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Acrocephalus 33 (154/155): 203−215, 2012
10.2478/v10100-012-0007-7
An overview of monitoring for raptors in Finland
Pregled monitoringa populacij ptic roparic na Finskem
Pertti Saurola
 Finnish Museum of Natural History, P.O. Box 17, FI–00014 University of Helsinki, Finland,  
 e−mail: saurola@cc.helsinki.fi
In Finland, population monitoring for both diurnal and nocturnal raptors has 
been almost entirely based on fieldwork carried out by voluntary raptor ringers. 
Responsible organisations include the Finnish Museum of Natural History, 
with economic support for administration from the Ministry of Environment, 
“Metsähallitus” (former National Board of Forestry) and WWF Finland. Since 
the early 1970s, numbers and productivity of four endangered species, the 
White-tailed Eagle Haliaeetus albicilla, Golden Eagle Aquila chrysaetos, Osprey 
Pandion haliaetus and Peregrine Falcon Falco peregrinus have been monitored 
by country-wide Comprehensive Surveys, with the aim of checking all known 
nest sites of these species every year. The Gyrfalcon F. rusticolus was included in 
this group in the late 1990s. Data for monitoring the populations of the other 
raptor species have been gathered by the Raptor Grid and Raptor Questionnaire 
projects. The Raptor Grid project produces annual population indices, which 
are calculated from the data collected from 10 × 10 km study plots (n = ca. 
130/year) and quite well reflect the annual population fluctuations and long-
term trends of seven common species of diurnal and six species of nocturnal 
raptors breeding in the southern part of Finland. For the rest of the species, 
which are either rare all over Finland or breed mostly in the north, outside the 
good coverage of the distribution of Raptor Grid study plots, conclusions on 
population changes are based on the total numbers of occupied territories and 
active nests reported annually by the Raptor Questionnaires.
Key words: monitoring for raptors, diurnal raptors, owls, Finnland
Ključne besede: monitoring populacij, ujede, sove, Finska
1. Introduction
Population monitoring is an absolute prerequisite 
and basis of conservation and sound management 
programmes. This means that regular surveys and well-
planned long-term monitoring programmes should be 
included in official duties of every government of our 
continuously and rapidly changing world, and implies 
that all monitoring programmes should be funded 
by public resources. However, in our real world, in 
the competition of resources, short-sighted economic 
interests and human welfare instigate worries about 
the future of other animal and plant species and 
our entire environment. Thus, nature conservation 
in Finland, as in most other countries, has largely 
been based on activities of idealistic and responsible 
individuals and NGOs.
Monitoring of raptor populations is important 
firstly because the raptors have suffered more than 
many other groups from negative impacts caused by 
people (e.g. persecution, contaminants and habitat 
destruction; Newton 1979) and secondly, because 
they are at the top of their food chains, with changes 
in their numbers, productivity and survival reflecting 
changes in the environment of other species, including 
man (Sergio et al. 2006).
In Finland, the Finnish Nature Conservation 
Society organised the first Peregrine Falcon Falco 
peregrinus and Golden Eagle Aquila chrysaetos 
surveys as early as in the 1950s (see Saurola 1976). 
In the early 1970s, Project Pandion, systematic and 
nationwide monitoring of the Osprey Pandion 
haliaetus was initiated on the basis of voluntary work 
by bird ringers (Saurola 1980), and the WWF 
204
P. Saurola: An overview of monitoring for raptors in Finland
Finland took the responsibility to monitor and save 
the vanishing population of the White-tailed Eagle 
Haliaeetus albicilla (Saurola 1976). Finally, in 1982, 
a new project to monitor “common” birds of prey 
based on voluntary fieldwork of ringers was launched 
(Saurola 1985a).
Description and evaluation of monitoring for 
diurnal and nocturnal birds of prey in Finland has 
been published recently and is not repeated here in 
detail (see Saurola 2006, 2008 & 2009). The main 
aim of this overview is to give answers to the questions 
raised by the EURAPMON to get a comparable 
view on the state-of-the-art of raptor monitoring in 
different parts of Europe. In addition, some selected 
examples of the updated results produced by different 
Finnish monitoring projects are given at the end of 
this contribution. Note: “raptor” includes here both 
diurnal and nocturnal birds of prey.
2. Questions raised by the EURAPMON
2.1. Main players
In Finland, the present main actors in monitoring for 
raptors are the Finnish Museum of Natural History, 
Ministry of Environment, “Metsähallitus” (former 
National Board of Forestry), WWF Finland and, the 
most important, raptor ringers. The Finnish Museum 
of Natural History is responsible for monitoring the 
Osprey population (started in 1971; Saurola 2011) 
and for two projects, the Raptor Grid (1982–) and 
Raptor Questionnaire (1986–) monitoring “common” 
raptors (Saurola 2006, Honkala et al. 2011). The 
Ministry of Environment has supported these projects 
by giving extra resources needed for the office work. 
Metsähallitus is responsible for monitoring the 
Golden Eagle (1971–; Ollila & Koskimies 2008, 
Ollila 2012), Peregrine Falcon (1974–; Ollila & 
Koskimies 2008) and Gyrfalcon F. rusticolus (1998–; 
Koskimies & Ollila 2009) since 1998; and WWF 
Finland is responsible for monitoring the White-tailed 
Eagle (1973–; Stjernberg et al. 2011).
Before obtaining a ringing licence, every Finnish 
bird ringer must have passed an exam, which proves that 
he is a high class field ornithologist and well aware of 
all aspects of bird protection (see Saurola et al. 2013). 
Thus, raptor ringers are an important voluntary resource 
with professional skills and are used for all fieldwork 
needed to monitor breeding raptors in Finland.
In addition to the monitoring projects carried 
out during the breeding season, migrating raptors 
have been counted systematically at the Hanko Bird 
Observatory located at the south-western corner of 
Finland’s mainland (Lehikoinen et al. 2008).
Several international meetings have been arranged 
between Nordic researches working on the Golden 
Eagle, White-tailed Eagle, Peregrine Falcon and 
Gyrfalcon. Co-operation between Estonian, Latvian 
and Finnish raptor researchers has been close. Recently, 
a workshop was arranged to improve the co-operation 
between Russian and Finnish raptor researchers (see 
Koskimies & Lapshin 2006).
The main users of the results of raptor monitoring 
are Finnish government officials, European 
Commission, raptor researchers and conservation 
NGOs, i.e. all those in need of information on the 
population status and trends of Finnish raptors.
2.2. National coverage
In principle, all monitoring for raptors is co-ordinated 
nation-wide (Saurola 2008). National network is 
the network of raptor ringers, which means that the 
coverage of activities is, in practice, much better in the 
southern than northern parts of the country.
The goal of monitoring for the Osprey, White-
tailed Eagle, Golden Eagle, Peregrine Falcon and 
Gyrfalcon populations is an annual Comprehensive 
Survey (Saurola 2008). In principle, all known 
territories all over the country are checked each year 
to obtain precise information on the annual breeding 
performance of these species.
Monitoring of the other, “common” raptor species, 
is patchy and concentrated to the southern half of the 
country. Voluntary ringers devoted to raptors have 
been encouraged to participate in the Raptor Grid 
monitoring project. This means that the ringers were 
asked (1) to join in teams, (2) to select a 10 × 10 km 
study plot based on the Finnish National Grid and 
(3) to try to locate active nests or at least occupied 
territories of raptors within their study plot by using 
the same searching effort from year to year (Saurola 
1985a & 2006). In 2011, for example, 130 Raptor 
Grid 10 × 10 km study plots were surveyed (Honkala 
et al. 2012) (Figure 1). On the basis of these data from 
Raptor Grid it is possible to calculate relevant annual 
population indices and long-term trends for common 
raptors breeding in the southern half of Finland 
(Saurola 2008) (see also Figures 4 and 5).
In addition, information (1) on the total numbers 
of potential territories checked, (2) on the totals of 
active nests and occupied territories found and (3) on 
the breeding performance (clutch size and brood size) 
assessed by the ringers have been collected annually by 
using the Raptor Questionnaire. Because the data have 
been collected by the territories of local ornithological 
205
Figure 1: The locations of the 10 × 10 km study plots 
based on the Finnish National Grid. The plots studied at 
least once in 1982–2010 are depicted in blue, and the ones 
studied in 2011 in orange. The grid lines shown are 100 × 
100 km (after HonKaLa et al. 2012).
Slika 1: Lokacije 10 × 10 km velikih popisnih ploskev, 
ki temeljijo na finski Nacionalni mreži. Ploskve, ki so bile 
v obdobju 1982–2010 preu~evane najmanj enkrat, so 
obarvane modro, v letu 2011 preu~evane ploskve pa 
oranžno. Prikazane mrežne ~rte so 100 × 100 km (po 
HonKaLa et al. 2012).
Acrocephalus 33 (154/155): 203−215, 2012
societies, the population fluctuations in space and 
time can be detected (see Figure 6). The total number 
of breeding attempts of raptors is highly dependent 
on the phase of the vole cycle. In a top vole year 
2009, 379 Raptor Questionnaires were filled by 256 
individual raptor ringers or teams. Altogether, 47,767 
potential nest sites of “common” diurnal and nocturnal 
raptors were inspected, 18,581 occupied territories 
detected as well as 12,259 breeding attempts verified 
and breeding performance reported (Honkala et al. 
2010). In contrast, in a poor vole year 2010, when the 
number of potential nest sites checked was 43,514, 
only 9,068 occupied territories and 5,357 active nests 
were found and reported (Honkala et al. 2011). The 
Raptor Questionnaire is vital (1) in obtaining at least 
some information on population changes of species 
not covered by the Raptor Grid project and (2) in 
monitoring annual productivity of all raptor species.
2.3. Key species and key issues
Population status and trends of all raptor species 
breeding in Finland have been monitored during the 
last 30 years (Saurola 2008). If it is necessary to select 
some “key species”, selection could be based e.g. on the 
specific IUCN category used in the national Red List.
The latest Finnish Red List of Birds included 14 
species of raptors (Rassi et al. 2010). Of these, the 
Black Kite Milvus migrans, Greater Spotted Eagle 
A. clanga, and Snowy Owl Bubo scandiacus were 
classified as Critically Endangered (CR); Montagu’s 
Harrier Circus pygargus and Gyrfalcon Endangered 
(EN); Honey Buzzard Pernis apivorus, White-tailed 
Eagle, Hen Harrier C. cyaneus, Buzzard Buteo buteo, 
Golden Eagle and Peregrine Falcon Vulnerable (VU); 
and Osprey, Eagle Owl B. bubo and Tengmalm´s Owl 
Aegolius funereus Near Threatened (NT).
All these 14 species could be defined as key species 
for monitoring for raptors, because these species 
need special protection and conservation measures. 
In addition, some of these species (e.g. the Osprey, 
White-tailed Eagle and Peregrine Falcon) can also 
be used as key indicators (sentinels) of the welfare 
of ecosystems in general (e.g. Helander et al. 2008, 
Henny et al. 2010).
At present, the most important “key issue” (threat) 
addressed by monitoring for raptors in Finland is land 
use, especially modern commercial forestry, which 
has reduced both the area of optimal habitat and 
availability of suitable nest sites needed by forest-
dwelling species (Saurola 1997, 2008 & 2011, Saurola 
& Björklund 2011). Many other human related factors 
like direct persecution, environmental contaminants, 
traffic, power lines, wind turbines, fishing and 
disturbances during the breeding period may have an 
additional negative effect on the population trends of 
Finnish raptors (e.g. Saurola et al. 2013).
International networking gives the opportunity to 
compare the population trends detected in Finland 
with the corresponding trends in neighbouring 
countries and other parts of Europe.
2.4. Strengths and weaknesses
The main strength of monitoring for raptors in 
Finland is the availability of professional level 
manpower for fieldwork comprised by voluntary 
206
P. Saurola: An overview of monitoring for raptors in Finland
ringers and other trained amateur ornithologists 
interested in raptors. Very important strength of the 
Finnish raptor monitoring is the production of series 
of regular annual monitoring reports. The motivation 
of fieldworkers is maintained by these reports (in 
Finnish with English summaries and captions), 
which demonstrate the yearly fluctuations and long-
term trends of different raptor species and the value 
of fieldwork carried out (e.g. Honkala et al. 2010, 
2011 & 2012, Stjernberg et al. 2011, Saurola 2011 
& 2012, Ollila 2012). Further, in addition to all 
ringing, recovery, recapture and resighting data, all 
monitoring data gathered by the Finnish Ringing 
Centre are stored in the Oracle Database Management 
System installed in the mainframe computer of the 
University of Helsinki and, thus, efficiently available, 
when needed for research and conservation (see 
Saurola et al. 2013).
The main weakness is the fact that the distribution 
of ringers is concentrated to the southern half of 
Finland. For this reason, the amount of data is not 
sufficient (1) to keep track of the status of the Critically 
Endangered Snowy Owl, and (2) for estimating 
reliable population trends of the “common” species 
breeding mainly in the northern half of the country 
like the Rough-legged Buzzard B. lagopus, Hen 
Harrier, Merlin F. columbarius, Hawk Owl Surnia 
ulula, Great Grey Owl Strix nebulosa and Short-eared 
Owl Asio flammeus. The Rough-legged Buzzard, Snowy 
Owl, Hawk Owl, Great Grey Owl and Short-eared 
Owl are at least partly nomadic species, which may 
change their breeding area thousands of kilometres 
depending on the fluctuations of their cyclic food, 
voles and lemmings. International co-operation is 
urgently needed to monitor population status and 
trends of these northern species, especially now, when 
the effects of global warming can only be speculated 
(Saurola 2009).
The other serious weakness is almost entire lack of 
reliable estimates of annual and long-term trends of 
survival of raptor species. To improve the situation, 
researchers and ringers have been encouraged to 
collect representative capture-recapture data needed 
to estimate survival of Finnish raptor species by using 
the new sophisticated statistical methods (see e.g. 
Saurola et al. 2003, Francis & Saurola 2008).
2.5. Priorities, capacity-building
At the moment, the highest priority to strengthen 
monitoring for both diurnal and nocturnal raptors in 
Finland should be given to the efforts to gather more 
relevant data from the northern half of the country. 
At least 20–30 new 10 × 10 km Raptor Grid study 
plots based on the National Grid and operating with 
standard effort from year-to-year should be urgently 
founded in the northern half of the country. Because 
this will not be possible only on the voluntary basis, 
extra national or/and international long-term funding 
is needed. Of course, more Raptor Grid study plots 
Figure 2: Annual numbers of occupied territories (squares), 
active nests (triangles) and successful nests (dots) of the 
Osprey Pandion haliaetus reported by the Finnish nationwide 
Project Pandion during the 1972–2012 period
Slika 2: Letna {tevila zasedenih teritorijev (kvadrati), 
aktivnih gnezd (trikotniki) in uspe{nih gnezd (pike) ribjega 
orla Pandion haliaetus, zabeležena v okviru vsedržavnega 
Projekta Pandion v obdobju 1972–2012
Figure 3: Average annual productivity of the Osprey Pandion 
haliaetus in 1972–2012; square – nestlings/occupied 
territory, triangle – nestlings/active nest, dot – nestlings/
successful nest
Slika 3: Povpre~na letna produktivnost ribjega orla Pandion 
haliaetus v obdobju 1972–2012; kvadrat – mladi~i/zasedeni 
teritorij, trikotnik – mladi~i/aktivno gnezdo, pika – mladi~i/
uspe{no gnezdo
N
um
be
r 
/ Š
te
vi
lo
Year / Leto
P
ro
du
ct
iv
ity
 / 
P
ro
du
kt
iv
no
st
Year / Leto
207
Table 1: Diurnal raptors (Falconiformes): highest national totals per year during 1986–2012 of occupied territories and 
active nests with eggs laid at least, as reported by ringers, and nestlings ringed
Tabela 1. Ujede (Falconiformes): najve~ja letna {tevila v obdobju 1986–2012 zasedenih teritorijev v državi in aktivnih gnezd 
vsaj z izleženimi jajci, o katerih so poro~ali obro~kovalci, in {tevilo obro~kanih mladi~ev
Species / Vrsta No. of territories/ Št. teritorijev
No. of nests/ 
Št. gnezd
No. of nestlings ringed/ 
Št. obročkanih mladičev
Honey Buzzard Pernis apivorus 432 149 206
White-tailed Eagle Haliaeetus albicilla 386 304 267
Marsh Harrier Circus aeruginosus 402 209 418
Hen Harrier Circus cyaneus 294 47 123
Goshawk Accipiter gentilis 1,613 1,267 2,311
Sparrowhawk Accipiter nisus 753 462 1,248
Buzzard Buteo buteo 1,131 650 1,131
Rough-legged Buzzard Buteo lagopus 231 167 558
Golden Eagle Aquila chrysaetos 345 209 147
Osprey Pandion haliaetus 1,167 951 1,489
Kestrel Falco tinnunculus 3,496 3,189 12,645
Merlin Falco columbarius 100 57 110
Hobby Falco subbuteo 565 208 189
Gyrfalcon Falco rusticolus    32 22 26
Peregrine Falcon Falco peregrinus 194 173 359
Acrocephalus 33 (154/155): 203−215, 2012
would be welcome also in southern Finland, except 
that those study plots would have to be based on 
voluntary fieldwork.
3. Selected examples of results
3.1. Data collected
A rough idea of the amount of data on raptors 
collected annually in Finland is given in Tables 1 
and 2. The tables show the highest numbers of (1) 
occupied territories and (2) active nests reported, and 
(3) nestlings ringed by Finnish ringers in the top year 
of the 1986–2012 period. The Greater Spotted Eagle, 
Montagu’s Harrier, Pallid Harrier C. macrourus (first 
breeding record in 2011) and Black Kite are excluded 
from these tables, given that these species have always 
been very rare breeders in Finland, which is, in fact, 
situated outside the normal distribution area of these 
species.
3.2. Comprehensive Surveys
The Osprey has been selected here as an example of a 
species monitored by the Comprehensive Survey. The 
present estimate of the Finnish Osprey population is 
1,300 pairs (Saurola 2011). In 2012, 2,046 potential 
nest sites of the Osprey were inspected; 1,133 
occupied territories were detected, 911 of the nests 
were active, meaning that eggs were laid, and 845 
successful with large young produced. The Finnish 
Osprey population remained at the same level through 
the 1970s, increased by 3% per year from 1982 to 
1994 and has since remained relatively stable (Figure 
2). The apparent increase during the very last years is 
at least partly due to the increased activity by ringers 
to construct artificial nests in the north-eastern part of 
the country. Also, the annual productivity increased 
steeply during the 1980s and has thereupon fluctuated 
at the same general level (Figure 3).
The positive trend can be attributed (1) to decreased 
persecution during migration and wintering (Saurola 
1985b, Saurola et al. 2013), (2) to decreased impact 
of environmental toxicants (P. Saurola unpubl.) and 
(3) to construction of artificial nests to compensate 
for the losses of nest sites caused by the modern 
forestry procedures (Saurola 1997). Almost 50% of 
Finnish Ospreys breed in artificial nests constructed 
by voluntary ringers (Saurola 2011).
In the 1960s and 1970s, the Finnish populations 
of the White-tailed Eagle, Golden Eagle and Peregrine 
Falcon were on a very low level and even close to 
the verge of extinction (Saurola 1976). Careful 
monitoring through the decades indicates that the 
populations of all these three species have recovered 
well and are at the moment on 60-year record levels 
208
shown by the numbers in Table 1 (T. Stjernberg & 
T. Ollila pers. comm.). Very little is known about 
the Finnish Gyrfalcons before the start of this millen-
nium; the population has probably fluctuated over the 
years around its present low level.
3.3. Raptor Grid
Annual population indices and long-term trends of six 
species of diurnal and six species of nocturnal raptors 
are shown in Figures 4 and 5.
Diurnal raptors
During the last three decades, the Honey Buzzard 
and Buzzard have been decreasing alarmingly 
steeply (Figure 4). The decreasing trend of the 
Goshawk Accipiter gentilis has not been as steep, 
but still statistically significant. Due to the modern 
commercial forestry, the amount of prime nesting 
habitat has continuously decreased and caused (1) 
increasing interspecific competition for high quality 
nesting sites between these forest-dwelling species 
and also (2) higher predation risk by the Goshawk 
on the nestlings of the other raptor species. Forestry 
has also caused a decrease of population densities in 
gallinaceous birds – important prey of the Goshawk 
(e.g. Sulkava 1964). Thus, the negative trends of these 
three medium-sized forest raptors are most probably 
connected with the effects of modern forestry (see 
Saurola 2008, Saurola & Björklund 2011).
In contrast to those typical forest-dwelling species 
mentioned above, the populations of three species 
breeding in more open habitats and independent of 
the effects of modern forestry have been increasing. 
The Kestrel F. tinnunculus and Marsh Harrier C. 
aeruginosus populations have increased steeply and the 
Hobby F. subbuteo moderately. The Kestrel has been 
recovering from the population crash in the 1960s 
and early 1970s. One of the important causes of the 
increase has been the effective nest box programme 
started by Erkki Korpimäki in the late 1970s (Valkama 
& Korpimäki 1999). According to the data from the 
Raptor Questionnaire, the number of artificial nests 
for small falcons (nearly all are nest boxes for Kestrels) 
has increased from 697 in 1986 to 7,003 in 2011. 
During the same period, the average productivity has 
increased because the nesting failures have decreased 
(see Figure 7).
The Marsh Harrier has extended its distribution 
towards the north during the last few decades, which 
have also been favourable for the Finnish population 
of the Hobby. No detailed studies on the causes of the 
success of these species are available.
Nocturnal raptors (Owls)
The Eagle Owl was increasing during the first decade 
of monitoring but, after that, has been on a continuous 
and very steep decrease (Figure 5). The increase phase 
can be attributed to (1) full protection since 1983, (2) 
increase of suitable open habitats (clear-cuts created 
by forestry) for nesting and hunting, and (3) year-
round stable and rich food supply of Brown Rats 
Rattus norvegicus at the numerous poorly managed 
rubbish dumps. Since the mid-1990s, 90% of the 
local rubbish dumps have been closed. This dramatic 
change in food supply has surely been one of the 
P. Saurola: An overview of monitoring for raptors in Finland
Table 2: Nocturnal raptors (owls, Strigiformes): highest national totals per year during 1986–2012 of occupied territories 
and active nests reported by ringers, and nestlings ringed
Tabela 2. No~ne ptice roparice (sove Strigiformes): najve~ja letna {tevila v obdobju 1986–2012 zasedenih teritorijev v državi 
in aktivnih gnezd, o katerih so poro~ali obro~kovalci, in {tevilo obro~kanih mladi~ev
Species / Vrsta No. of territories/ Št. teritorijev
No. of nests/ 
Št. gnezd
No. of nestlings ringed/ 
Št. obročkanih mladičev
Eagle Owl Bubo bubo 1,106 537 854
Snowy Owl Bubo scandiacus 21 15 20
Hawk Owl Surnia ulula 182 120 399
Pygmy Owl Glaucidium passerinum 1,339 963 4,797
Tawny Owl Strix aluco 1,189 905 2,844
Ural Owl Strix uralensis 2,545 1,786 4,722
Great Grey Owl Strix nebulosa 145 103 200
Long-eared Owl Asio otus 1,486 1,135 554
Short-eared Owl Asio flammeus 581 298 532
Tengmalm’s Owl Aegolius funereus 3,643 2,265 6,691
209
Acrocephalus 33 (154/155): 203−215, 2012
Figure 4: Annual population indices (dots) of six species of diurnal raptors, calculated from the numbers of occupied territo-
ries recorded on the Raptor Grid study plots during 1982–2012. Vertical bars indicate standard errors. Thick line = log-linear 
regression line, except in the panel of the Marsh Harrier = 7-point LOESS smoother (WiLLiam 1978). Note: the indices of the 
Kestrel Falco tinnunculus from the 1982–1996 period are biased by the increasing number of nest boxes and not included in 
the estimation of population trend.
Slika 4: Letni populacijski indeksi (pike) {estih vrst ujed, izra~unani iz {tevila zasedenih teritorijev, zabeleženih v obdobju 1982–
2012 na popisnih ploskvah Mreža ptic roparic. Stolpi~i ponazarjajo standardne napake. Debela ~rta = log-linearna regresijska 
krivulja, razen pri rjavem lunju = 7-to~kovni LOESS smoother (WiLLiam 1978). Opomba: indeksi postovke Falco tinnunculus iz 
obdobja 1982–1996 so pristranski zaradi pove~anega {tevila gnezdilnic in niso vklju~eni v oceno populacijskega trenda.
Honey Buzzard Pernis apivorus
Po
pu
la
tio
n 
in
de
x 
/ 
Po
pu
la
ci
js
ki
 in
de
ks
Marsh Harrier Circus aeruginosus
Goshawk Accipiter gentilis Buzzard Buteo buteo
Kestrel Falco tinnunculus Hobby Falco subbuteo
Year / Leto
V. Dumbović Mazal & K. Mikulić: Preliminary overview of monitoring for raptors in Croatia
210
Figure 5: Annual population indices (dots) of six species of nocturnal raptors, calculated from the numbers of occupied 
territories recorded on the Raptor Grid study plots during 1982–2012. Vertical bars indicate standard errors. Thick line = 
log-linear regression line. Note: the indices of the Pygmy Owl Glaucidium passerinum from the 1984–1993 period are biased 
by the increasing number of nest boxes and not included in the estimation of population trends.
Slika 5: Letni populacijski indeksi (pike) {estih vrst sov, izra~unani iz {tevila zasedenih teritorijev, zabeleženih v obdobju 
1982–2012 na popisnih ploskvah Mreža ptic roparic. Stolpi~i ponazarjajo standardne napake. Debela ~rta = log-linearna 
regresijska krivulja. Opomba: indeksi malega skovika Glaucidium passerinum iz obdobja 1984–1993 so pristranski zaradi 
pove~anega {tevila gnezdilnic in niso vklju~eni v oceno populacijskih trendov.
Eagle Owl Bubo bubo
Po
pu
la
tio
n 
in
de
x 
/ 
Po
pu
la
ci
js
ki
 in
de
ks
Pygmy Owl Glaucidium passerinum
Tawny Owl Strix aluco Ural Owl Strix uralensis
Long-eared Owl Asio otus Tengmalm’s Owl Aegolius funereus
Year / Leto
211
Acrocephalus 33 (154/155): 203−215, 2012
most important factors causing the steep decline of 
the Finnish Eagle Owl population during the last two 
decades (Valkama & Saurola 2005, Saurola 2009).
The populations of the Tengmalm’s Owl and Long-
eared Owl A. otus have fluctuated widely according to 
their cyclic main prey, voles. In general, it is challenging 
to detect statistically significant long-term trends from 
widely scattered data set. In the Tengmalm’s Owl, the 
decline is clear and significant, whereas in the Long-
eared Owl, the trend is similar but not significant, 
mainly because of the record year 2009. The population 
decline of the Tengmalm’s Owl has been attributed 
to the continuous degradation of forest habitat in 
Finland (for references, see Saurola 2009, Korpimäki 
& Hakkarainen 2012). No hypotheses have been 
proposed to explain the possible negative trend of the 
semi-nomadic Long-eared Owl population. Perhaps 
the “trend” happens to be an uncommon result of the 
nomadic life style of the species.
Of the two resident Strix-species, the population 
of the Ural Owl has been increasing, while the 
population of the Tawny Owl S. aluco has remained 
at the same level during the last decades. The welfare 
of both of these species and particularly of the Ural 
Owl is nowadays quite highly dependent on several 
thousands of nest boxes constructed by ringers. 
Some of the cavities made by the Black Woodpecker 
Dryocopus martius are large enough for the Tawny 
Owl. In contrast, most of the best natural nest sites 
of the Ural Owl, chimney-like old rotten stumps 
were some decades ago (not anymore) “cleaned” away 
by the foresters, and big old trees with large cavities 
had disappeared from Finnish forests long time ago. 
The Ural Owl is ready to breed in open stick nests 
constructed by the Goshawk and buzzards, but the 
breeding output seems to be much worse than in nest 
boxes.
Systematic monitoring of the breeding population 
and regular ringing activities at the bird observatories 
during the autumn have brought the pattern of the 
population changes of the Pygmy Owl Glaucidium 
passerinum to a new light (Saurola 2008 & 2009). The 
“increase” of the population during the first part of the 
monitoring period is heavily biased by the increase of 
the number of nest boxes constructed for the Pygmy 
Owls, because the detection probability of pairs 
breeding in nest boxes is higher than the ones breeding 
in woodpecker cavities. Since the mid-1990s, the data 
reflect the real changes in the population and indicate 
how the Pygmy Owl population has increased during 
favourable circumstances to a very high level and after 
that crashed owing to the strong autumn invasion in 
2003 and 2009 detected at coastal bird observatories 
(e.g. Lehikoinen et al. 2011). The present data show 
that Pygmy Owls, which participated in the mass 
invasion, disappeared from the Finnish population. 
There are no data to show whether these owls started 
to breed elsewhere or did they simply die during the 
invasions, which seem to be an important part of the 
population regulation of the Pygmy Owl (Saurola 
2008).
3.4. Raptor Questionnaire
Nests and territories
By the end of each breeding season, ringers have to 
report on the Raptor Questionnaire their data on 
breeding of all other raptor species except of the 
species monitored by the Comprehensive Surveys. 
Data from the Raptor Questionnaire are dependent 
on the variation of the activities of ringers and, if not 
corrected, may be in the worst case biased and even 
misleading. Keeping this potential source of error in 
mind, these data are “better-than-nothing”, giving 
valuable additional information on numbers of nests 
and territories found in different parts of the country 
of all common species, also of the ones monitored by 
the Raptor Grid project, e.g. the Goshawk (Figure 6, 
but note that these data have not yet been corrected by 
taking the variation in ringers’ activities into account).
For monitoring raptor species that are either rare or 
relatively common but breed in the northern or north-
eastern parts of the country, the present distribution 
and number of Raptor Grid study plots are not 
representative enough for calculating annual indices 
(see Table 1 and 2). For these species, data from Raptor 
Questionnaire are the only piece of information on 
annual fluctuations and long-term trends (Figure 6).
The Hawk Owl is a true nomadic species: two 
nestlings ringed in Finland were encountered east 
of the Ural Mountains, almost 3,000 km to the east 
of their natal sites, and three others were found in 
southern Norway, 1,200–1,400 km southwest of 
their natal sites (Saurola 2002). Thus, the occurrence 
of Hawk Owls in Finland shown by the data from 
Raptor Questionnaire is as irregular as expected: 
during 1986–2012, the number of nests found per 
year has varied between 1 and 120!
According to the present knowledge, the Great 
Grey Owl is a semi-nomadic species: a part of the 
population is nomadic and the other part resident 
(Stefansson 1997, Saurola 2002). During 1986–
2012, the annual total of active nests reported by the 
Raptor Questionnaire has varied a great deal, from 
4 and 103 nests, but the population seems to have 
remained at the same general level. Contributions 
212
P. Saurola: An overview of monitoring for raptors in Finland
1
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0
50
100
150
200
Figure 6: The annual numbers of all 
occupied territories (columns) and 
active nests (lower red parts of the 
columns) of the Goshawk Accipiter 
gentilis, Hawk Owl Surnia ulula and 
Great Grey Owl Strix nebulosa by the 
areas of local ornithological societies 
and for the entire country during 
1986–2012. Data from the Raptor 
Questionnaire. Note: within the species, 
the scale in the panels is the same for 
all local areas, but different in the panel 
for the entire country.
Slika 6: Letna {tevila vseh zasedenih 
teritorijev (stolpi~i)  in aktivnih gnezd 
(spodnji rde~i deli stolpi~ev) kragulja 
Accipiter gentilis, skob~je sove Surnia 
ulula in bradate sove Strix nebulosa 
po obmo~jih lokalnih ornitolo{kih 
dru{tev in za celotno državo v obdobju 
1986–2012. Podatki so iz Anketnega 
vpra{alnika o pticah roparicah. Opomba: 
lestvica na grafikonih za vrste je enaka 
za vsa lokalna obmo~ja, vendar razli~na 
na grafikonu za celotno državo.
Hawk Owl 
Surnia ulula
1
9
8
6
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86
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98
20
00
20
02
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04
20
06
20
08
20
10
20
12
0
50
100
150
Great Grey Owl  
Strix nebulosa
19
8
6
19
8
8
19
9
0
19
9
2
19
9
4
19
9
6
19
9
8
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0
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1
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250
500
750
1000
1250
1500
1
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0
1
20
50
100
150
200
Goshawk  
Accipiter gentilis
213
Acrocephalus 33 (154/155): 203−215, 2012
from local areas to the national total have been 
very different from year to year, depending on the 
local fluctuations of vole populations and perhaps 
movements of the owls.
Productivity
In addition, the Raptor Questionnaire has produced 
important information for monitoring annual 
productivity of all raptor species, e.g. of the Kestrel 
and Ural Owl shown here as examples in Figure 7. 
The annual average productivity of the Kestrel seems 
to have increased during 1986–2012, although the 
annual fluctuations are large. This can probably be 
attributed to the increasing number of pairs breeding 
in nest boxes, where the risks of nesting failures caused 
by avian and mammalian predators are lower than 
in open stick nests. The average annual productivity 
of the Ural Owl varies also much from year to year 
according to the fluctuations of voles. Although the 
Ural Owl population has been slowly increasing, no 
long-term trend in productivity can be detected.
4. Concluding remarks
(1) Monitoring the three Critically Endangered 
and one Endangered raptor species is based on 
sporadic and random field observations made by 
amateur ornithologists around the country; only 
the Gyrfalcon has been systematically surveyed.
(2) The Finnish monitoring projects produce 
reliable data to assess the annual population 
size (or index), long-term population trend and 
productivity of almost all Vulnerable and Near 
Threatened raptor species; for the Hen Harrier, 
the distribution and number of Raptor Grid 
study plots should be much more representative.
(3) More resources and international co-operation 
are needed for reliable monitoring of species, 
which belong to the Finnish Red List category 
Least Concern (LC) and breed in northern 
Finland.
(4) More effort should be devoted to collect capture-
recapture data for survival monitoring.
Acknowledgements: This article is based on voluntary 
fieldwork by enthusiastic and experienced Finnish 
raptor ringers. Juha Honkala, the administrator of 
the raptor monitoring data of the Finnish Ringing 
Centre, has depicted the Figures 1 and 6. Tuomo 
Ollila updated the information on the Golden 
Eagle, Peregrine Falcon and Gyrfalcon, and Torsten 
Stjernberg did the same for White-tailed Eagle data.
5. Povzetek
Monitoring populacij tako dnevnih kot nočnih 
ptic roparic na Finskem skoraj v celoti temelji na 
terenskem delu prostovoljnih obročkovalcev teh ptic. 
Nad monitoringom bdi Finski prirodoslovni muzej, 
gmotna sredstva za administrativno delo pa prispevajo 
,
,
,
,
Figure 7: Average annual productivity of the Kestrel Falco tinnunculus and Ural Owl Strix uralensis during 1986–2012; filled 
circles – young/successful nest, open circles – young/active nest (Kestrel) or young/occupied territory (Ural Owl). Vertical 
bars indicate standard errors.
Slika 7: Povpre~na letna produktivnost postovke Falco tinnunculus in koza~e Strix uralensis v obdobju 1986–2012; polni 
krogci – mladi~i/uspe�no gnezdo, prazni krogci – mladi~i/aktivno gnezdo (postovka) ali mladi~i/zasedeni teritorij (koza~a). 
Stolpi~i ponazarjajo standardne napake.
N
o.
 o
f 
yo
un
g 
/ 
[t
. m
la
di
~e
v
Year / Leto
Kestrel Falco tinnunculus Ural Owl Strix uralensis
214
P. Saurola: An overview of monitoring for raptors in Finland
Ministrstvo za okolje, “Metsähallitus” (nekdanji 
Nacionalni odbor za gozdarstvo) in finski WWF. 
Od začetka 70. let 20. stoletja skrbijo za monitoring 
številčnosti in produktivnosti štirih ogroženih vrst 
– belorepca Haliaeetus albicilla, planinskega orla 
Aquila chrysaetos, ribjega orla Pandion haliaetus in 
sokola selca Falco peregrinus – v okviru popisov, ki 
potekajo po celotni državi, in sicer z namenom, da 
se vsako leto preverijo vsa znana gnezdišča teh ptic, 
med katere je bil v 90-ih letih vključen tudi arktični 
sokol F. rusticolus. Podatki o populacijah drugih 
vrstah ptic roparic se zbirajo v okviru projektov, 
imenovanih Mreža ptic roparic in Anketni vprašalnik 
o pticah roparicah. S prvim projektom se vsako leto 
pridobijo podatki o letnih populacijskih indeksih, 
ki so izračunani na osnovi opažanj na 10 × 10 km 
velikih popisnih ploskvah (n = ca. 130/leto) in dobro 
odsevajo letna populacijska nihanja ter dolgoročne 
trende sedmih pogostih dnevnih in šestih nočnih 
ptic roparic, ki gnezdijo v južnem delu Finske. Za 
druge vrste, ki so bodisi redke po vsej Finski bodisi 
gnezdijo predvsem na severu države, zunaj območja 
dobre pokritosti ozemlja s popisnimi ploskvami Mreže 
ptic roparic, ugotovitve o populacijskih spremembah 
slonijo na skupnem številu zasedenih teritorijev in 
aktivnih gnezd, o katerih vsako leto poročajo v okviru 
anketnega vprašalnika.
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Arrived / Prispelo: 27. 3. 2013
Accepted / Sprejeto: 1. 7. 2013