Ynglefugle i Vejlerne - Dansk Ornitologisk Forening

ules in the Ministerial Order, especially when consi dering
that the lakes, marshes and meadows of Vejlerne
by no means are pristine habitats, but in fact are articial
but high quality nature pockets created and managed
by humans.
The Field Observatory was closed in late 2003. This
terminated a 25 years period with intensive documentation
of the breeding bird populations of Vejlerne, and
one might wonder, what happens now? One of the primary
goals of this paper is to document that there has
been, and still is, a major need for continued compilation
of information about the breeding birds of Vejlerne, and
of factors affecting them in what happens to be one of
the most important breeding areas for marsh and meadow
birds in Denmark – and for some species even in
Western Europe.
Annotated species list
The species are reviewed in systematic order.
The treatment of each species generally follows this
templet: a review of historical evidence of breeding
birds in Vejlerne, prior to 1978. Then follows a statement
of the popula tion size and trends during the Field
Observatory years 1978-2003, including a description of
methods applied in eld work and working up data. The
causes of any population trends are discussed, and for
species with enough material, different circumstances
like phenology and breeding success are examined in
depth. The last section addresses the relative importance
of Vejlerne in a national perpective for Denmark, and for
a few populations even at the international level. When
appropriate this part for some rare or endangered species
is also concluded with management recommendations to
secure a prosperous future for these birds in Vejlerne.
Abbreviations for localities used in some tables are
referring to a list on page 27.
Little Grebe has been a fairly rare breeder throughout
the study period. Half of the years from 1978 to 2003
are void of observations that indicate breeding attempts.
Small chicks have only been observed at a few occasions,
and most of these were from Han Vejle, fewer
from smaller open ponds within the reedbeds. Little
Grebes prefer small shallow ponds, usually with less
than 1 ha open water surface, and small sized lakes with
these characteristics are rare in Vejlerne. The total breeding
population of Vejlerne is estimated at 0-5 pairs.
Great Crested Grebe has been known as a common
breeder in Vejlerne since the 1930s. Prior to the Field
Observatory period only one attempt to count the total
breeding population has been done, in 1971 when c. 180
breeding pairs (nests and pairs attending chicks) were recorded.
This year a single colony at Arup Vejle held 89
nests in the immediate vicinity of a large Black-headed
Gull colony. The breeding population in 1978-2003 has
uctuated (Fig. 10). Truly comparable data have only
been compiled since 1989, when we introduced a permanent
counting procedure. The breeding population
in the rst years following this counting procedure was
Breeding birds in Vejlerne 215
the highest ever recorded, followed by a gradual decline
from 1991 to 1996, when the lowest numbers of the recent
15 years of survey was recorded. Since 1996 numbers
have gradually increased. It is likely that the total
population in the top-years around 1990 was more
than 200 breeding pairs. The distribution of the breeding
birds within Vejlerne has changed con si de rab ly during
1989-2003 (Fig. 11). Some of these rather dramatic
changes in local breeding populations can be linked
to changes in the distribution of larger Black-headed
Gull colonies. The gulls have abandoned both Vesløs/
Arup Vejler and Glombak, and Great Crested Grebe
numbers have declined. In contrast numbers of Great
Crested Grebes have increased at Bygholm Vejle, and
many of the new breeders have settled in association
with new Black-headed Gull colonies established after
1994, when the water levels were raised in that locality.
Increasing numbers in Tømmerby Fjord and falling
numbers in Østerild Fjord, however, can not be linked to
changing numbers of Black-headed Gulls because neither
of these two lakes has held any but small colonies of
Black-headed Gulls. In these lakes the population trajectories
are more likely linked to changes in feeding conditions
following changes in lake water quality, and associated
changes in sh populations.
In the 1990s the total Danish population was estimated
at slightly more than 4000 pairs. If considered
a single locality, Vejlerne is one of the most important
breeding sites for the species in Denmark, holding c. 5%
of the national total. The trends for the breeding populations
of Great Crested Grebes in the different localities
within Vejlerne suggest that the ecological conditions of
the lakes are unstable, hence the overall population of
Great Crested Grebes might be considered vulnerable.
An important factor for successful breeding amongst
Great Crested Grebes and other grebes is the presence
of Black-headed Gull colonies; hence creation of optimal
breeding conditions for the latter remains a high
priority.
Red-necked Grebe held a large population in Vejlerne
until the mid 1960s, probably 60 to 100 pairs. Following
the water level lowering in the Eastern Vejler after
1965, a major decline in numbers was observed, and by
1978-79 only 13-14 pairs remained. Since then the population
has been steadily increasing, and by 1999 reached
the highest recorded size of c. 140 pairs. The majority
of these bred in smaller lakes within the reedbeds
of the northern parts of Bygholm Vejle. Large year-toyear
variations in breeding numbers have been evident
throughout the 25 years with continuous monitoring (Table
4, Fig. 12). Parts of the apparent population decline
after 1999 might be an artifact due to poorer coverage,
because reed fringe mappings of nests were discontinued
in Bygholm Vejle after 1999. An important factor affecting
the breeding population of Red-necked Grebes is the
water level (Fig. 12), but winter temperatures also affect
the size of the population, in terms of a statistically signicant
and negative correlation between the breeding
population and a coldness index of the preceding Da