Reconstructing trends in bird population numbers by integrating data and information sources.

Abstract

From a scientific but also policy perspective it is important to know long-term changes in a population. However, in only a few countries standardized monitoring schemes allow us to reconstruct population trends which cover periods of at least half a century. Although ‘official’
monitoring schemes deliver trend indices covering a quite recent period often we do have various other sources of data and information. We present a method to use and integrate these sources.
In the case of a reconstruction of Dutch breeding bird population numbers since 1915 we used as common denominator the yearly estimate of the population. The main sources of information were monitoring scheme data, old repeated census data, atlas data and expert judgement. We used an estimate for the total population in a certain year for which we also had a (relative) population index in order to ‘translate’ all relative numbers into absolute population numbers. This resulted in a completely filled year-species matrix for 1960 to 2013 and estimates for 1915 and 1950. The results show a remarkable increase of the number of species since 1915. Population
numbers developed very contrastingly between habitats. Numbers in open habitats like farmland, heathlands and dunes dropped dramatically and numbers in heavily vegetated habitats like
marshlands, shrubs and forests increased and more or less compensated for the losses. The total biomass however has increased with almost 50 % since the 1950s, the average birds gets heavier. The underlying reasons involve very obvious factors like the change in land cover and land use. Intensification of farming activities and loss of total area resulted in a huge loss of farmland birds.
Forest areas have expanded since the start of the 20 th century and are on average older resulting in growing forest bird communities. Urbanization resulted in a rapid increase of generalists. At
least partly, effective conservation measures, explain the ‘comeback’ of large species that were hunted or persecuted until the early 1950s. There is also a likely correlation with fertility in major habitat types like farmlands and wetlands, large herbivores and piscivores profited from this.

Full text

80 VOGELWELT 137: 80–88 (2017)
1. Introduction
Determining a population trend for a particular species
is one of the most used and practical ways to describe the
fate or ‘health state’ of a population. Population trends
can be regarded as the result of dynamics in under-
lying demographical processes and are considered a
good indication of the state of a species (G
2007,  T 2011). For a limited number of
taxonomic groups meanwhile large-scale monitoring
schemes are developed and implemented in order to
infer population trends on national levels. For birds,
these schemes exist already for decades in many parts of
Europe (see www.ebcc.info). anks to the involvement
of thousands of voluntary observers (citizen scientists)
in the Netherlands, we have sucient data to calculate
reliable national population trends ( T
2011). It is obvious that the power to detect changes
increases with the length of the time series (B et al.
Reconstructing trends in bird population numbers by integrating
data and information sources
Ruud P. B. Foppen, Chris A. M. van Turnhout, Arend van Dijk, Arjan Boele, Henk Sierdsema &
Fred Hustings
Foppen, R. P. B., C. A. M. van Turnhout, A. van Dijk, A. Boele, H. Sierdsema & F. Hustings
2017: Reconstructing trends in bird population numbers by integrating data and information
sources. Vogelwelt 137: 80–88.
From a scientic but also policy perspective it is important to know long-term changes in a
population. However, in only a few countries standardized monitoring schemes allow us to
reconstruct population trends which cover periods of at least half a century. Although ‘ocial’
monitoring schemes deliver trend indices covering a quite recent period oen we do have various
other sources of data and information. We present a method to use and integrate these sources.
In the case of a reconstruction of Dutch breeding bird population numbers since 1915 we used
as common denominator the yearly estimate of the population. e main sources of informa-
tion were monitoring scheme data, old repeated census data, atlas data and expert judgement.
We used an estimate for the total population in a certain year for which we also had a (relative)
population index in order to ‘translate’ all relative numbers into absolute population numbers.
is resulted in a completely lled year-species matrix for 1960 to 2013 and estimates for 1915
and 1950. e results show a remarkable increase of the number of species since 1915. Population
numbers developed very contrastingly between habitats. Numbers in open habitats like farmland,
heathlands and dunes dropped dramatically and numbers in heavily vegetated habitats like
marshlands, shrubs and forests increased and more or less compensated for the losses. e total
biomass however has increased with almost 50 % since the 1950s, the average birds gets heavier.
e underlying reasons involve very obvious factors like the change in land cover and land use.
Intensication of farming activities and loss of total area resulted in a huge loss of farmland birds.
Forest areas have expanded since the start of the 20th century and are on average older resulting
in growing forest bird communities. Urbanization resulted in a rapid increase of generalists. At
least partly, eective conservation measures, explain the ‘comeback’ of large species that were
hunted or persecuted until the early 1950s. ere is also a likely correlation with fertility in major
habitat types like farmlands and wetlands, large herbivores and piscivores proted from this.
Keywords: monitoring, breeding birds, trend reconstruction, Netherlands
2004). Also from a scientic and policy perspective it
is important to know how populations have evolved
in the long term. Analyses to detect underlying causes
and processes of change are more powerful if the time
series are longer. For nature conservation and policy
purposes oen comparisons with references in the past
are needed, e.g. as reference criterion for Red Lists (see
i. e. www.iucnredlist.org).
Population trends are usually depicted as (rela-
tive) indices, with one year chosen as reference (usu-
ally index = 100) and population sizes in other years
expressed relative to this base year. However, for many
purposes absolute population estimates are essential.
A degree of rareness will be largely dependent on
absolute population number and this for instance is
an important criterion of Red Lists. Determining or
estimating reliable absolute population numbers across

VOGELWELT 137: 80–88 (2017) 81
a whole bird community (from rare to common spe-
cies) involves a good understanding of distribution and
densities. Modern atlas projects supply these data and
oen one of the nal results of an atlas project are esti-
mates of the national population sizes (SOVON 1987,
2002). National atlas projects are, however, repeated
at best with large time intervals and so this will not
give us estimates of absolute abundances annually. Here
Data source –
Datenquelle
Explanation – Erklärung Number of species
concerned – Anzahl
der betroenen Arten
Reference
– Quellen-
angabe
Monitoring scheme data
breeding birds, 1990-cur-
rent –Monitoringpro-
gramm Brutvögel, 1990 bis
heute
is monitoring scheme delivers indices per year for
almost all regular breeding birds. Reference year is 1990.
– Dieses Monitoringprogramm liefert jährliche Index-
werte für nahezu allen regelmäßig brütenden Vogelarten.
Referenzjahr ist 1990.
190 B et
al. (2016)
Monitoring scheme data
breeding birds, 1984-1989
–Monitoringprogramm
Brutvögel, 1984-1989
Early years of monitoring scheme delivers reliable indi-
ces for common species – Die frühen Jahre des Monito-
ringprogramm liefern belastbare Indexwerte für häuge
Brutvogelarten
105 B et
al. (2016)
Repeated surveys 1960-
1984 –
Wiederholte
Erfassungen 1960-1984
A time series analysis conducted in 2004 based on col-
lated repeated surveys in sample areas resulted in indices
with variable length. An internal review process selected
the time frame considered reliable. – Eine 2004 durch-
geführte Zeitreihenanalyse basierend auf zusammengetra-
genen wiederholten Erfassungen von Probeächen lieferte
Indexreihen von unterschiedlicher Länge. Eine interne
Begutachtung wählte als belastbar erachtete Zeitreihen aus.
113 SOVON
(2002),
 T-
 et al.
(2010b)
Atlas project 1973-1977 –
Atlasprojekt 1973-1977
e rst breeding bird atlas. Delivered population esti-
mates for all species. Year for estimate set at 1975. – Der
erste Brutvogelatlas lieferte Populationsschätzungen für
alle Arten. Als Jahr der Schätzung wurde 1975 festgelegt.
198 T
(1979)
Atlas project 1978-1983 –
Atlasprojekt 1978-1983
All-year round atlas. Delivered population estimates
for breeding birds for all species. Year for estimate set at
1982. – Der Jahresatlas lieferte Populationsschätzungen
für alle Brutvogelarten. Als Jahr der Schätzung wurde
1982 festgelegt.
198 SOVON
(1987)
Atlas project 1998-2000 –
Atlasprojekt 1998-2000
e second breeding bird atlas. Delivered population
estimates for all species. Estimate is average popula-
tion size 1998-2000. – Der zweite Brutvogelatlas lieferte
Populationsschätzungen für alle Arten. Die Schätzung
repräsentiert die mittlere Populationsgröße 1998-2000.
198 SOVON
(2002)
Avifauna Netherlands –
Avifauna der Niederlande
In this avifauna a large number of sources from (grey)
literature with estimates of total national population
size for a certain year outside atlas periods are gathered
and rated, data covering 1900-2001. – Diese Avif aun a
führt eine Vielzahl (auch grauer) Literaturquellen zu
nationalen Bestandsschätzungen, für Jahre außerhalb der
Atlaszeiträume, zusammen und bewertet diese. Die Daten
umfassen den Zeitraum 1990-2001.
>50 B et
al. (2001)
Breeding birds in Nether-
lands early 20th century –
Brutvögel der Niederlande
im frühen 20. Jahrhundert
A reconstruction of population sizes of all breeding birds
in the early 20th century (1900-1930). Year for estimate
set at 1915. Estimates are given in logarithmic classes.
– Rekonstruktion der Populationsgrößen aller Brutvogel
arten im frühen 20. Jahrhundert (1990-1930). Als Jahr
der Schätzung wurde 1915 festgelegt.
198 P-

(2003)
we describe a method to reconstruct long-term trends,
expressed as absolute population numbers each year,
for all regular breeding bird species in the Netherlands.
e objective was to go back at least until the year 1950,
which is the reference year in Dutch Red Listing meth-
odology (I & B 2007). We will show some of
the results focusing on summary statistics for the total
breeding population and habitat-specic subsets.
Table 1: Overview of data sources used to reconstruct the population sizes. – Überblick über die genutzten Datenquellen zur
Rekonstruktion der Vogelpopulationsgrößen.

82 R. P. B. FOPPEN et al.: Reconstructing trends in bird population numbers by integrating data and information sources
2. Methods
2.1 Data sources
Start of the reconstruction was lining up the data sources
available per year for all regular breeding bird species in the
Netherlands. We considered a species regular if breeding
occurred in at least ten consecutive years (Red List crite-
rion). In the present analysis we only consider native species.
is resulted in a list of 198 species. Table 1 depicts the most
important data sources.
2.2 Reconstruction analysis
We decided to use the yearly national estimate of the popula-
tion size as the common denominator. We used the yearly
population indices from 1990 onwards. If available, we
extended the time series with indices back to 1984 and back
to the rst reliable year with an index (earliest year is 1960).
e relative indices were converted into absolute numbers
by a factor based on the average index over the period 1998-
2000 and the absolute population estimate during that period
derived from the breeding bird atlas. In case no indices were
available before 1984 we used all available total population
estimates between 1960 and 1984 instead. In case both were
available, we used these absolute estimates for validation.
Because of the importance of 1950 as reference year, we
estimated the 1950 population size for all species. In most
cases without additional information, we assumed that the
population size equaled the size of 1960. As estimates for
1915 we used the midpoint of the range indicated by P-
 (2003).
is resulted in a matrix with absolute estimates for
1915, 1950 and 1990-current for almost all species and in
the period 1960-1990 for a variable number of years. To be
able to perform analyses across species we ‘imputed’ the year x
species matrix by applying an interpolation statistic. Between
two years with a calculated or a given absolute estimate, we
imputed a population size assuming a constant yearly popu-
lation growth rate. For instance assume 1960 has estimate
100 and 1970 has 50 than the imputed gures per year for
1961-1969 are based on a yearly constant decline of 6.7 %.
Table 2 depicts the various methods depending on the avail-
able data sources.
For data processing and easy updating the gures, we use
a database structure and link the various data sources by que-
ries. Our monitoring program delivers a yearly update from
1990 onwards. ese data are stored in a separate database
and so by running the queries every year an extra year can
be added quickly. Essential is also the conversion from index
to absolute number. is is also stored in a separate database.
So, if we have more reliable population estimates in a certain
year, for instance in two years time our new atlas project will
supply new estimates, we can easily process these data and
calculate new population numbers for years with indices.
3. Overview and summary statistics:
how many species and how many
birds per year?
e rst and maybe most intriguing question is how
many birds we had and have in the Netherlands. e
number of breeding species has gradually increased
from 1960 onwards and reached a more or less con-
stant level of around 185 from 1993 (Fig. 1). Apparently
in 1950 the number of species was higher and quite
some species disappeared in the 1950s but from the
1970s onwards were substituted by others: the number
of species since then has increased about 10 %. e
extinction and (re)colonisation events corroborate this
picture (Fig. 3). Since the 1960s regular (re)colonisa-
tions have taken place but only few extinctions.
e total number of breeding pairs uctuates between
9.5 million and 13.5 million and shows remarkable dif-
ferences over the years (Fig. 2). e numbers dropped
between early 20th century and the late 1960s by 25 %.
Since then the numbers recovered and the highest num-
bers were reached in the early 1990s.
If we link the species to their preferred habitat a clear
picture emerges: drastic declines for farmland birds and
heathland/dune species and gains for forest, shrub and
marshland species (Fig. 4). is picture also remains
when we calculate geometrical means based on the
yearly indices for these species groups. e largest
decreases then are in the heathland/dune group and the
largest increase is for marshland birds. e enormous
losses for farmland birds (estimated loss around almost
four million pairs between 1915 and 2013, which is a
decline of 75 %) resulted in a large balance shi between
habitats (Fig. 5). We estimate that in 1915 almost half
of all the breeding birds in the Netherlands were farm-
year – Jahr 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 …. 2013
Estimate total pop. –
Populationsschätzung
9000 ….
Population Index –
Populationsindex
23 31 27 25 30 29 46 38 60 55 59 54
Method – Methode calc* calc calc calc calc calc calc calc calc calc estim** …. calc
Final number –
resultierende Schätzung
3572 4806 4124 3788 4584 4424 7059 5787 9151 8407 9000 …. 8254
*calculation based on atlas estimate and corresponding population index for years 1998-2000 – * Berechnung basiert auf Populations-
schätzung aus dem Atlas und dem entsprechenden Populationsindex der Jahre 1889-2000
**estimate of the total population, in this case an atlas project – ** Schätzung der Gesamtpopulation, in diesem Fall durch ein Atlasprojekt
Table 2: Example of how dierent data sources are treated to determine total population estimates per year. – Beispiel dafür,
wie unterschiedliche Datenquellen genutzt wurden, um jährliche Populationsschätzungen zu generieren.

VOGELWELT 137: 80–88 (2017) 83
150
155
160
165
170
175
180
185
190
1
915
1
950
1
960
1
962
1
964
1
966
1
968
1
970
1
972
1
974
1
976
1
978
1
980
1
982
1
984
1
986
1
988
1
990
1
992
1
994
1
996
1
998
2
000
2
002
2
004
2
006
2
008
2
010
2
012
Number of breeding species – Anzahl der Brutvogelarten
Fig. 1: Total number of breeding species in the Netherlands, all native bird spe-
cies combined for period 1915-2013 based on yearly estimates. – Gesamtzahl der
niederländischen Brutvogelarten, Kombination aller heimischen Vogelarten für den
Zeitraum 1915-2013 basierend auf jährlichen Schätzungen.
0
2000000
4000000
6000000
8000000
10000000
12000000
14000000
16000000
1
915
1
950
1
960
1
962
1
964
1
966
1
968
1
970
1
972
1
974
1
976
1
978
1
980
1
982
1
984
1
986
1
988
1
990
1
992
1
994
1
996
1
998
2
000
2
002
2
004
2
006
2
008
2
010
2
012
Absolute number of breeding pairs –
Gesamtzahl der Brutpaare
Fig. 2: Absolute number of breeding pairs
in the Netherlands, all regular native bird
species combined (regular = breeding > 10
years in a row). – Absolute Anzahl von
Brutpaaren in den Niederlanden, Kom-
bination aller regelmäßig vorkommenden
heimi schen Vogelarten (regelmäßig vork-
ommend = Brutvogel >10 Jahre in Folge).
8
6
4
2
0
2
4
6
8
<1960
1962
1964
1966
1968
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
2008
2010
2012
2014
number of species – Anzahl der Arten
establishment – Besiedlung
extinction – Aussterben
Fig. 3: Frequency of extinction events
and year of establishment of new species
in the Netherlands, indicated are number
of species per year or period, < 1960 =
1915-1960. – Häugkeit des Aussterbens
von Arten und Jahre der Etablierung
neuer Vogelarten in den Niederlanden,
angegeben ist die Anzahl der Arten pro
Jahr oder Periode, < 1960 = 1915-1960.
land birds, nowadays this is hardly
15 %. On the other hand, forest
birds (i. e. Blackcap Sylvia atricapilla,
Wren Troglodytes troglodytes, Robin
Erithacus rubecula) and generalists
(i. e. Blackbird Turdus merula, Wood
Pigeon Columba palumbus) have
increased their share, resp. from 9
to 23 % and 9 to 31 %).
A good illustration of the changes
between 1915 and 2013 is to compare
the top 10 of most abundant species
(Table 3). In 1915 the list mainly
consists of species from farmland
and other open landscapes (Sky-
lark Alauda arvensis, House Martin
Delichon urbicum, Barn Swallow
Hirundo rustica, Tree Sparrow Passer
montanus and Linnet Carduelis can-
nabina). In the most recent period
this list is mainly consisting of gener-
alists, species found in many habitats
(farmland, marshland, urban areas
and forest) like Blackbird, Chanch
Fringilla coelebs, Great Tit Parus
major and Wood Pigeon and forest
birds like Wren, Blackcap and Chi-
cha Phylloscopus collybita.
We also looked at the total biomass
of breeding birds during these last

84 R. P. B. FOPPEN et al.: Reconstructing trends in bird population numbers by integrating data and information sources
Fig. 5: Proportions of total breeding
bird population per habitat group in
the Netherlands for period 1915-2013.
Indicated are proportions for 1915 and
every 10-year period since 1950. – Pro-
portionen der Gesamtzahl der Brutvo-
gelpopulation der Niederlande je Habi-
tatgruppe für den Zeitraum 1915-2013.
Dargestellt sind die Proportionen für 1915
und jeden Zehnjahreszeitraum seit 1950.
0
1000000
2000000
3000000
4000000
5000000
6000000
1900 1920 1940 1960 1980 2000 2020
number of breeding birds – Anzahl der Brutpaare
Generalists –
Generalisten
Urban –
städtische Prägung
Estuaries-Coastal –
Ästuar-Küste
Heathland-Dunes –
Heide-Dünen
Marshland –
Feuchtgebiete
Shrubs –
Gebüsch
Forest –
Wald
Farmland –
Agrarland
Fig. 4: Total number of breeding birds in the Netherlands for specialists of a certain habitat type and generalists, period 1915-
2013, indicated is the estimated number of breeding pairs, the lines from 1960 onwards are based on TrendSpotter Soware
(S et al. 2007). – Gesamtzahl der Spezialisten (für bestimmte Habitattypen) und Generalisten unter den Brutvogelarten
der Niederlande, Zeitraum 1915-2013, dargestellt ist die geschätzte Anzahl von Brutpaaren, die Linien ab 1960 basieren auf der
Soware TrendSpotter (S et al. 2007).
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
1915 1950 1960 1975 1985 1995 2005 2015
Generalists –
Generalisten
Urban –
städtische Prägung
Estuaries-Coastal –
Ästuar-Küste
Heathland-Dunes
–
Heide-Dünen
Marshland –
Feuchtgebiete
Shrubs –
Gebüsch
Forest –
Wald
Farmland –
Agrarland
proportion breeding birds – Anteil Brutvögel

VOGELWELT 137: 80–88 (2017) 85
100 years. A slightly dierent picture emerges (Fig.
6). e total biomass shows an increase, mainly in the
period 1970-85, and is more or less constant since then.
e contribution per habitat clearly shows that biomass
of farmland birds suered a large decrease (from 33 %
to 7 % of total), as expected on basis of the drop in
numbers. However, the relative contribution of farm-
land biomass is not as big as in population numbers.
e largest biomass is delivered by the marshland/
waterbirds, quite understandable regarding the large
proportion of large and middle sized birds like ducks,
swans, geese and herons. is group shows the largest
increase and by now makes up 50 % of the total biomass,
in 1915 that was only 28 %.
4. Discussion
By integrating a number of important data sources on
bird numbers and trends we were able to reconstruct
the fate of the breeding birds in the Netherlands over
the last century. e estimates from 1915 on the species
level oen should be treated with care. Also the number
of recorded species is probably underestimated because
0
500000
1000000
1500000
2000000
2500000
3000000
3500000
4000000
4500000
5000000
1
915
1
950
1
960
1
962
1
964
1
966
1
968
1
970
1
972
1
974
1
976
1
978
1
980
1
982
1
984
1
986
1
988
1
990
1
992
1
994
1
996
1
998
2
000
2
002
2
004
2
006
2
008
2
010
2
012
Total biomasse breeding birds in kg –
Gesamtbiomasse der Brutvögel in kg
Fig. 6: Total biomass for all breeding birds
in the Netherlands in the period 1915-
2013. Indicated is the total amount in kg.
is is calculated by multiplying the num-
ber of breeding pairs x average weight x 2.
– Gesamte Biomasse (in kg) aller Brutvögel
der Niederlande im Zeitraum 1915-2013
errechnet über die Multiplikation der Anzahl
der Brutpaare x durchschnittliches Körper-
gewicht x 2.
Top 10 species in 1915 – die 10 häugsten Arten 1915 Top 10 species at present (2013) –
die 10 aktuell (2013) häugsten Arten
Blackbird Turdus merula – Amsel Blackbird Turdus merula – Amsel
Starling Sturnus vulgaris – Star Starling Sturnus vulgaris – Star
House Sparrow Passer domesticus – Haussperling House Sparrow Passer domesticus – Haussperling
Tree Sparrow Passer montanus –Feldsperling Blackcap Sylvia atricapilla – Mönchsgrasmücke
Skylark Alauda arvensis – Feldlerche Chicha Phylloscopus collybita – Zilpzalp
Linnet Carduelis cannabina – Bluthäning Wren Troglodytes troglodytes – Zaunkönig
Barn Swallow Hirundo rustica –Rauchschwalbe Wood Pigeon Columbus palumba – Ringeltaube
House Martin Delichon urbicum – Mehlschwalbe Chanch Fringilla coelebs – Buchnk
Common Whitethroat Sylvia communis – Dorngrasmücke Great Tit Parus major – Kohlmeise
Willow Warbler Phylloscopus trochilus – Fitis Willow Warbler Phylloscopus trochilus – Fitis
Table 3: Comparison of the 10 most abundant species breeding in the Netherlands in 1915 and 2013 (present), ranked based
on the estimated total number of breeding pairs. – Vergleich der 10 häugsten Brutvogelarten der Niederlande in den Jahren
1915 und 2013 (heute), Reihenfolge basierend auf Schätzungen der Gesamtzahl der Brutpaare.
of the much lower number of observers. Species like
for instance Wood Sandpiper Tringa glareola, Aquatic
Warbler Acrocephalus paludicola and Great Snipe Gal-
linago media easily could be missed in the large moors
in the north of the country.
We do believe however that on the community level,
so per habitat type, the comparisons with other periods
for number of species and population sizes are valid. We
are also condent on the sign of the trends, increase or
decrease, for most species. e estimates for 1950 are
more robust. Although there were no atlas or coordi-
nated monitoring activities for a large number of spe-
cies data were collected and estimates were made based
on counts and surveys. e population sizes for com-
mon birds oen are based on the 1960 estimates. From
1960 onwards repeated surveys and local monitoring
activities delivered good estimates of change for these
species. Monitoring methodologies for breeding birds
were more or less equal with the ocial coordinated
monitoring (territory mapping method) from 1990
onwards. As far as we know this is the longest time span
covered for a national population estimate concerning
all breeding bird species. Indeed there have been long

86 R. P. B. FOPPEN et al.: Reconstructing trends in bird population numbers by integrating data and information sources
term reconstructions of population development, i.e. in
the United Kingdom covering >200 years (G et
al. 1996) and in Germany (50-150 years) (G
et al. 2015), but this concerns semi-quantitative changes
in population numbers.
A major advantage of this exercise is that we now
have a transparent and reproducible procedure to
assess the Red List status of birds. e previous Red
List assessments applied a semi-quantitative approach
and/or expert judgement to determine the size of the
breeding population and the rate of change relative to
the reference year 1950 (i.e 0-25 % decrease, 26-50 %,
etc.). With the present database we have a formal cal-
culation of the absolute numbers and rate of change.
Previous analyses of the Dutch breeding bird commu-
nity were based on much shorter periods and on com-
parisons of two time frames, for instance atlas periods
(e.g. P 2003). e most robust study consisted
of a formal analysis of trends for breeding birds in a 25
year period: 1975-2000 ( T et al. 2007).
e results showed a homogenization of the breeding
bird community. is is, amongst others, reected by
the fact that waterbirds nowadays occur in formerly
rarely occupied regions i.e. because city development
involves the inclusion of water rich environments. In
contrast, species of shrubs and trees were able to colo-
nize formerly open farmland and semi-natural habitats,
like lowland peat areas. A probable consequence of the
increase of trees and bushes following urbanisation. e
results also indicated that habitat preference and migra-
tion strategy were among the best explanatory variables
to explain trends ( T et al. 2010a). Long
distance migrants do worse than short distance migrants
or residents and marshland and forest species do much
better than farmland and heathland species. Our results
show that many of these processes started already in
the early 20th century. So the 1975-2000 period seems
to be a representative period for the past century in the
Netherlands. Birds of open habitats, farmland, dunes,
heathlands, fens and moors in general show large
declines. Birds of forests and wetlands show the largest
gains. We even consider some of the former strict for-
est birds like blackbird and woodpigeon as generalists
nowadays because they occur in many dierent habi-
tats. e underlying reasons involve very obvious factors
like the change in land cover and land use but ongoing
proper scientic analyses are needed to assess the rela-
tive contributions of all the possible and likely pressure
factors. Forest areas have expanded since the start of
the 20th century and are on average older (Table 4, 
T et al. 2010a,  T 2011). Urban
areas increased with a factor ten since 1900 and this for
instance resulted in a rapid increase of generalists with
a preference for wooded habitats, increasingly found
in urbanized areas. Large open natural areas like fens,
moors were converted to farmland areas. Total farm-
land area peaked in the 1960s and decreased with a
constant pace since then. Together with an enormous
intensication of agricultural activities this resulted
in deterioration of farmland habitat quality for birds
and consequently in major population losses. A simple
description of the observed changes is that we converted
2D landscapes in to 3D landscapes.
Although the losses in sheer population numbers
were enormous within the major part of the country
(>60 %) this was compensated by increases in forest
and urbanized areas.
e average densities of the total bird communities
are much higher in these 3D landscapes than in open
2D landscapes. In an average forest area easily densi-
ties of 5-10 breeding pairs per ha can be found while
this is 1-2 for bird communities in high quality open
farmland habitats. e median density in hundreds of
census plots during 1984-2000 in the Netherlands was
774 per 100 ha in forests and 128 in farmland areas
(unpublished results SOVON).
Total biomass of bird species has increased consider-
ably since the 1970s. It seems likely that this is related
to the higher productivity in many habitats, but also
is a result of ‘wildlife comeback’ (D et al. 2013).
At the end of the 19th century and the beginning of
the 20th heavy persecution and exploitation of birds
impacted population sizes and occurrences of many,
mainly bigger, species like waterbirds (hunting), herons
(hunting, trade of nestlings) , crows (persecution), rap-
tors (persecution) and waders (hunting and trade of
eggs) (D R 2015). e start of conservation, i.e. by
the constitution of BirdLife in the Netherlands in 1899
resulted gradually in less persecution, better protection
Landcover type –
Bodenbedeckung
1900 1960 1990 2010
Agricultural grassland
– Grünland
16,077 18,445 17,531 15,662
Arable land –
Ackerland
10,760 13,169 12,380 11,466
Heathland – Heide 5,359 872 582 582
Forest/shrub –
Wald/Gebüsch
4,196 3,656 4,611 4,320
Marshland –
Feuchtgebiet
208 83 42 374
Dune/Sands –
Düne/Strand
623 415 415 374
Open water –
oenes Gewässer
3,240 1,869 1,412 1,745
Urban – städtisch 748 2,991 4,570 7,021
Rest – Rest 374 0 0 0
Table 4: Change in total surface of landcover types 1900-
2010 in the Netherlands. Indicated are km2 (source: Anton
S, Alterra). – Veränderungen der Gesamtäche ver-
schiedener Bodenbedeckungstypen in den Niederlanden zwis-
chen 1990-2010 in km2 (Quelle: Anton S, Alterra).

VOGELWELT 137: 80–88 (2017) 87
laws and conservation measures. All this dispropor-
tionally benetted the bigger species and consequently
contributed to a shi in average weight. ere is also
a likely correlation with fertility within major habitat
types like farmlands and wetlands. Fertilization because
of more intensied agricultural production has caused
more nutritional rich conditions, i.e. crops, grasslands,
freshwater marshland. Middle sized and large birds that
are piscivores or herbivores are likely to have proted
more than smaller species.
e present data set also oers opportunities to quan-
tify periods of decrease. is is an interesting approach
to see whether decrease phases in population dynami-
cal sense are predictable and consequently can be used
to infer conservation implications.
is paper only describes some of the major sources
of change. e reconstruction of population number
over a large period of time allows us to perform more
formal analyses taking into account all kinds of pres-
sure factors from the past, currently but also in the
near future. By grouping species, for instance looking
at food guilds and other traits, we can develop hypoth-
esis for certain pressure factors. We aim to include not
only obvious factors like climate and landcover changes
but also eects of pesticides in the past but also pres-
ently (see e.g. H et al. 2014). Furthermore,
for migrants not only changes in habitats within the
breeding range are relevant but also circumstances in
stopover and wintering sites (e.g. Z et al. 2015).
A main problem is to acquire reliable and quantita-
tive data on all possible pressure factors over such long
time spans and geographic regions. Only then we will
be able to unravel the prime causes of change in bird
populations.
5. Zusammenfassung
Foppen, R. P. B., C. A. M. van Turnhout, A. van Dijk, A. Boele, H. Sierdsema & F. Hustings 2017: Rekonstruktion von
Vogelpopulationstrend durch Integration von Daten und Informationsquellen. Vogelwelt 137: 80–88.
Sowohl aus wissenschalicher als auch aus politischer Perspek-
tive ist die Verfügbarkeit von Informationen über langfristige
Populationsveränderungen wichtig. Gleichzeitig sind nur in
wenigen Ländern standardisierte Monitoringprogramme
vorhanden, die die Rekonstruktion von Bestandsentwicklun-
gen über Zeiträume von mindesten 50 Jahren ermöglichen.
Während „ozielle“ Monitoringprogramme Trendindexwerte
für jüngere Zeiträume liefern, sind darüber hinaus häug viel-
fältige weitere Daten und Informationsquellen vorhanden.
In diesem Kontext beschreibt der Beitrag eine Methode zur
Nutzung und Integration dieser vielfältigen Quellen. Im Falle
der Niederlande wurde so, mit Hilfe jährlicher Populations-
schätzungen als gemeinsamen Nenner, die Entwicklung der
Brutvogelpopulationen seit 1915 rekonstruiert. Als zentrale
Informationsquelle wurden Daten aus Monitoringprogram-
men, alte Daten von mehrfach untersuchten Probeächen,
Daten aus Brutvogelatlanten und Experteneinschätzungen
genutzt. Schätzungen des Gesamtbestandes eines bestimmten
Jahres, für das auch (relative) Populationsindexwerte vorlagen,
wurden genutzt um alle relativen Werte in absolute Bestands-
zahlen zu „übersetzen“. Dies ermöglichte die Berechnung und
Vervollständigung einer Jahr-Art Matrix für den Zeitraum
1960 bis 2013 sowie Schätzungen für die Jahre 1915 und
1950. Die Ergebnisse zeigen einen beachtlichen Anstieg der
Artenzahl seit 1915. Habitatspezisch entwickelten sich Vogel-
populationen sehr unterschiedlich. Die Bestände von Arten
des Oenlands wie Ackerland, Heiden und Dünen nahmen
dramatisch ab, während Arten der stark bewachsen Habitate,
wie Feuchtgebiete, Gebüsche und Wälder in ihren Beständen
zunahmen und die Verluste mehr oder weniger kompensier-
ten. Die Gesamtbiomasse aller Vogelarten hingegen hat seit den
1950er Jahren deutlich, um nahezu 50 %, zugenommen. Der
durchschnittliche Vogel ist heute also schwerer als früher. Den
größten Anteil an der Gesamtbiomasse haben Feuchtgebiets-
bzw. Wasservogelarten, da hier mittelgroße und große Arten,
wie Enten, Gänse, Schwäne und Reiher, eine großen proporti-
onalen Anteil der Artengruppe ausmachen. Die Gruppe zeigt
auch den stärksten Biomassezuwachs. Während Wasservögel
1915 28 % der Gesamtbiomasse ausmachten, waren es 2013
50 %. Die Gründe für die beobachteten Veränderungen sind
zum Teil oensichtlich Faktoren wie der Wandel der Landbe-
deckung und Landnutzung. Die landwirtschaliche Intensi-
vierung und der Verlust von Ackerland resultierten in einem
gewaltigen Verlust von Agrarlandarten. Geschätzte 4 Millionen
Brutpaare Agrarvogelarten verschwanden zwischen 1915 und
2013, was einer Abnahme um 75 % entspricht. Demgegenüber
haben Waldächen seit dem Beginn des 20. Jahrhunderts zuge-
nommen und die Wälder sind im Durchschnitt auch älter, was
zum Populationswachstum innerhalb der Gilde der Waldvögel
geführt hat (z.B. Mönchsgrasmücke, Zaunkönig, Rotkehlchen).
Die verstärke Urbanisierung über den betrachteten Zeitraum
führte zudem zu einem rapiden Anstieg der Generalisten unter
den Vogelarten (z.B. Amsel, Ringeltaube). Zumindest teilweise
erklären auch eektive Schutzmaßnahmen das „Comeback“
einiger größere Vogelarten, die bis Anfang der 1950er Jahre
gejagt oder verfolgt wurden. Vermutlich gibt es zudem eine
Korrelation zwischen Nährstoreichtum in wichtigen Habi-
taten wie Ackerland und Feuchtgebieten und potenziellen
Proteuren unter den mittelgroßen und großen herbivoren
und piscivoren Vogelarten.

88 R. P. B. FOPPEN et al.: Reconstructing trends in bird population numbers by integrating data and information sources
6. References
B, J., K. P. B, E. H. D, C. M. F & C. J.
R 2004: Goals and strategies for estimating trends
in landbird abundance. J. Wildl. Manage. 68: 611–626.
B, R. G., F. H & C. J. C 2001:
Schaarse en algemene vogels in Nederland (Avifauna van
Nederland 2). GMB/KNNV uitgeverij, Haarlem/Utrecht.
B, A., J.  B, F. H, K. K,
J. W. V & T. V  M 2016: Broedvogels in
Nederland in 2014. SOVON rapport 2016/04. SOVON
Vogelonderzoek Nederland, Nijmegen.
D R, J. 2015: Bird and People in the Netherlands 1500-
1920. Dissertation Vrije Universiteit Amsterdam.
D, S., C. I, L. MR, I. B, R. P. B.
F, B. C & M. B 2013: Wildlife Come-
back in Europe, the recovery of selected mammal and
bird species. Final report to Rewilding Europe by ZSL,
Birdlife International and the European Bird Census
Council, UK, ZSL.
G, D. W. ,M. I. A & A. F. B 1996: Population
trends of breeding birds in the United Kingdom since
1800. Brit. Birds 89: 291–305.
G, J. 2007: Citizens, science and bird conservation.
J. Ornithol. 148: 77–124.
G, C., H.-G. B, H. H, O. H, T.
R  & P. S 2015: Rote Liste der Brutvögel
Deutschlands, 5. Fassung. Ber. Vogelschutz 52: 19–67.
H, C. A., R. P. B. F, C. A. M.  T,
H.  K & E. J 2014: Declines of insec-
tivorous birds are associated with high neonicotinoid
concentrations. Nature 511: 341–343.
I, H. H.  & D. B 2007: Harmonization of Red Lists
in Europe: some lessons learned in the Netherlands when
applying the new IUCN Red List Categories and Criteria
version 3.1, Endangered Species Research 3: 53–60.
P, J. 2003: Breeding birds in e Netherlands in
the 20th century. Limosa 76: 141–156.
S, L., H. V, M.  R & A.  S
2007: Smoothing and trend detection in waterbird moni-
toring data using structural time-series analysis and the
Kalman lter. J. Ornithol. 148: 351–357.
SOVON 1987: Atlas van de Nederlandse Vogels. SOVON,
Arnhem.
SOVON 2002: Atlas van de Nederlandse Broedvogels 1998-
2000. Nederlandse Fauna 5. Nationaal Natuurhistorisch
Museum Naturalis, KNNV Uitgeverij en European Inver-
tebrate Survey-Nederland, Leiden.
T, R. M. 1979: Atlas van de Nederlandse broedvogels.
Natuurmonumenten, ’s-Graveland.
 T, C. A. M. 2011: Birding for science and con-
servation. Explaining temporal changes in breeding bird
diversity in the Netherlands. esis Radboud University
Nijmegen, e Netherlands.
v T C. A. M., R . P. B. F, R. S. E. W. L,
H. S & H. E 2007: Scale-dependent homog-
enization: Changes in breeding bird diversity in the Neth-
erlands over a 25-year period. Biol. Cons. 134: 505–516.
 T, C. A. M., R. P. B. F, R. S. E. W. L,
A.  S & H. S 2010a: Life-history and eco-
logical correlates of population change in Dutch breeding
birds. Biol. Cons. 143: 173–181.
 T, C. A. M., W. H & R. P. B. F
2010b: Long-term population developments in typical
marshland birds in the Netherlands. Ardea 98: 283–299.
Z L., R. G. B, J.   K, M. S &
E. W 2015: Moreau’s paradox reversed, or why
insectivorous birds reach high densities in savannah trees.
Ardea 103: 123–144.
Ruud P. B. Foppen, Sovon Dutch Centre for Field Ornithology, P. O Box 6521, NL-6503 GA Nijmegen,
e Netherlands; E-Mail: ruud.foppen@sovon.nl