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S. SVENSSON, 2004 - Monitoring long term trends of bird populations in Sweden.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):123-130
MONITORING LONG TERM TRENDS OF BIRD POPULATIONS
IN SWEDEN
S. Svensson
ABSTRACT Swedish common birds have been monitored for more than
twenty years using territory mapping, point counts and a combination of
point counts and line transects. Most work has been based on the
participation of volunteers and arbitrary choice of plots and routes. Hence
there are geographical and habitat biases. Starting in 1996, a new sampling
system was introduced, representatively covering the whole country with a
grid of permanent routes with fixed positions. We report population trends
over the previous two decades. The Tree Pipit has declined in recent years.
The long term decline of the Starling has continued. The Chaffinch has been
very stable in all parts of the country. The two subspecies of the Willow
warbler show different trends: trochilus has declined recently whereas
acredula has not. Among the woodpeckers, Green Woodpecker and
Wryneck have declined but Great Spotted and Black Woodpeckers have
been fairly stable. The Stock Dove has declined drastically and the Wood
Pigeon remained stable. Both Lapwing and Snipe are strongly declining.
Department of Ecology, University of Lund, Ecology Building,
S-223 62 Lund, Sweden
INTRODUCTION
When monitoring bird populations, different census methods are used in different countries
and in different monitoring programmes. When the methods are standardized, so that the same
method is always used at the same site every year, it matters little which method is used.
Results that come from a mixture of methods are equally as useful as results from a single
method as long as a standard is maintained through time for each individual count. The main
weakness of most monitoring programmes is not the actual counting method but the sampling
strategy. In many programmes, e.g. the Swedish one, the sampling strategy has not been
considered seriously. The census sites have been chosen arbitrarily, usually by the census takers
without any rules. There is of course an awareness of the difficulties that may arise as a
consequence of the fact that the samples are not representative in terms of habitat and
geographical location. The North American Breeding Bird Survey randomly selects the starting
point and direction of a route within a one by one degree block. This stratified random sampling
ensures representative counts on a continental scale but does not eliminate habitat bias.
In this report I will first briefly describe the Swedish bird monitoring system, which in
1995 had been in operation for 25 years, then provide some examples from the summer point
counts, and finally discuss improvements of the sampling strategy now being introduced.
The Swedish bird monitoring system
Apart from several species specific monitoring programmes, mainly of rare or endangered
birds, the multi-species programmes may be grouped into four categories: (1) winter counts of
waterfowl (the IWRB counts in autumn and mid-winter), (2) migration counts (standardized
trapping at Falsterbo and Ottenby bird stations, and counts of visible migration at Falsterbo),
(3) winter bird counts (all species, point routes, five times every winter), and (4) breeding bird
counts.
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The breeding bird counts belong to three categories: (1) point counts by volunteers all over
the country since 1975, (2) territory mapping by volunteers all over the country since 1969, and
(3) territory mapping and 10 km combined point and line transects in eleven specially selected
"baseline" or "integrated" environmental monitoring areas (since 1980).
Distribution of point count routes in Sweden
The distribution of routes over Sweden is given in Fig. 1, both for summer and winter, and
the proportion of routes within different regions, compared to proportions of the total land area
for each region, are shown in
Table 1. It is evident that the number
Figure 1. The number of census routes in different
of routes is heavily biased in favour
parts of Sweden (left is summer, right is
of the southern provinces. The three
winter). The three regions used for some of
southern regions contain 73 % of all
the index calculations are indicated in the
summer routes and 83 % of all winter
summer map. The southern border of the
routes but they cover only 34 % of
northern region coincides approximately with
Sweden's land area. And even within
the southern limit of the boreal zone.
the regions, there is uneven coverage.
The three major population centers,
i.e. the Stockholm-Uppsala,
Gothenburg, and Malmö/Helsingborg
areas, host a much higher proportion
of routes than the rest of the regions.
Table 2 provides information
about the habitats of the routes in
comparison with the true proportion
of different habitats in southern
Sweden, where most of the routes are
located. The table clearly shows that
there is a strong habitat bias. Only
farmland is sampled in the proportion
that it covers. Deciduous forest is
greatly overrepresented in relation to
coniferous forest, and mires and
clear-cut forest areas are underrepresented.
The habitat bias is even more
prominent in the northern provinces,
which can easily be seen from the
route distribution maps (most
farmland is located along the coast; coniferous forest and mire predominate in the interior). The
Table 1. Proportion of routes in different regions of Sweden in relation to the proportion of
the total land area of Sweden in each region
Region
Northern Norrland
Southern Norrland and Dalarna
Svealand except Dalarna
Northern Götaland
Southern Götaland
Percentage of routes
Summer
Winter
4
4
23
13
23
32
30
26
20
25
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Percentage of land area
37
29
13
11
subalpine birch forests and alpine habitat in the western parts of northern Sweden are not
sampled at all. The concentration of routes in the most densely populated areas incorporates
another bias, that of proximity to urban areas, and also a road bias, since some routes are
censused with a car as a means of transport between the stops.
Temporal coverage and turnover of routes
The number of routes has increased since the start of the project in 1975. The number of
summer routes has increased from below 100 to more than 250. The number of routes declined
during the period 1981-'86 (Fig. 2A), primarily due to low recruitment activity and insufficient
feed-back to the volunteers. When the Nature Conservation Agency successively increased its
support of bird monitoring during the 1980s it became possible to supply all participants with
annual progress reports and newsletters, giving detailed results and lists of participants
acknowledging what they had done. This contributed substantially to recruitment.
Figure 2. Number and turnover of summer routes. (A) Total number of routes (black bars) and
number of routes common in adjacent years (shaded bars). (B) Proportion of routes
still active after one year. (C) Proportion of routes still active after five years. (D)
Correlation between change in number of routes in relation to the proportion of new
routes in the previous year.
Table 2. Division of the routes among major habitats in comparison with the percentages of
these habitats in Götaland and Svealand (the southern half of Sweden where the
majority of the routes are located).
Habitat
Coniferous forest
Deciduous forest
Farmland
Mire, heath, clear-cut forest
The routes (%)
38
30
23
9
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True values (%)
51
10
22
17
Since the population trends are estimated by a chain index based on the data from common
routes of successive pairs of adjacent years, the number of "effective" routes is lower than the
total number of routes (Fig. 2B). In 1975-'81 between 70 and 80 % of the routes were censused
at least two years in a row. This figure increased to between 80 and 90 % for the period
1982-'94. Hence, more than 80 % of all route counts have been used for analysis.
Fig. 2C shows the proportion of routes, started in a certain year, that were still active five
years later, thus quantifying a more long-term turn-over rate. The proportion of routes still
active after five years increased from only 30 % in 1975 and 1976 to a stable value about 65%
from 1982 onwards. The ten year turn over rate stabilized in a similar way at 55 %. Hence these
figures may be used as an estimate of expected route age.
If the change in number of routes is plotted against the percentage of new routes recruited
each year (Fig. 2D), one can see that the total number of routes has always increased if at least
30 % new routes has been added. If the number of new routes amounted to less than 15 % there
was always a decline in the number of routes. In the interval 15-30 % new routes, the changes
were small. Thus, the Swedish system of volunteer route runners must recruit that proportion of
new routes every year in order to replace discontinued routes. It should be noted, however, that
the Swedish system requires that the same person carries out the census every year. If a new
person takes over a route, that route is considered to be new. If old routes could be taken over
by new observers and maintained in the system, routes would be active longer.
RESULTS & DISCUSSION
Two different indices
I have only used chain indices based on common routes in adjacent years. But this index
may be calculated in two ways. The first method is to total the number of individuals each year
and then calculate the change between the two sums. The second method is to calculate the
change for each route, then report the average of all these changes. I have found that most often
the two methods give very similar results.
The example of Tree Pipit is given in Fig. 3 for two different geographical zones of
Sweden (cf. Fig. 1). The indices based on totals and on averages are almost identical. In
addition, the fluctuations in the two regions are also very similar, indicating that the changes
have been parallel over the whole of southern Sweden. The decline since the late 1980s is quite
alarming. The population size is now only half of the average level of the period 1975-'90. The
next few years will show whether it is an unusually steep but temporary drop or a long term and
more serious event.
The Starling (Fig. 4) is an example where the two indices do not agree. The Starling is
known to be declining in Sweden, particularly in the northern, boreal part of the country.
However, when I divided Sweden into two regions, it was only in the southernmost region
where a decline was evident when the index was based on annual totals of common routes. But
when using an index based on average change, the populations of both regions showed declines.
Such a difference could occur if routes with few Starlings show a stronger decline than routes
with many Starlings. If that were the case, the proportion of routes with Starling, must have
declined more in the northern than in the southern of the two regions. In fact, in the southern
zone, there was no decline at all of the proportion of routes with at least one Starling, whereas
that proportion declined very much in the northern zone.
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Figure 3. Population indices calculated by
two different methods for Tree
Pipit Anthus trivialis in two
different zones (see Fig. 1).
Figure 4. Population indices for Starling
Sturnus vulgaris for two different
zones (see Fig. 1), calculated in three
different ways.
Figure 5. Population indices for Willow
Warbler Phylloscopus trochilus in
southern and northern Sweden (see
Fig. 1).
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Chaffinch and Willow Warbler
The Chaffinch and the Willow Warbler are the two most abundant species in Sweden. The
Chaffinch (Fig. 6) is the most stable of all species, with no trend in any region of Sweden. The
larger variation in the northern part of the country may reflect the smaller sample there.
The Willow Warbler shows a different picture compared to that of the Chaffinch (Fig. 5).
The two southern zones 1-7 and 8-12 show the same trend, first an increase over several years,
then a decline. There is no significant difference between the two curves, so they have been
combined. But there is a clear difference between southern Sweden and the northern, boreal
zone. For the first fourteen years the two parts of Sweden had the same trend, a slow increase,
but then the northern population continued to increase whereas the southern population
declined. This difference is interesting because the two populations are different subspecies,
trochilus and acredula, distinguished by morphology, colour, migration routes and wintering
areas.
Woodpeckers
Among the forest birds, the woodpeckers are important components. Certain species have
proved to be sensitive indicators of the state of the forests, for example the Middle Spotted,
White-backed, Three-toed and Lesser Spotted Woodpeckers. The first species is now extinct in
Sweden, the second one has become extremely rare and the other two are declining. There are
four common woodpeckers, the Black, Green and Great Spotted, which are true woodpeckers,
and the Wryneck.
Fig. 7 shows the population trends of these species. For the Black and Great Spotted
Woodpecker, there is no clear evidence of long term change, whereas Green Woodpecker and
Wryneck have declined. I have included in the figures the proportion of routes where the
species has been recorded irrespective of numbers. These measures, whcih exclude some of the
excess variation between years caused by factors such as weather, calling activity and
phenological differences, show the same trends as the indices of the totals. The two species that
have not declined are primarily restricted to coniferous forests where they are able to excavate
nest holes even in healthy pines and spruces. Thus, as long as there is a sufficient number of
trees with a sufficient diameter, these species are able to cope with forestry. This suggests that
food is not a limiting factor.
The two declining species occur in deciduous woodland, the Green Woodpecker preferring
deciduous trees such as Aspen for nesting and the Wryneck is a secondary hole nester. They
also share a common trait in food choice, being ant-eaters to a large extent. My guess is that the
main factor for the decline of the Green Woodpecker is the declining amount of old deciduous
trees. The decline of the Wryneck is more difficult to explain. One factor could be the
expansion of forested areas, but this has not happened at the rate shown by the decline of the
Wryneck. The Wryneck differs from all the other woodpeckers in being a tropical migrant.
Hence, one cannot exclude the possibility that limiting factors operate in the winter quarters.
But in that case the Wryneck has reacted very differently from most other tropical migrants,
which have done well during the last twenty years.
The pigeons
The Wood Pigeon and the Stock Dove both migrate to about the same areas of
southwestern Europe. They differ in breeding ecology, the Stock Dove being a hole nester
whereas the Wood Pigeon is an open nester. From the mid 1980s, population trends of the two
species have been very different (Fig. 8). The Stock Dove has declined to a level below half of
what it was in the 70s, but the Wood Pigeon has remained at the same level.
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Figure 6. Population indices for Chaffinch
Fringilla coelebs for three different
zones and all Sweden.
Figure 8. Population indices for Wood Pigeon
Columba palumbus and Stock Dove
Columba oenas.
Figure 9. Population indices for Lapwing
Vanellus vanellus and Snipe
Gallinago gallinago.
Figure 7. Population indices and mean number of birds per route for four woodpeckers: Black
Woodpecker Dryocopus martius, Green Woodpecker Picus viridis, Great Spotted
Woodpecker Dendrocopos major, and Wryneck Jynx torquilla.
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As yet there is no clear explanation for this. It is unlikely that the number of suitable nest
holes has declined greatly, at least not at the rate shown by the decline of the Stock Dove. One
possibility is predation by the Marten. The evidence is partly indirect - a strong increase and
expansion of the Marten. More direct evidence comes from a study plot that has been censused
since 1965. This plot had a population of about 5-7 pairs of Stock Doves until 1988. After the
appearance of the Marten in the wood the population declined to 3, 2, 0, 1, 0, and 0 pairs in
1989-'94. This interpretation was supported by a parallel decline of the Jackdaw, from about 20
pairs in 1965-1988 to 14, 14, 1, 0, 0, and 0 pairs in 1989-'94. There was considerable Marten
predation on nest-boxes which had hinged roofs without locks, implicating Marten predation as
the cause of the Stock Dove decline in this plot. The fact that both the Stock Dove and the
Wood Pigeon winter in about the same areas seems to preclude differential mortality caused by
hunting or other winter factors. I suggest that the decline of the Stock Dove in Sweden has been
caused mainly by increasing predation by the Marten.
Farmland birds
Apart from a few species, most woodland birds, both resident and migratory species, have
done well during the last twenty years. The situation is very different for farmland birds. Most
of them are declining, for example Skylark, Linnet, Wheatear, Starling, White Wagtail,
Red-backed Shrike and Pheasant. The decline has been still more drastic for two wetland birds
that often breed associated with farmland, the Lapwing and the Snipe (Fig. 9). The Snipe
population has declined at an almost constant rate over the two decades, and the South Swedish
population is now only about a quarter of what it was in the 1970s. The Lapwing population
also declined rapidly until the mid 1980s but has since stabilised at a lower level. I believe that
the cause of these declines is drainage, both of true farmland and of woodlands. The drainage of
farmland has been much improved during the last decades when many of the old drainage
piplines were replaced by modern ones. Drainage of woodland is today a standard procedure
integrated with clear-cutting before new trees are planted. Populations of these species breeding
on mires in northern Sweden have not declined since the 1960s, though hard data about this
come from a small number of census plots.
A new bird monitoring system
I have focussed on the two main problems of the present monitoring system: the uneven
geographical coverage of the country and the low degree of habitat representativeness of the
individual plots. In order to create a satisfactory sampling design, a new way of distributing the
count sites will be introduced in 1996. There will be a basic system with one sample in each of
the 50 × 50 km squares of the Swedish national map grid, in total just below 200 samples.
Within each square there will also be three supplementary samples that may be used in regions
with many ornithologists, in areas where the habitat pattern is so patchy that one sample is not
sufficient, or in areas where for some other reason it is judged appropriate to have more than
one sample in a square. By this system we will achieve both goals, a more even distribution of
routes, and samples that are not biased by observer choice.
The counting method will be a combination of a point count and a line transect. A square
route, 2 × 2 km, will have the same standard location in all squares. There will be eight points
along the route, one kilometre apart, and consequently eight line transects, each one kilometre,
between the points. From each point, all birds seen or heard will be counted irrespective of
distance but excluding birds that have been counted from a previous point. From the lines, all
birds will also be counted irrespective of distance, excluding all double counts between
kilometre sections. This new system will not replace the old one with routes selected by the
observers, but it will be given priority and, if possible, financial support, particularly in
northern Sweden.
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