Ynglefugle i Vejlerne - Dansk Ornitologisk Forening

The decrease of this population is supposed to be a
result of general changes in agricultural practices, but in
the case of Vejlerne it is a natural assumption, that the
very sudden disappearance from Bygholm Vejle after
1965 is due to the water level lowering that took place
when the new sluise was built, which also reduced populations
of other colony breeding species like Black-headed
Gull and Avocet.
Common Tern has been reported as a breeding bird in
Vejlerne since the 1920s, but prior to the Field Observatory
period there has been some uncertainty with respect
to identication of this species and Arctic Tern.
In 1978-2003 most years a population of 20-40 pairs
of Common Terns bred in Vejlerne, with an increasing
trend to 40-50 pairs in 1999-2003 (Fig. 67). The island
Melsig in Arup Vejle has been the main locality through
most of the period, but since 1999 the largest part of the
population moved to Bygholmengen, linked to the abandonment
of Melsig by Black-headed Gulls.
The number of Common Terns breeding in Vejlerne
accounted for 2-3% of the Danish population during
the latest national survey (18-32 pairs out of c. 1000 in
1993-96). Vejlerne constitutes one of the last remaining
stable breeding areas in northwestern Jutland.
Arctic Tern bred in Vejlerne before 1978, but because
of the widespread previous confusion regarding Arctic
and Common Tern identication, it is not until after
the start of the Field Observatory that we have a precise
knowledge of this species' occurrence as a breeding bird
in the area. Counts show an increasing trend in the population
till 102 it reached pairs in 2003 (Fig. 68). Almost
the whole population has been registered at Bygholmengen
(98%). There is a signicant correlation showing
that more Arctic Terns breed in years with a high water
level in May.
From a national population of 8000-9000 pairs the
Vejlerne number accounts for a little less than 1%.
Little Tern presumably bred commonly at Limfjorden
before the embankment of Vejlerne, but never after has
it been a common breeder in Vejlerne. Breeding pairs
were reported in 1964, 1965 (three pairs) and 1973 – all
at Bygholmengen – while after 1978 1-3 pairs have often
been mapped showing breeding behavior, especially at
Østerild Fjord near Arupdæmningen. However, no nests
have been found, nor hatched or edged young.
Whiskered Tern. During 22 to 25 May 1992 two pairs
of Whiskered Terns had taken residence at the periphery
of a Black Tern colony in Kogleakssøen. Both pairs
built nests, and copulation, food delivery and territorial
behavior towards the Black Terns was observed. Nevertheless,
the breeding attempts were hurriedly abandoned.
This is the only case when more than one Whiskered
Tern has been registered in Vejlerne. Denmark is outside
the normal breeding range of this species, but Vejlerne
has adequate habitat, should more "prospectors" arrive.
Breeding birds in Vejlerne 227
Black Tern did not encounter possibilities for breeding
in Vejlerne before some years after the embankment,
where freshwater fauna and ora had established in the
lakes. However, already in the late 1920s this species
was very common here, and from mid-1930s a population
estimate of 450-550 pairs was presented. Henceforth
no specic information is available until 1964-67, when
the population was estimated at c. 200-250 pairs. Soon
after, in 1970-73, it had been reduced to c. 100 pairs,
as an effect of the construction of the new sluice Centralslusen
in 1965, which lowered water levels in Østlige
Vejler.
After 1978 and through most of the Field Observatory
period the population has continued decreasing (Table
36, Fig. 69). It was at its lowest point in 1995-2000 with
fewer than or just about 30 pairs. Hereafter numbers increased
somewhat; in 2001-03 the population reached
40-46 pairs. Data for edged juveniles have been collected
since 1981, with substantial fluctuations. The
years 1990-95 were characterized by a very poor
breeding success (two years without any breeding result
whatsoever), but in 1996-99 young production increased
considerably. The last years of the covered period again
saw few edged juveniles.
Colonies have been located at many places; none of
the localities have been used every year (Table 36). Most
frequently used localities have been Kogleakssøen (20
years), Tømmerby Fjord (17 years) and Bygholm Nord
(9 years). No direct relationship between population size
and water level at any of these localities has been established,
but there was a signicant positive correlation
between the production of young per pair and the water
level in May in Bygholm Nord 1981-2003. Four of
the best years in terms of young production (all from
1997-2001) are between the ve years with highest water
levels.
Production of edged juveniles has had a measurable
effect on the population the following years. The material
has been analyzed for the connection between the
breeding population and the production of edged young
in six seasons previous to the actual breeding season (Tabel
37, includes results from monitoring 2004-06). Signicant
correlations were found for the number of edged
young 3-5 years earlier, which indicates that recruitment
to the breeding population at Vejlerne to a large extent
originates from local production of young.
A large proportion of the Black Terns in Vejlerne have
bred in association with Black-headed Gull colonies.
The 1996-2003 colonies with breeding success, however,
were located independently from colonies of other
species.
Through the 20 th Century Vejlerne has constituted one
of Denmark's most important localities for this species,
and since 1960 the percentage breeding here has been
c. 50%. In late 1990s the population reached a critically
low level, but the modest increase during the last years
could maybe give reason for a cautious optimism. Several
years with many edged juveniles since 1996 give
hope that the population in coming years may be stable
or even slightly increasing. The long life span of Black