Ynglefugle i Vejlerne - Dansk Ornitologisk Forening

226 Ynglefugle i Vejlerne
25.000 pairs. In years thereafter a rapid reduction took
place, and from mid-1960s to mid-1970s the population
was in the magnitude of 5000-6000 pairs.
The counts conducted by the Field Observatory since
1978 have exhibited large uctuations (Fig. 63, Table
35). During most of the period the populations numbered
4000-5000 pairs, with a marked culmination of more
than 6000 in 1986-89. The trend has been a decline during
the latest ten-year period; after 2000 not over 4000
pairs have been counted. Colony locations have moved
about a lot (Table 35). Some years there was a displacement
from Østlige Vejler to Vestlige Vejler; later on a reverse
movement took place, with c. 95% of the population
in Østlige Vejler in 1998-2003. It is, however, not a
question of returning to the same locations (where the
majority previously were in connection with the great
lakes, among others Lund Fjord and Selbjerg Vejler).
Now, Bygholm Vejle has (again) become more attractive
to Black-headed Gulls, especially north of Krapdiget after
its reestablishment in 1995. A signicant correlation
between population size and the water level in April has
been found for Bygholm Nord, and the six best years (in
terms of population size) were all during 1997-2002, the
same years that had the highest April water level.
At the island Melsig in Arup Vejle, formerly an important
locality during the whole 20 th Century, Herring Gull
(since c. 1960) and Cormorant (since 1991) colonies
have reduced the breeding space for other species, which
caused the Black-headed Gulls to reduce the colony size
at rst, and eventually give up this breeding site. In 1999
no breeding attempts were made at this island by this
species, for the rst time within living memory.
In the long term the large population reduction is the
result of changes in the utilization of the area, especially
that the former open grazed marshes in Bygholm Vejle
and around the large lakes in Østlige Vejler throughout
the 20 th Century gradually have been replaced by dense
reedbeds, in the extent that grazing has been stopped.
The presence of Black-headed Gulls in Vejlerne is
utterly important for a number of other breeding bird
species, primarily grebes, ducks, Little Gull and Black
Tern, which very often breed associated with colonies of
Black-headed Gulls, and accordingly, the decrease has
affected those other species. In planning the future management
of Vejlerne it is thus important to seek to create
optimal conditions for this species, e.g. by creating
new islands, deepening channels around existing islands
close to land, and maintaining and potentially enlarging
areas of open wet marsh with relatively deep water, by
mechanical mowing or by grazing.
Around 2000 the Danish Black-headed Gull population
was c. 110.000-125.000 pairs. The number breeding
in Vejlerne accounted for c. 3% of this national total, and
Vejlerne as a whole was the seventh-largest locality.
Common Gull did not breed in Vejlerne in the 1930s.
When it arrived is not known, but it was present prior
to 1950. In the 1960s the population had grown to c.
130-140 pairs, while it had declined to 85 pairs in 1973.
Since 1978 numbers have been rather uctuating, be-
tween c. 40 and more than 100, with the largest population
in the rst and last part of the covered period (Fig.
65), and with the majority breeding at Bygholmengen
and at Melsig in Arup Vejle.
The Danish Common Gull population was c.
25.000-30.000 in the mid-1990s, hence in a national
perspective the numbers breeding in Vejlerne are quite
insignicant.
Lesser Black-backed Gull has been breeding since
1991 among Herring Gulls on island Melsig in Arup
Vejle, with a maximum count of 18 pairs in 1997. The
establishment in Vejlerne is part of the species' expansion
in Jutland, where most "bird islands" have been colonized
during the last 30 years.
Herring Gull bred for the rst time in Vejlerne about
1960, and in the period until 1973 the population rose
to 90 nests on Melsig in Arup Vejle. Field Observatory
counts of this colony (Fig. 66) exhibited a stable level of
25-30 pairs 1983-90, followed by an increase to 250-350
pairs from 1997 till the end of the covered period. In addition
1-6 pairs have bred on Bygholmengen, irregularly.
On Melsig the Herring Gulls were controlled by laying
poison on the nests from 1976 to 1989. After discontinuance
of this practice the population responded by accelerated
growth, and in the 1990s the Herring Gulls managed
to hold their own in competition against Cormorants
about nesting space on the island.
Greater Black-backed Gull has bred in Vejlerne in
1964, 1997 and 1999-2003, with 1-5 pairs. The spe cies
is spreading in the whole Limfjorden area, and other localities
have been colonized during the 1990s. It can
hardly be expected, though, that a colony of this species
in Vejlerne will have the potential to reach any substantial
size.
Gull-billed Tern was a common breeding species in
Vejlerne, when the rst ornithologists visited the area
early in the 20th Century. Throughout the 1930s and
1940s the species was present, but reports do not suggest
a large population. During a visit in 1949 a colony
with more than 100 pairs (with Black-headed Gulls and
Little Gulls) was seen on Bygholmengen. There was still
a large population in 1963-66 with 50 pairs as the maximum
reported, but subsequently there was a rapid decline,
and the last breeding record was in 1968 with 2-3
pairs. Until 1982 records of "nomadic" birds continued
with a few most years, and an ultimate breeding attempt
was made in 1979.
The population trend of Gull-billed Tern in Vejlerne
has paralleled the development in the rest of the country,
where a drastic decline from originally 300-500 pairs
took place after 1950. Until mid-1960s Vejlerne constituted
one of the most important breeding areas for the
species. After mid-1980s the very few Danish breeders
have all been found in the Wadden Sea, and currently
the species is close to disappearing from Denmark as a
breeding bird.