by
Finn Salomonsen
Chief Curator of Birds
The Zoological Museum
University of Copenhagen
Copenhagen, Denmark
Most species of seabirds that regularly breed in Denmark are declining, for a variety of reasons: shooting; oil pollution; toxic chemicals; reclamation of land; collecting of eggs; disturbance at breeding sites by visitors, motorboats, camping, etc.; destruction by predators; and others. On the other hand, the numbers of certain other species are increasing as a result of climatic changes (six species), protection (three species), and increase in food supply (three species of gulls). In addition to breeding birds, a total of about 3 million birds occur in Danish waters as passage migrants or winter visitors. More than half of the European winter populations of a number of marine waterfowl species winter in Denmark. Large numbers of seabirds spend the summer in Danish waters, including several hundred thousand immature gulls and just as many molting waterfowl.
The seabird fauna of the Faroe Islands is very rich, the immense number of birds being attracted by the local abundance of macroplankton and fish. The seabirds are harvested by man, formerly by fowling (capturing and shooting), now primarily by shooting. Until about 1910, more than 400,000 birds were taken annually by fowling. The Faroese game act is now very restrictive, and most seabird populations appear to be almost stable. However, a census in 1972 indicated that common murres (Uria aalge) have declined by about 20% to a population of about 600,000. Shooting and snaring appear to be the primary causes of the decline; oil pollution and toxic chemicals do not seem to be contributing to the population decrease.
In Greenland seabirds provide an important source of human food; however, because of the increase in human population and in the use of guns and speedboats for hunting, and the absence of a game act, serious overshooting of seabirds is taking place. A new game act passed in 1977 should largely alleviate this overharvest. Oil pollution and toxic chemicals do not yet play an important part in influencing the number of seabirds, though offshore oil drilling is being initiated in West Greenland. A recently established gigantic national park, covering 200,000 km2 of ice-free land, is the largest nature reserve in the world.
The Danish Monarchy consists of three parts far removed from each other, scattered in the North Atlantic—namely Denmark proper, the Faroe Islands, and Greenland. They differ so much from each other in climate and in bird life that they must be treated separately in this paper. The Faroes possess a provincial government and also a sort of home rule. Greenland also has a provincial government, but all statutory provisions, including acts concerning hunting or wildlife protection, must be passed by Danish authorities, usually by the Ministry of Greenland.
Insofar as seabirds are concerned, it is important that Greenland is an arctic country, whereas the Faroes and Denmark are boreal. In both Greenland and the Faroes the breeding birds are most significant, from an ecological point of view, whereas in Denmark the passage migrants and winter visitors are far more important.
There are other differences as well. In Greenland and the Faroes the seabirds mostly breed in colonies on high and steep cliffs, and the structure of these breeding places is not disturbed by man. In Denmark, on the other hand, the seabirds usually breed on glacial deposits, now forming meadows, low islets, salt marshes, etc., and these habitats have unfortunately been largely changed in the last hundred years by draining and reclamation. This practice has taken place in Denmark on a much larger scale than in most other countries and has, therefore, to a high degree diminished the life conditions of seabirds.
Denmark is situated on the continental shelf of western Europe; all seas surrounding the country are shallow (less than 100 m deep), apart from the Skagerrak, north of Jutland, which is much deeper. The shallow depth, combined with the rapid flow of water between the Baltic and the North seas causes much upwelling, which forms excellent life conditions for plants and animals. It is well known that the fishery in Danish waters, especially in the North Sea, is very rich. This richness of the seas provides suitable conditions for a high diversity of seabirds and ecological types.
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).
| 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.
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.
A variety of factors tend to reduce the numbers of seabirds. The most important ones are outlined below, with comments on what has been done or what is expected to be done to reduce the impact of these activities on seabirds and protect this endangered resource.
The shooting of seabirds in Denmark is considerable, because the seabirds are extraordinarily numerous, and the number of sportsmen is very large, amounting to about 135,000 (a larger number per capita than in any other country).
The Danish game statistics are excellent—well known to be much more accurate than in most other countries (see Salomonsen 1954; Strandgaard 1964). According to Danish bag records, almost one million ducks, geese, and coots (Joensen 1974:31) and about 100,000-200,000 gulls (Salomonsen 1954:125) are shot each year. The average annual bag of each species of wildfowl is given in Table 2 and the open season for each species of seabirds in Table 3. The open season for dabbling ducks is long, extending from 16 August to 31 December, which means that local birds are persecuted almost as soon as birds-of-the-year are able to fly. This has resulted in a dabbling duck breeding population that is much smaller than what the available food supply could support, and in the large-scale development of artificial rearing of mallards for later shooting. A 5-month hunting season on specialized birds like loons, grebes, and various auks is not good management practice and should be carefully reviewed.
Four other important facts about the shooting of seabirds in Denmark merit inclusion here: (1) there is no bag-limit for any species; (2) in general, all marine areas within territorial limits are open to all Danish sportsmen, and the admission is free; (3) motorboats with a maximum speed of 10 knots are allowed for shooting in the period 1 October-30 April; and (4) the shooting of seabirds is permissible from 1.5 h before sunrise to 1.5 h (in December 1 h) after sunset, whereas for most other birds shooting is prohibited between sunset and sunrise.
Shooting is a national tradition in Denmark, and the large number of sportsmen has considerable political power. Too much influence is given to the representatives of the hunters' organizations, which have the decisive force in game committees dealing with protective measures. It is difficult, therefore, to change the existing system.
Shooting of seabirds, especially various waterfowl, is popular and intensive. The number of ducks taken by Danish sportsmen is probably in the order of 10-15% of the total kill on the West European Flyway (Joensen 1974:171). Excessive duck shooting can, in some cases, be controlled by banding in the breeding areas; the ensuing results then give rise to strong protests from the Scandinavian countries against the extensive persecution. As stated above, Denmark has (in relation to its size) the largest number of sportsmen of any nation in the world and the most intensive shooting. The number of sportsmen shooting ducks and shorebirds per 100 km2 is 278 in Denmark, 28 in Sweden, 37 in Finland, 10 in Poland, 83 in Holland, 164 in Britain, and 129 in Western Germany; the number of ducks shot per 100 km2 is 1,856 in Denmark, 39 in Sweden, 68 in Finland, and 129 in Western Germany (Nowak 1973). This shooting is undoubtedly of importance to dabbling duck populations, which are popular as shooting objects everywhere in Europe.
Insofar as marine ducks are concerned, it can be seen in Table 2 that appreciable numbers are shot in Denmark. The same is true for other Scandinavian countries, whereas shooting on the high seas is rather modest in most other European countries. The Danish bag undoubtedly makes up a significant proportion of the total number of marine ducks killed each year, but when the total number of ducks in European waters is considered, the shooting pressure in Denmark appears to be of only minor importance. However, the shooting, particularly when undertaken from motorboats, is so noisy and makes such a disturbance over large areas that the time for seabirds to rest and forage is significantly reduced. It must also be noted that the number of pleasure craft is steadily increasing in the present period of prosperity, and that increasing numbers of sportsmen will probably make use of the free shooting in territorial waters, since it is becoming more and more expensive to lease hunting areas.
To restrict seabird shooting, the Danish Ornithological Society has recently (1975) submitted a proposal to the Danish Government, of which the following points are relevant:
• The open season for dabbling ducks and geese should begin 15 September except for pintail (Anas strepera), shoveler (A. clypeata), wigeon, and pochard—species which should not be hunted until 1 October;
• the open season for all diving ducks, as well as for coot, should end 31 December;
• the open season for the great cormorant should be restricted to the period between 15 September and 31 October;
• murres, razorbill, great-crested grebe (Podiceps cristatus), and all species of loon should be fully protected;
• it should be prohibited to shoot from motorboats less than 1 km from the shoreline, as well as in certain narrow sounds and fjords;
• it should be prohibited to shoot from shooting-punts less than 100 m from the shoreline;
• it should be prohibited to sell waterfowl and shorebirds shot, except for eider ducks and mallards (Anas platyrhynchos); and
• no shooting should be allowed between sunset and one hour before sunrise.
Oil Pollution
Oil pollution incidents constitute one of the greatest dangers to seabird populations in Danish waters. The enormous masses of seabirds present in these waters throughout the year, combined with the fact that Danish waters contain some of the heaviest shipping traffic in the world would give rise to anxiety for oil disasters. The majority of all tanker traffic from the Atlantic and the North Sea to the Baltic passes through the Cattegat and the narrow straits of the Sound, the Great Belt, and the Little Belt, to supply a population of about 100 million people. Up to 100,000 ships pass through these waters each year, half through the Sound.
There have been severe oil pollution disasters every year since about 1935, accompanied by enormous mortalities of seabirds, particularly marine ducks. The Danish Game Biology Station, which has studied these disasters (Joensen 1972a, 1972b, 1973b), has noticed that the number of seabirds involved has increased in recent years, in spite of increased control by Danish authorities.
Unfortunately, it appears that small amounts of oil in the sea, originating from cleaning the tanks of vessels, or from the release of a few tons of oil, are enough to create mass mortality of seabirds when large concentrations of birds are present in the vicinity. Such incidents have passed unnoticed in spite of control measures. In no case has the source of the pollution been traced (Joensen 1972b:27). There has not yet been a real "oil disaster" in the Danish waters similar to the Torrey Canyon catastrophe. If such a disaster takes place, the destruction of seabirds will be enormous and immeasurable.
| Species | Oil incident no. | |||||
|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | Totals | |
| Gavia stellata | 1 | 9 | 1 | 4 | 15 | |
| G. arctica | 2 | 2 | 4 | 8 | 16 | |
| Gavia sp. | 4 | 1 | 5 | |||
| Podiceps grisegena | 4 | 1 | 8 | 8 | 21 | |
| P. cristatus | 1 | 1 | ||||
| Phalacrocorax carbo | 20 | 20 | ||||
| Anas platyrhynchos | 2 | 2 | 4 | |||
| A. clypeata | 2 | 2 | ||||
| Aythya marila | 6 | 2 | 8 | |||
| Clangula hyemalis | 35 | 2 | 26 | 6 | 4 | 73 |
| Melanitta nigra | 387 | 241 | 521 | 262 | 77 | 1,488 |
| M. fusca | 197 | 33 | 417 | 223 | 119 | 989 |
| Somateria mollissima | 1,683 | 1,081 | 947 | 1,713 | 19 | 5,443 |
| Bucephala clangula | 3 | 3 | 13 | 9 | 28 | |
| Mergus serrator | 48 | 28 | 28 | 2 | 106 | |
| Cygnus olor | 10 | 17 | 1 | 28 | ||
| C. Cygnus | 1 | 1 | ||||
| Fulica atra | 1 | 1 | 2 | 5 | 9 | |
| Larus sp. | 13 | 13 | ||||
| Alca torda | 1 | 12 | 1 | 14 | ||
| Uria aalge | 1 | 1 | ||||
| Cepphus grylle | 1 | 2 | 16 | 19 | ||
| Total birds examined | 2,380 | 1,362 | 1,996 | 2,324 | 242 | 8,304 |
| Estimated minimum number of birds killed | 10,000 | 5,000 | 12,000 | 15,000 | 1,500 | 43,500 |
| Percent of total birds contributed by three species[71] | 95.3 | 99.5 | 94.4 | 94.6 | 88.8 | 95.4 |
As a result of five of the major oil pollution incidents in the Cattegat from 1969-71, a total of 43,500 birds were killed, of which 8,304 were examined and enumerated (Table 4). Altogether, 21 or 22 species were involved, but 95% of all birds examined were diving ducks: common eider and black and velvet scoters. At present, it has not been possible to identify any decrease in the number of these ducks in Danish waters due to oil pollution. However, if these disasters continue, it can be expected that duck populations of northern Europe and the Baltic area will be severely reduced, and that an overall decline will take place from which the birds may not be able to recover.
A particularly disastrous year was 1972, when large numbers of ducks were killed as a result of rather small oil spills. A tanker disaster in March 1972 off the eastern coast of Jutland, in the northern Cattegat, and another in December 1972 in the Danish Waddensea, both took place in areas critical to major concentrations of sea ducks. A total of more than 60,000 birds were killed, of which about 95% consisted of the same three species of diving ducks mentioned above. These tragic events represent a further increase in the annual mortality of birds caused by oil, and there is reason to believe that a critical upper limit is rapidly being approached.
It appears, however, that the measures taken by pollution control and naval authorities have greatly improved in recent years. In January 1973, when a Polish merchant vessel collided with a Swedish tanker in the Sound, about 300 tons of heavy fuel oil were released into the sea. Several Danish and Swedish ships working in cooperation succeeded in dispersing the oil, and no serious effect on seabird populations took place (Joensen 1973b:118). It seems that the best way of cleaning up such oil disasters is through a mechanical removal of the oil, but this is a very expensive and difficult procedure.
Chemical pollution is probably the most ominous threat to seabirds at present. Since all toxic chemicals used in agriculture ultimately end up in the sea, and many large factories release their industrial wastes directly into the sea, the effects of this pollution on marine organisms is attracting a growing interest. Many students have worked on these problems, and the results that concern birds were summarized by Bourne (1972:205). It is known that organochlorine residues have been found in seabirds in all the oceans of the world, including Antarctic waters and Arctic seas (Bogan and Bourne 1972:358). The chemicals most often found in birds are DDE (a metabolite of DDT) and PCB's (polychlorinated biphenyls), a mixture of related chemical compounds often originating from industrial wastes. In addition, some mercury will always be found, sometimes in increased concentrations. The present restrictions on the use of DDT and PCB in Denmark have not yet resulted in a corresponding decrease in the amount of these pesticides in birds.
It is well known that marine pollution reaches a peak in the Baltic. This high level of pollution is reflected in seabirds. For example, analyses have shown that eggs from the colony of common murres on Christiansø in the Baltic contain about 100 times as much DDE and 50 times as much PCB as eggs of murres from the Faroe Islands in the Atlantic Ocean (Dyck 1975).
A similar difference exists in the mercury content in birds examined in the two areas. Feathers of a large sample of black guillemots and murres from the Cattegat and the Baltic had higher mercury levels than those from the Faroe Islands and Greenland. It is interesting that this difference existed over a hundred years ago, as evidenced by the analysis of feathers in museum specimens. The Baltic populations of both species show very significant increases in the mercury content in 1965-70, as compared with the values earlier in this century. Since 1970 there has been a sharp decrease in mercury content, and in 1973 the level was almost as low as it was early in the century. These results indicate that the strict control of mercury discharges enforced in Sweden has resulted in a quick recovery of nearly normal conditions in the Baltic (Somer and Appelquist 1974). However, recent studies by Koeman et al. (1975:286) appear to show that mercury does not accumulate to the same extent in seabirds as it does in seals.
High concentrations of chlorinated hydrocarbon residues accumulate in carnivorous birds and upset the normal breeding behavior by making the eggshells too thin and fragile to survive (Peakall 1970:73; Mueller and Leach 1974:289). In Denmark, shells of herring gull eggs from the Baltic population were thinner, lighter, and more heavily contaminated with DDE and PCB than were shells of eggs from other colonies (Jørgensen and Kraul 1974:173). This further emphasizes the pollution of the Baltic Sea.
Massive mortalities of common murres, such as the one reported in the Irish Sea in the fall of 1969 which was apparently caused partly by malnutrition and PCB poisoning (Parslow and Jefferies 1973:87), are unknown in Danish waters.
It should be added that the pollution of seawater with cadmium, so very dangerous for man, has been high in recent years owing to the increased use of this element in industry, but no analysis of its importance for seabirds in Danish waters has yet been made.
It should also be mentioned that pollution of fresh water in lagoons or lakes near the sea can often cause serious declines in numbers of certain seabirds. This is well illustrated by recent events in the sanctuary Nakskov Indrefjord on the island of Lolland. This landlocked fjord once supported numerous breeding populations of ducks, grebes, and terns, but in recent years a number of species (e.g., eared grebe; common teal; garganey, Anas querquedula; pintail; and black tern, Chlidonias nigra) have failed to breed and practically all other species have declined in numbers. The main reason for these changes is a severe pollution from the admission of raw sewage from tributaries (Bloch et al. 1972). After several outbreaks of botulism in recent years, procedures to improve conditions are now being developed.
The most dangerous threats to seabirds are those discussed above. Authorities are aware of these dangers and attempts are being made to improve conditions. Some results have been achieved in the combat against oil pollution, and the control of shooting is reaching an acceptable level. Game management agencies in Denmark and other Scandinavian countries (Norway, Sweden, and Finland) are cooperating on the request of the parliamentary body of the Nordic Council. If game biologists in these countries could agree on proposed changes in the game acts, owing to the marked decline of a number of bird species, the parliamentary basis for such a legal step would be absolutely certain.
However, it must be admitted that the impact of man on the environment is enormous, especially in a country like Denmark, which possesses no raw materials, and where agriculture has transformed the whole country. In such a country, the birds have to "face the music," and by this sharing of resources with man, they will inevitably decrease in number. It is the responsibility of biologists and politicians, without emotional biases, to find the balance between the requirements of the two spheres of interest.
Many other dangers that threaten seabirds, some of which are unrelated to human activities, are listed here.
• Land reclamation.—Reclamation of land has reduced extensive areas of shallow water, lagoons, marsh land, etc., from seabirds for foraging or breeding places. Draining and diking of coastlands, estuaries, and saltings have had the same effect. This activity is now almost stopped, as these projects are no longer subsidized by the government.
• Egg-collecting.—According to the present game act, collecting gull eggs is permitted until 24 May. This creates much disturbance on the breeding grounds, and eggs of terns and shorebirds are also taken. This practice should be halted. The "Bird Island Group" of the Danish Ornithological Society, in a symposium in 1972, prepared some rules for the protection of seabirds, among which is a proposal to stop egg-collecting.
• Common property.—The Nature Conservancy Act regards all land not fenced in, even small uninhabited islets, as common property. People have free access to such areas with the result that seabirds breeding in colonies, or separately on islands, are disturbed by visitors arriving by boat. At the same time, noisy motorboats, bathing parties, or camping visitors frighten the birds, making successful breeding almost impossible. Even ornithologists, bird-banding teams, and bird photographers add to the destruction. The "Bird Island Group" of the Danish Ornithological Society has proposed a general prohibition against visitors on important bird islands from 1 March to 15 July to protect the breeding seabirds.
• Destruction by predators.—Fox, ermine, and stone-marten do not play an essential role. Rats are more important, even on small islands, and have caused destruction of tern and gull colonies. Rat numbers do not decline until a severe winter with much ice occurs, or until high tide kills them all. Large gulls also cause a great deal of destruction, but crows and magpies are unimportant as predators in seabird colonies. Numbers of nonbreeding mute swans or greylag geese may sometimes be a nuisance, trampling eggs and nestlings in seabird colonies.
• Forestry practices.—The prevailing practice of the forestry industry in Denmark of not preserving old trees with holes has considerably diminished the breeding habitat of hole-nesting species like the common merganser. Artificial nest-boxes have now been established in several areas.
• Sea conditions.—During high water, or rough sea, salt water may flood colonies of breeding seabirds nesting on low islets, often reducing the production of young.
• Aircraft disturbance.—Disturbances are also caused by noise from jet aircraft flying low, especially in military training areas where air traffic may be heavy.
• Commercial fisheries.—Modern commercial fisheries are depleting so-called industrially important fish stocks such as sand eels (Ammodytes), herrings, and other small fish over large areas of the sea for the production of fish meal. This fishing has undoubtedly been the main reason for the decline in the number of terns—especially sandwich terns which depend on these small fish species for food.
• Unknown factors at sea.—Large numbers of pelagic seabirds, particularly fulmars, kittiwakes, and gannets, are washed up on the western coast of Jutland in certain years (e.g., 1959, Joensen 1961:212). These birds died at sea, for unknown reasons, and apparently as a result of food shortages or oil pollution.
The threats to seabirds mentioned above are all well known to conservationists, who are attempting to reduce the impact of these factors on seabirds where possible. Insofar as legal protection is concerned, it must be admitted that there are no marine sanctuaries in Denmark, although several discussions have taken place reviewing the possibility of establishing some in critical areas. There are, however, a number of sanctuaries on islands where seabirds breed. In the Sanctuary Act of 1936 these areas were called "Scientific Reserves" because they were the site of scientific investigations of bird life. All admission was forbidden, at least during the breeding season, and all shooting was prohibited, with few exceptions. These sanctuaries were administered by the government's Nature Conservancy.
The following Scientific Reserves are important for seabirds: Hirsholmene Islands (in Cattegat off Frederikshavn), Knotterne Islands (small islets east of Laesø Island), Vejlerne (diked in, landlocked fjords, densely covered with vegetation, at the Lim Fjord), Tipperne Peninsula and Klaegbanken Island (in Ringkøbing Fjord, western Jutland), Varsø Island (Horsens Fjord, eastern Jutland), and Græholm Island (Christiansø Archipelago, in the Baltic off Bornholm). A detailed description of these sites and their erection, bird life, and ornithological value was given by Salomonsen (1945). More recently, two additional Scientific Reserves have been established: Aegholm Islet (south of Sealand), and Hesselø Island in the southern part of Cattegat.
In addition to these scientific sanctuaries, there are game reserves and governmental forest reserves in Denmark. The game reserves are administered by the Ministry of Agriculture, which is also responsible for hunting legislation. The purpose of game reserves is to support and protect the stock of game, which includes migrating birds. Shooting is usually prohibited, but a restricted shooting season is allowed at some reserves. More than 50 game reserves are now present and functioning. Regulations differ widely from reserve to reserve, but entry to some of them is not allowed in the breeding season. Many reserves are important for breeding or migrating waterfowl and some seabirds. In fact, a total of 26 game reserves contain seabirds, the most important of which are the following: Ulvedybet (landlocked fjord at the Lim Fjord), Hjarbaek Fjord (landlocked fjord with brackish water at the Lim Fjord), Felsted Kog (landlocked fjord at Nissum Fjord), Jordsand (large stretches, almost 11,000 ha, of the Danish Waddensea), Stavns Fjord (at Samsø Island), Esrum Lake (in northern Sealand), and Kalvebod Beach (at Amager Island, near Copenhagen).
In the Nature Conservancy Act of 1969, differences between scientific and game reserves were abolished, although regulatory provisions that were in force for the scientific sanctuaries were maintained. Unfortunately, the amalgamation of the two types of reserve has given more power to the hunters' associations, which constitute the majority of the administrative body of the reserves, the so-called Game Commission ("Vildtnævnet"). However, any change in status of the original scientific reserves will not be tolerated by conservationists and other environmental groups in Denmark.
The number of seabirds in the Faroe Islands is greater than in any other region of the North Atlantic, and is closely related to the extraordinary richness of the plankton. The high phytoplankton production is due to a strong vertical mixing of the water in the northeast Atlantic, especially at the slopes of the submarine ridges, where both tidal currents and oceanic currents are usually strong. The resulting upwelling enriches the upper layers of water with large quantities of nutrient salts for the phytoplankton, and this, in turn, produces a teeming life of macroplankton and fish on which the seabirds are dependent (Salomonsen 1955).
The enormous seabird population of the Faroes is apparent from the first description of the islands, "De mensura orbis terrae," a document written in the year 825 by the Irish monk Dicuilus, who described the most characteristic feature of the Faroes as being the fact that "the islands were full of various kinds of marine birds." This richness has remained to the present, and has provided an important source of food for the resident human population, particularly in former times. There are few, if any, countries in the world in which wild-fowling and other exploitations of birdlife have played such a major role as in the Faroes. A number of elaborate and varied bird-catching methods were invented, and these have remained essentially the same for at least the last 500 years. Bird-fowling at great heights on precipitous sea-cliffs was a dangerous venture, and each year lives were lost. The main thing, however, was that food obtained from fowling meant life and death for local inhabitants and so was undertaken in such a well-balanced way that the seabird populations did not decrease or disappear. Some fowling still takes place, but on a reduced scale, since most men are now engaged in the fishery during the summer. Shooting is now of much greater importance than in former times.
The Faroese game acts (from 1897, 1928, and 1954) are very severe and show a broad consideration for birdlife. Practically all terrestrial birds, including shorebirds, are protected, and existing regulations permit people to catch or shoot only common murres, razorbills, puffins, shags (Phalacrocorax aristotelis), fulmars, gannets, parasitic jaegers (Stercorarius parasiticus), and gulls, as well as a few "pest" species like crows (Corvus corone) and ravens (C. corax). The legal right of fowling on a "fowling cliff" belongs to the registered owner of the land on which the cliff is situated. There are some sound restrictive laws for these cliffs. For example, shooting within 3.2 km of any seabird colony is prohibited.