Vreemde vogels in de Nederlandse avifauna: verleden, heden en wat voor een toekomst

Strangers in the Dutch avifauna: past, present and how about the future In this paper the rise in numbers and distribution of alien bird species in the Netherlands is described. This group of alien bird species have settled or have tried to do so, with help of mankind (release or escape). Among these species a few are native to the Netherlands in winter, but their breeding here is (partly) linked to human efforts. Examples are White-fronted Geese, Barnacle Geese and Red-crested Pochard. In 1994, seven species had established distinct feral populations (see Lensink 1996). This will soon happen for five other species (table 1). Establishment of feral populations by eight species Is expected in the near by future. For another eight species, I expect successful settlement will never occur. The White-fronted Goose has been breeding in the Netherlands since 1980. After 1987 the number rose, mainly due to a prohibition on keeping geese in captivity for use as call bird in game sports. The species mainly breed along the great rivers (figures 1,2, 3). During winter, the Dutch birds mingle with the numerous geese from the arctic that winter in the Netherlands. After some incidental breeding attempts in the 19705, the Bar-headed Goose has been breeding in the Netherlands since 1985 (figures 4,5, 6,7). They mainly breed in the central part of the country. After breeding, some birds leave the Netherlands, probably with Greylag Goose, for Spain. A proportion of the birds stay along the rivers and in the SW-Netherlands. The Barnacle Goose breeds mainly in the SW-Netherlands. Some also breed along the rivers and elsewhere (figures 8,9). The first breeding pairs were found in 1986, and thereafter the numbers rose. Since 1970 during summer, escaped and wounded birds wander through the Netherlands (figure 10). In winter most of the birds mix with the geese from the high arctic. The Upland Goose attempts to settle along the rivers (figures 12, 13, 14, 15). In winter they gather in the riverine areas in the central part of the Netherlands. The settlement of the Red-crested Pochard in the 1940s probably originates from captivity. Certainly, in recent years, a small fraction of the 30-50 breeding pairs in the Netherlands are escapes. In winter a number of the birds stay in the Netherlands (figures 16, 17). The numbers seen each winter do not show any dear trend, although the numbers were quite low in the 1980s. The Feruginous Duck has bred incidentally in the Netherlands. These birds probably originated from captivity, but there is no detailed information or proof available. The majority of observations occur in winter, with no clear trend in the past 25 years (figures 18. 19). Since 1984 the Goldeneye has been breeding in the Netherlands (figures 20, 21). In the beginning It was thought that this was a natural invasion by the species, however this is thought doubtful today. Among professional wildfowl keepers it is a numerous species. Between the natural distribution range and the birds in the Netherlands (400 km) no breeding, not even incidental, occurs. The Ruddy Duck breeds occasionally in the Netherlands (figure 22). The range expansion from England to the Netherlands, which was foreseen more than 10 years ago, has not come about so far. There is no evidence of any threat to native species. The Red Partridge bred in the Netherlands only around 1975 (figure 23). Birds were certainly released for sports. Other species (table 1) have made breeding attempts, but no settlement has followed. In general, at this very moment, no negative effects of exotic birds species are observed. However, in the field population developement can progress very quickly, as was seen in the last decade. Most exotic bird species are not mentioned in any law concerning plants and animals. This might become a problem in the case of a rapid, harmful expansion by a species. An appeal is made for more governmental concern about the potential dangers of exotic bird species.