• 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.