| Species | Census, January 1973 | Estimated winter populations of the Western Europe Flyway | Average annual bag in Denmark |
|---|---|---|---|
| Anas platyrhynchos | 127,000 | 1,550,000 | 380,000 |
| A. crecca | 500 | 260,000 | 76,000 |
| A. querquedula | 11 | [67] | [68] |
| A. acuta | 100 | 70,000 | 13,000 |
| A. strepera | 5 | [67] | [69] |
| A. penelope | 3,000 | 485,000 | 44,000 |
| A. clypeata | 17 | 63,000 | 9,000 |
| Tadorna tadorna | 13,000 | 105,000 | [69] |
| Aythya ferina | 7,100 | 235,000 | 5,000 |
| A. fuligula | 94,700 | 530,000 | 35,000 |
| A. marila | 80,900 | 145,000 | 8,000 |
| Clangula hyemalis | 11,000 | [67] | 11,000 |
| Melanitta nigra | 148,100 | [67] | 18,000 |
| M. fusca | 6,700 | [67] | 9,000 |
| Somateria mollissima | 450,800 | [67] | 136,000 |
| Bucephala clangula | 67,000 | 142,000 | 25,000 |
| Mergus serrator | 11,700 | 40,000 | 8,000 |
| M. merganser | 23,200 | 75,000 | 6,000 |
| M. albellus | 206 | 5,000 | [69] |
| Cygnus olor | 48,900 | 120,000 | [69] |
| C. cygnus | 5,700 | 17,000 | [69] |
| C. bewickii | 1,113 | 6,000 | [69] |
| Fulica atra | 142,500 | [67] | 70,000 |
| Totals | 1,243,252 | 3,848,000 | 853,000 |
A comprehensive investigation of the nonbreeding waterfowl in Danish waters was recently undertaken by the Game Biology Station Kalø (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, undertaken in all winters since 1967, usually 1.0-1.5 million birds have been recorded. Since such censuses usually give minimum numbers, and certain species-especially marine ducks—generally go unrecorded, the normal winter population (November to February) of ducks, swans, and coots in Danish waters can scarcely be less than 2 million birds (Joensen 1974:156). In Table 2, bird numbers in Denmark are compared with the estimated winter populations in western Europe, based on the investigation of Atkinson-Willes (1972). When all the winter censuses in Denmark are compared with those for Europe, as was done by Joensen (1974:156), it is evident that Danish waters support about half of all greater scaup (Aythya marila), common goldeneye (Bucephala clangula), red-breasted merganser, mute, whooper (Cygnus cygnus), and tundra swans (C. bewickii) wintering in Europe; about one-third of the population of tufted duck (Aythya fuligula) and common merganser; and probably also one-third of the population of common eider and coot (Fulica atra).
The wintering population of common eider is very large. According to banding records it makes up the greater part of Baltic breeding birds; however, it is not possible to calculate its percentage contribution to the total European winter population since its size is unknown in most European countries. Although most of the surface-feeding ducks disappear from Denmark waters in winter, extremely large numbers occur there during the fall migration period. For example, it has been estimated that for species like common teal (Anas crecca) and wigeon (A. penelope) about one-third of the West European Flyway population passes Denmark in the fall. Possibly some of the surface-feeding ducks listed in Table 2 for January 1973 were recorded in fresh water and not from the seas, but at the time the census was taken most freshwater lakes were frozen and, therefore, unavailable for water birds.
These breeding seabirds and the off-season visitors do not constitute the total population in Danish waters. Large numbers also occur in summer as nonbreeding birds; most are in two categories: (1) several hundred thousand pre-adult (up to 4-5 years of age) gulls (mostly great black-backed, herring, and lesser black-backed gulls), which feed inshore or at the coast, and (2) large concentrations of waterfowl that carry out a molt migration in Danish waters, particularly in shallow areas. Black scoter (Melanitta nigra), velvet scoter (M. fusca), common eider, and whooper swan are especially numerous, totaling hundreds of thousands of individuals, and probably constituting the majority of the European molting populations of these species. Less numerous, but still totaling thousands of molting birds, are sheld-duck (Tadorna tadorna), common goldeneye, red-breasted merganser, and possibly some other diving ducks. About 3,000 surface-feeding ducks of various species, most of which undoubtedly are local breeding birds undergo wing molt in Danish waters. Comprehensive descriptions of the molt migration, particularly in Denmark, were published by Salomonsen (1968) and Joensen (1973a, 1974).
It may then be concluded that very large numbers of seabirds are found in Danish waters in all periods of the year; most feed in the inshore zone and some offshore, but none in the pelagic zone.
Increase of Seabirds
Seabirds are affected by several factors related to human activities, most of which pose a threat to them and will eventually reduce their numbers. Some factors, however, tend to increase bird numbers, like climatic changes which, as reported by Salomonsen (1963), have given rise to the immigration to Denmark of great cormorant (in 1938); eared grebe, Podiceps nigricollis (about 1870); red-crested pochard, Netta rufina (1940); common pochard, Aythya ferina (about 1860); tufted duck (about 1900); and common murre (1929). They all still breed in Denmark, having more or less increased in number.
Another reason for increases of certain species is legal protection. Among protected seabirds are the sheld-duck, which has been completely protected since 1931, and particularly the mute swan, of which only 2 or 3 pairs were breeding in Denmark when the species was completely protected in 1926. Since then, mute swans have increased enormously, reaching at least 2,740 pairs in 1966 (Bloch 1971:43), of which large numbers were breeding colonially on small islets of boulders or on sand reefs off the coast (Bloch 1970:152). The gannet has also increased considerably as a fall visitor since about 1945, apparently due to protection in England and other countries.
Finally, some gull populations have increased in size because of an increase in the food supply, consisting especially of wastes from commercial fisheries and garbage dumps. In Denmark, this unnatural food source has caused an enormous increase since about 1925 in herring gulls (from less than 500 pairs to 60,000 pairs), lesser black-backed gulls (all three subspecies, fuscus, intermedius, and graelsii have immigrated to Denmark), and great black-backed gulls (immigrated to Denmark in 1930). Improved waste disposal practices in recent years have not yet offset the rate of growth of these gull populations. The increase of common eiders, which also started in about 1925, is probably related to the increases in the larger gulls.