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W. HAGEMEIJER & I. TULP, 2004 - Monitoring meadow birds in the Netherlands: monitoring meets policy. In: Anselin, A. (ed.) Bird
Numbers 1995, Proceedings of the International Conference and 13th Meeting of the European Bird Census Council, Pärnu, Estonia.
Bird Census News 13 (2000):57-65
MONITORING MEADOW BIRDS IN THE NETHERLANDS:
MONITORING MEETS POLICY
W. Hagemeijer & I. Tulp
ABSTRACT. Several species of meadow birds breed in large numbers in
wet grasslands in The Netherlands. Wet grasslands make up 28-30% of the
total surface area of The Netherlands and are mainly used to graze cattle and
for grass production to serve as winter food. The combination of economic
and natural values poses high demands to management practices. In Dutch
Nature Conservation Policy, wet grassland is an important habitat to be
conserved, with meadow birds as one of its major assets. Since these areas
harbour a considerable proportion of the European breeding population of
many species of meadow birds, conservation of this habitat is of paramount
importance to maintain population sizes. Based on historical meadow bird
counts trends of six species of meadowbirds were calculated back to the
sixties. To unravel the influence of agricultural management schemes on
these trends were more closely evaluated for the period 1975-1992. Actual
densities were calculated from recent counts. The results indicated that all
species had higher densities in reserves and Environmentally Sensitive Areas
as compared to intensively used grassland. Trends for Black-tailed Godwit
and Redshank show a sharp decline until 1975 and seem to have stabilised
since then. Numbers of Ruff and Snipe are rapidly declining. Oystercatcher,
and Lapwing to a lesser extend, show an increase throughout the studied
period. We discuss problems with regard to the representativeness of the
data on which these calculations are based. Results should therefore be
interpreted with care. A new monitoring scheme for meadow birds, aiming
to result in a randomised and stratified sample is currently under
construction by SOVON
SOVON Dutch Centre for Field Ornithology, Rijksstraatweg 178,
NL-6573 DG, Beek-Ubbergen, The Netherlands
INTRODUCTION
In the last centuries natural grasslands in Europe (steppe, marshes), breeding ground to
several species of waders, disappeared due to human activity. Following deforestation and
agricultural developments these species started to inhabit new areas: grassland meadows kept
for the grazing of cattle or as hayfields. In the Netherlands these species have adapted very well
to this artificial situation and have been classified as 'meadow birds' ever since. Internationally
these species might be better known as (wet) grassland birds. Particularly in the North and West
large areas of grassland polders are used to keep dairy cattle. The low level, soil type of the
polders and the Dutch climate have hampered a good drainage for a long time. In the second
half of this century, management of agricultural grasslands has been intensified through
increased fertilization and water level control aiming at an increase in the productivity of the
land. Meadow birds have been able to adapt to these developments and increased their
populations, until the moment that the negative effects of intensification started to overrule the
positive effect of increasing biomass availability.
Beintema (1986) hypothesised that for each meadow bird species optimal feeding
conditions arise in the development from extensively to more and more intensively managed
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grassland. With increasing fertility of the soil, edible soil biomass for full grown birds (worms)
increases, resulting in (potentially) higher breeding densities. On the other hand changes occur
in the arthropod fauna, that cause a decline in the mean prey size and therefore chicks will have
more difficulty to find enough prey per time unit.
The optimum intensity of the agricultural management is different for each species. The
smaller species are thought to reach their optimum at lower management intensity than the
heavier species. The fact that Oystercatchers Haematopus ostralegus and Curlews Numenius
arquata, the heaviest among the meadow birds, have only recently (1950-1970, Hulscher 1972;
van den Bergh 1986; Beintema 1995) colonized the meadows as breeding habitat is explained
by the idea that sufficiently high biomass levels have only recently evolved.
The character that distinguishes Dutch polders from similar habitats in other countries is
that, being situated in the delta of some of the major rivers of Europe, the soil remains moist
throughout the nesting season. The soft wet soil enables adults and chicks to find food in the
soil, grass growth is retarded and meadows are only accessible for cattle and machines late in
the breeding season because of the limited mechanical carrying capacity of the wet soil. So the
danger of destroying nests or chicks by tramping or mowing stays limited.
The Netherlands harbour a substantial proportion of the total breeding population of
meadow birds in Europe. Percentages are given in Table 1. For all three species The
Netherlands hold the highest population size in Europe (excl. Russia for Lapwing)
(Hagemeijer & Blair 1997).
Table 1. Percentages of the population sizes of three species of meadow birds in The Netherlands.
Values are given as % of total European population and of European population
excluding Russia and of the EU. (European values after Hagemeijer & Blair 1997, EU
values after Beintema et al. 1995)
Species
Black-tailed Godwit
Limosa limosa
Oystercatcher
Haematopus ostralegus
Lapwing
Vanellus vanellus
Percentages of population sizes in The Netherlands
of European population
of EU population
Excl Russia (%)
Incl Russia (%)
(%)
63
36-58
86
38
33-36
56
18
2-5
33
Because drainage-control of the meadows improved and the fertility of the soil increased,
multiple grass crops per season became practice and farmers started mowing, on average one
month earlier. In addition, cattle density increased and consequently caused a greater risk of
trampling. These developments reduced survival chances of nests and chicks considerably.
Although meadow birds have partially adapted by starting to breed on average two weeks
earlier than in the beginning of the century (Beintema et al. 1985), they were not able to fully
compensate for the negative influences. This might be explained by the detrimental effects of
cold weather and low food availability early in the season.
New laws, enacted to reduce mineral emission, imply injection of manure into the soil. The
use of very heavy machines for this purpose further increases the risk of destroying nests and
chicks. Besides the changes in grassland management, a lot of grassland area has been turned
into arable land. The total amount of grassland surface area has declined remarkably during this
century (20-25 % decline).
In the Nature Policy Plan of The Netherlands, published in 1990 by the Dutch government, the
study project 'Future Perspectives of Meadow birds' was announced. Its goal was to develop a
model to estimate the changes in populations of breeding meadow birds in grassland areas in
The Netherlands in relation to changes in grassland management.
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The first phase of this project consisted of:
-collection of information on trends (1960-'92) in breeding numbers of meadow birds in
grassland areas, where possible specified by type of management. Both primary and secondary
meadowbirds (Beintema 1995) were subject of the study, with a focus on Black-tailed Godwit
Limosa limosa, Lapwing Vanellus vanellus, Redshank Tringa totanus, Oystercatcher, Ruff
Philomachus pugnax, Snipe Gallinago gallinago, and Curlew
-collection of information on actual densities (breeding pairs/km2; 1988-'92), if possible for
different management types and geographical location.
In this paper some trends and densities will be presented, as well as some examples of
modelling population sizes under different management scenarios.
DATA COLLECTION AND ANALYSES
The material used to reconstruct population developments consists of data from historical
counts. For some areas these counts started in the early sixties. For many species however
sufficient amounts of reliable information was only available from 1970 onwards. Population
developments are therefore shown starting in 1970. For more recent periods, data were
available systematically collected by volunteers within the scope of the SOVON (van Dijk
1996). Furthermore data collected in census work of provincial governments were used. Data
had to result from counts using standardized methods and the location and surface area had to
be exactly known as well as the years of counting and the bird numbers. Plots had to be counted
at least twice. The criteria used to select data for inclusion in the analyses are given in detail in
Hagemeijer et al. (1996). Selected data included both very small and very large areas.
The census areas were assigned into three different categories: (1) intensively managed
agriculture areas with no management restrictions, (2) farmland with voluntary restrictions, the
so-called Environmentally Sensitive Areas (ESA's), restriction here relate to late mowing and
reduced use of fertilizers and (3) nature reserves. Criteria used for this assignment are given in
detail in Hagemeijer et al. (1996). The assignment appeared to be very difficult because
management information was available only in a format that was incompatible format to the
format of the bird data. This has resulted in a rather large heterogeneity of plots within each
category (e.g. an 'intensive farmland' can contain up to 30 % of its surface being ESA or
reserve). Table 2 shows the number of plots and area sizes in the dataset per management
category, used for calculating the densities. Sample sizes for calculating year-indices are given
in Fig. 1.
Table 2. Number of census areas and their area per management category as used
for calculations of the current densities
Category
Intensive agriculture
ESA
Reserve
area (ha)
grassland (ha)
155 443
6 429
16 374
Number of areas
643
58
94
Total area
1 024 759
24 838
14 012
To calculate the current densities the most recent census per area was used, and densities
were calculated as the number of breeding pairs km2. National density per species was weighted
for the total area of the three different management categories in The Netherlands.
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Figure 1 : Year-indices for the six meadowbird species in the period 1970-1992. The dots
represent an average value for an average census area. Numbers are indexed and
presented relative to 1992. 95 % confidence limits are indicated by lines
Note the different scales along the Y axis.
Total number of areas and number of birds counted are indicated. Likelihood ratio
tests are performed to examine how well the model fits the data.
X2 values and degrees of freedom for each species are: Oystercatcher: X2 = 3713, df=
2203, p<0.001, Lapwing: X2 =10796, df= 2261, p<0.001, Ruff: X2 = 1131, df= 457,
p<0.001, Snipe: X2 = 967, df= 616, p<0.001, Black-tailed Godwit: X2 = 8628, df=
2239, p<0.001, Redshank: X2 = 3283, df= 2196, p<0.001.
Note the different scales along the Y axis.
Population indices of meadow birds were reconstructed by performing a loglineair poisson
regression on the data matrix (Ter Braak et al. 1994; Pannekoek & van Strien 1994).
Calculations were performed by the program TRIM 0.95 (Statistics Netherlands). This program
can calculates linear trends and year-effects, with the possibility to include covariates in the
linear as well as the year-effect model. The last model (year effects with covariates) requires a
very complete dataset however and was not applicable on this data matrix. The influence of
different management types was evaluated by adding the categories as covariates to the linear
model.
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TREND AND YEAR-INDICES
Year-indices for the six meadowbird species in the period 1970-1992 are given in Fig. 1.
Oystercatcher and Lapwing show a similar development: a constant level until the early eighties
followed by an increase of 30-50 %. Breeding numbers of both Ruff and Snipe decreased
dramatically especially in the seventies, but after that continued to decrease. The steep decrease
in numbers of Black-tailed Godwits in the early seventies stabilised later and even showed a
moderate increase. A similar pattern is found for Redshanks, although their numbers seem to
remain on a constant level in the eighties, rather than increase.
Figure 2 : Trends and year-indices for Black-tailed Godwit for two consecutive periods:
1960-'75 and '75-1992. The index for 1975 was set to 100.
The amount of information available for the Black-tailed Godwit allows to give year-toyear indices starting in 1960. Fig. 2 shows the trends and year-to-year indices for this species
when the data are split in two periods: 1960-'75 and 1975-'92. The reason for the split at 1975 is
that the management under evaluation (ESA, 'relatienotabeleid' in Dutch) was implemented in
1975. The early period is characterized by a marked decline (100 %, index change from 200 to
100). The latter period shows a slight increase (max. 15%, index change 100-115). Note the
difference in the width of the confidence interval. This is due to the fact that the latter period
contains a much smaller proportion of missing values as compared to the earlier period. Indices
differ somewhat between Fig. 1 and Fig. 2. This is the result of the fact that a larger dataset is
used for Fig. 2 and to the fact that is Fig. 2 the index for 1975 was set to 100, whereas in Fig. 1
this was done for 1970. The overall trend is similar however.
The effects of different management categories could only be analysed using the linear
model. Insufficient data were available per category to calculate year effects. Fitting a linear
model with management as covariate to the data for the 6 species of Fig. 1 results in trends
shown in Fig. 3. For the Godwit the 'national' trend (NL linear) shows a moderate increase (the
same as in Fig. 2, after '75). In the intensively farmed areas the trend is nearly horizontal,
whereas the categories ESA and Reserve show an increase. Lapwing shows a very similar
picture to the Black-tailed Godwit (ESA and Reserve better than intensive). The development in
Oystercatcher numbers shows a large increase in reserves and an increase of 20 % in other
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Figure 3 : Linear trends for Black-tailed Godwit, Lapwing, Oystercatcher, Ruff, Snipe and
Redshank in the period 1975-'92, discriminated by management category.
The output of TRIM (model 4) is indexed. The index for 1975 was set to 100. (The
lines merely show the extent of the change over the period, they do not indicate the
relative abundance between the categories).
Wald tests were performed to indicate the significance of the covariate.
Oystercatcher X2=166, df=2, p<0.0001; Lapwing X2=286, df=2, p<0.0001; Ruff
X2=12, df=2, p<0.005; Snipe X2=45, df=2, p<0.0001; Black-tailed Godwit X2=54,
df=2, p<0,0001; Redshank X2=3.1, df=2, n.s.
categories. Redshank shows no significant differences between the categories. Ruff and Snipe
show sharp declines, Snipe less steep in reserves.
DENSITIES
Densities of meadowbirds for The Netherlands are given in Table 2. Lapwings and Blacktailed Godwits are the most common meadowbirds in Dutch grassland areas. Oystercatcher and
Redshank show moderate densities, while Ruff and Snipe are very rare breeders. Curlews have
only recently started to colonize grassland areas, their common breeding areas are found in the
dunes, and occur in low densities. The results for the density calculations carried out per
management category are given in Table 3. All species show highest densities in reserves or in
ESA's.
Results are presented in detail in Hagemeijer et al. (1996).
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Average densities per km2 in the Netherlands, weighted for area of each management
category as present in 1992.
Species
Oystercatcher
Lapwing
Ruff
Snipe
Black-tailed Godwit
Curlew
Redshank
area(ha)
176 528
177 046
99 848
103 235
177 714
92 128
176 472
n
area (ha) density
Average interval
95%-conf. interval
n (ha)
Surface area
Reserve
95%-conf. interval
ESA
Average density
11.4
23.9
0.1
0.5
17.3
10.5-12.4
22.2-25.5
0.1-0.2
0.3-0.7
15.9-18.7
640
641
389
414
646
154 543
154 996
81 340
84 670
155 663
18.4
36.3
0.1
0.9
33.1
15.3-21.6
30.7-41.9
0.1-0.3
0.4-1.5
26.1-40.2
58
59
54
58
59
6,362
6,429
5,820
6,362
6,429
26.2 12.8-39.5 94
37.4 31.6-43.2 94
0.7
0.3-1.2 74
2.2
1.2-3.3 75
33.0 28.1-37.9 94
15 621
15 621
12 688
12 202
15 622
0.6
6.5
0.4-0.8
5.9-7.2
393
638
78 665
154 571
0.6
12.3
0.2-1.0
9.6-15.0
46
59
5,086
6,429
1.9
9.9
8 376
15 472
0.6-3.2
7.6-12.2
(ha)
Species
n areas
792
794
517
547
799
499
791
Average densities and 95 % confidence intervals per km2
for the three management categories.
Intensive agriculture
Oystercatcher
Lapwing
Ruff
Snipe
Black-tailed
Godwit
Curlew
Redshank
95%-conf.
10.6-13.0
22.5-26.2
0.1-0.2
0.3-0.7
16.3-19.5
0.4-0.8
6.0-7.5
interval
Table 4.
Average
11.8
24.3
0.2
0.5
17.9
0.6
6.7
density
Table 3.
60
93
DISCUSSION
ESA and reserves seem to be effective measures for the conservation of some species of
meadowbirds. The highest densities are found in areas with most restrictions on agricultural
use. This finding cannot be solely contributed to the difference in management. The selection of
areas appointed to become a reserve is by density of meadowbirds. So, even before the adjusted
management becomes into practice differences in densities already exist. The fact that the
trends in population size are more positive in reserves is a better indication for the effectiveness
of the management. Although the question remains whether this is not also a result of better
conditions (source-sink differences).
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For Black-tailed Godwit and Lapwing trends are more positive in ESA and reserve areas
than in intensively farmed land (Fig. 3). For Snipe there is a smaller decline in reserves than in
other management types. ESA seems to be less effective for this species. Ruff shows a decline
in all categories and the analysis indicates a significantly better situation in intensively farmed
land as compared to ESA and Reserves. Numbers are extremely low however and we consider
this result an artefact. For its conservation in The Netherlands, the Ruff is almost totally
depending on the population in reserves.
The representativeness of the data used for calculating year-indices and trends is a serious
problem. The data do not result from a stratified, random sampling effort. They turn out to be
heavily skewed towards the better areas for meadow birds, especially so for the category
'intensive'. This is the result of volunteers choosing their own census plot. The consequence is
that areas with low numbers of birds are underrepresented. For ESA and Reserve this
presumably does not seriously hamper the assessment of national trends but for 'intensively
farmed land' results present a too positive picture.
The same problem arises in calculating the densities. The intensive agricultural areas are
not represented in the dataset. Most areas that were censused represent the better meadowbird
areas of The Netherlands. Therefore the presented densities show a too optimistic picture. These
figures are therefore not suitable for extrapolation on a national scale. In order to be able to
develop policies for the conservation of grasslands in general and meadowbirds in particular, a
prediction of future population sizes is calculated using adjusted densities. Professional
judgement values were used to substitute the figures for densities in intensively managed
agricultural land (without 'meadow bird friendly' measures). The results are given in Table 5.
Estimates of population size on basis of the densities for 'intensively farmed land' gave much
higher numbers for most species as compared to other sources (Hötker 1991, Hustings 1992).
Table 5.
Modelling population sizes of Black-tailed Godwit. Using densities from the above
study for ESA and reserves and professional judgement values for intensively farmed
land, population sizes are calculated for 1995 and for the future, after full realization of
goals for surfaces of ESA and reserve.
For goals see Den Boer 1995 (adapted for division into high and low).
Intens. high = Intensively farmed land in the higher parts of The Netherlands (dry,
sandy soils in E of country);
Intens. low = same for low parts ('wet', clay and peat, W of country).
Prot. nest = nest protection by volunteers against impact of agricultural practices.
Intens
high
Densities
(n/100 ha)
Model 1995
area (ha,
x1000)
pop size
Model 'goal'
area (ha)
pop size
Intens
low
Prot.
nest
ESA
Reserve
3
7
18
36
57
535
500
125
12
13
16 100
35 000
22 500
4 300
7 700
228
200
425
80
60
6 840
14 000
76 500
28 800
34 200
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Total
85 600
161 140
Policies are based on, and evaluated by the use of simple calculations like this. It is of
paramount importance therefore to be able to retrieve reliable density and trend information.For
the interpretation of these figures, e.g. in policy documents one must keep in mind some
potential drawbacks. For instance, densities in reserves are generally high, but new reserves will
be obtained in sub-optimal areas since the best are reserve already and will therefore not hold
the same densities.
In order to be better able to assess representative trends and densities in the future, a
national monitoring scheme for meadowbirds is under construction by SOVON. Important
parameters to be measured in this scheme are (1) changes in numbers, (2) reproductive success
and (3) survival and population structure. In order to be able to answer questions regarding the
effectiveness of management and conservation measures, it is crucial to follow a stratified
approach, stratifying the sample according to the parameters to be measured and the factors to
be analysed. The Dutch government partially finances the scheme under construction.
Hopefully, in the near future, we will be able to calculate reliable trends and figures for most
meadow bird species. More importantly these figures then ought to be used in studies
supporting policy documents in order to develop cost-effective plans for meadow birds
conservation.
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