Ynglefugle i Vejlerne - Dansk Ornitologisk Forening

ut this positive development was followed by declines
that could be attributed to drainage and destruction of
wetlands, persecution of migrating Marsh Harriers, and
the introduction of pollutants such as DDT and PCB,
which had very adverse effects on productivity of raptors
and other top-predators during the 1950s and 1960s.
In 1967 Marsh Harriers were protected from hunting
year-round in Denmark, and the worst environmental
pollutants became subject to bans over a wider international
scale, including much of the Danish Marsh Harriers
y-way, and especially the latter is considered responsible
for the major increase in Marsh Harrier numbers
seen over most of Europe since then. The increase
in Vejlerne was somewhat delayed compared to other
parts of Denmark, suggesting local factors have been involved
as well. The population in the reedbeds of Bygholm
Nord 1978-2003 is strongly and positively correlated
to the water level in March-May (strongest correlation
in March, when birds arrive and settle in territories;
Fig. 40). Signicant correlations between the population
and precipitation in April and temperatures in March are
also evident, but weaker than those seen for the water
level inuence on numbers.
A negative correlation exists between the breeding
population and reed cutting intensity. The best correlation
is found with winter reed cutting 1½ years prior to
the breeding season, but correlations with reed cutting
immediately before, 2½ and 3½ years before breeding
has also been found.
The rst Marsh Harriers arrive between February 12
and March 31 (average date March 12). The period with
records of Marsh Harriers in Vejlerne is given in Fig. 41,
and a tendency to gradually earlier arrival as well as later
departure over the study period from 1978 to 2003 is
recognized. The arrival date is determined by temperature,
the warmer the earlier arrival. On average Vejlerne
has hosted Marsh Harriers 238 days per year, and the
length of the staging period is positively correlated with
the breeding population size. The breeding Marsh Harrier
population of Vejlerne is, despite its small size, composed
of birds with the longest staging period on their
breeding site in Denmark. The first fledged juvenile
Marsh Harriers has been seen between June 28 and July
17, on average July 6, and the birds of Vejlerne breed
earlier than in other investigated Danish populations.
Until the 1960s Vejlerne held more than 10% of the
Danish population. Later population growth was higher
outside than within Vejlerne, and in the mid-1990s
when the Danish population was estimated at 650 pairs,
Vejlerne only had 2-5% of these. With the local increase
in the population at Vejlerne after 2000, the proportion
of the Danish total might have increased as well, but
new national surveys that could document this is not
available.
Montagu's Harrier was previously a regular but rare
breeder in and adjacent to Vejlerne. The species disappeared
from the area in the late 1960s, apart from a single
breeding attempt in 1997.
Breeding birds in Vejlerne 221
Quail was rst recorded in the area around 1970. Since
1983 singing males have been recorded almost an nually
in the vicinity of Vejlerne. Large inter-annual uctuations
in numbers of birds involved is seen in Vejlerne
(Table 27), and is parallel to similar variations in numbers
in other parts of Denmark. After 2000 numbers
have remained generally high. Some years Vejlerne host
10-20% of all Quails recorded in Denmark.
Water Rails have been noticed for their nocturnal "choruses"
in Vejlerne since the 1930s, but only after 1978 attempts
have been made to actually quantify the population
size. The records compiled by the Field Observatory
staff in terms of mapped territories must, however, only
be considered as an index of population size, because
some parts of the reed beds are so large that the species
is practically impossible to count. The data available
nevertheless demonstrate a major increase in the population
size from 1978 to 2003 (Table 28, Fig. 42). Most of
the mapped population has been found in the three large
continuous reedbeds, i.e. Bygholm Nord, Selbjerg Vejle
and Tømmerby Fjord (with respectively 32%, 29% and
18% of the population).
The water level in the reeds of all three areas has major
importance for the population size which is positively
correlated with water levels in March-May (Table
29, Figs. 43-45). Hence most Water Rails are found in
springs with high water levels. The rails apparently are
also negatively inuenced by reed cutting as I found a
signicant and negative correlation between the population
size and reed cutting intensity in the three preceding
winters. The population also seems to be inuenced by
the winter temperatures prior to the breeding season, a
somehow surprising result, because the Rails are thought
to winter in milder regions of Europe (on either side of
the British Channel and in France).
It is likely that the true population of Water Rails in
Vejlerne is more than 1000 pairs. The size of the Danish
population is unknown, but Vejlerne is without any
doubt the single most important site for the species in
Denmark.
Spotted Crakes have been noticed in variable numbers
in Vejlerne since the 1930s, but systematic surveys
were not initiated before 1978 (Table 31, Fig. 47).
The observed uctuations in numbers are more dramatic
than that of any other breeding bird in Vejlerne. The
development in Vejlerne generally has been parallel to
that observed in the rest of the country, partly because
Vejlerne by far has the largest population, and this represents
40-60% of all singing Spotted Crakes recorded
in Denmark.
Most crakes have been mapped in swampy marshes
with tussocks in the transition zone from meadows to
reedbeds in Østlige Vejler. It is unknown how big a proportion
of the recorded birds have been true breeders.
The rst singing bird has been recorded between April 2
and May 26, on average April 17.