphus grylle | 325 |
| Alca torda | 400 |
| Uria aalge | 1,100 |
| Total | 243,530 |
More than 90% of the herring gull population breeds on small islands, and a large proportion occurs in a few large colonies. It never breeds in freshwater localities, but is exclusively found as a breeding bird in coastal habitats. The population has particularly increased 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; Christiansø 9,000 pairs; Hirsholmene, 2,500 pairs; Jordsand, 1,800 pairs; Samsø, 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 Christiansø sanctuaries (in 1973 and 1974, respectively), to improve conditions 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 Christiansø 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 Baltic countries, northern Russia, and northwestern 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 thousands, or probably millions, of gulls; numerous ducks (especially diving ducks); swans and brants, Branta bernicla; jaegers, Stercorarius spp. (four species); loons, Gavia 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, especially gulls, tubenoses, phalaropes, and others which appear as casual or accidental visitors and which are not further mentioned in this paper.
Table 2. Total numbers of ducks, swans, and coots recorded in Denmark during a winter census in January 1973 (based on ground counts and aerial surveys), compared with estimated flyway populations wintering in western Europe and annual bird harvest in Denmark (after Joensen 1974:23, 155, 168).