55
Acrocephalus 40 (180/181): 55–67, 2019
10.1515/acro-2019-0003
Area use and important areas for Common Tern Sterna hirundo 
inland populations breeding in Slovenia and Croatia 
Raba območij in pomembna območja celinske populacije navadne čigre 
Sterna hirundo v Sloveniji in na Hrvaškem v času gnezdenja 
Davorin Tome1, Miloš Martinović2, Jelena Kralj2, Luka Božič3, Tilen Basle3, Luka Jurinović4
1 Nacionalni inštitut za biologijo, Večna pot 111, SI-1000, Slovenia, e-mail: davorin.tome@nib.si
2 Hrvatska akademija znanosti i umjetnosti, Zavod za ornitologiju, Gundulićeva 24, 10000 Zagreb, Croatia, 
e-mail: martinovic@hazu.hr, jkralj@hazu.hr
3 DOPPS  – BirdLife Slovenia, Tržaška cesta 2, SI-1000 Ljubljana, Slovenia, e-mail: luka.bozic@dopps.si, tilen.
basle@dopps.si 
4 Hrvatski veterinarski institut, Centar za peradarstvo, Heinzelova 55, 10000 Zagreb, Croatia, e-mail: luka.
jurinovic@gmail.com 
During the breeding periods of 2018 and 2019 we investigated the extent 
of areas Common Terns Sterna hirundo use while searching for food. We 
used GPS-UHF tags to follow the movements of 23 terns from Slovenia (7 
individuals) and Croatia (16 individuals). We investigated the movements 
of birds from three breeding sites, i.e. Lakes Ptuj, Siromaja 2 and Rakitje. 
Conclusions are based on 43,105 locations which were collected with a 
frequency of one reading per 20 minutes during the day and one reading 
per 4 hours during the night. 
In Slovenia, terns used a 60 km long and narrow area over Stara struga 
Drave (former river-bed of the Drava River) between Ormož and Maribor 
as well as eleven fishponds / lakes in its surroundings, most of them in 
the Pesnica valley. The most distant location was 30 km of straight line 
from breeding islands, but it was visited only once by a single tern. The 
areas with the most locations, hence important areas, were Lake Ptuj, 
Drava at Ptuj, Stara struga Drave between Ptuj and Rošnja and about 
20 km distant Lakes Radehova and Gradiško. These were probably the 
most important feeding areas for Common Terns breeding on Lake Ptuj. 
In Croatia, terns were found along the Sava almost exclusively, with only a 
few visits more than 2 km from the river. The most distant locations were 
over 60 km away from the breeding grounds, but terns visited them only 
rarely. Most locations of terns nesting on Siromaja were within a 5 km 
radius, while terns from Rakitje were making regular flights to waters up 
to 23 km from their colony. The area with the most locations visited by 
terns from both colonies was the Sava at Hrušćica. Besides, birds from 
the Rakitje colony were frequently recorded on the Sava near Savica and 
waterbodies within 5 km of the colony. These were probably the most 
important feeding areas for Common Terns breeding around Zagreb.
Key words: Common Tern, foraging areas, GPS-UHF tags, home range, 
Drava, Sava
Ključne besede: navadna čigra, območja prehranjevanja, GPS-UHF 
oddajniki, Drava, Sava
56
D. Tome et al.: Area use and important areas for Common Tern Sterna hirundo inland populations breeding in Slovenia and 
Croatia
1. Introduction
Bird abundance and distribution is limited by 
suitable habitats. Among the most important factors 
affecting habitat suitability are quality of nest-site, 
food supply and “enemies”, being predators, parasites 
or competitors (Newton 2007). Common Terns 
Sterna hirundo nest close to large water bodies on 
flat, scarcely vegetated surfaces (Nisbet 2002). Sea 
coast seems like a very spacious breeding area for 
them, but only very specific habitats meet all criteria 
for nest-site selection. In Croatia, for example, 
although abundant with sea coast, only 200–500 
pairs are presumed to nest along the sea (Kralj et al. 
2013). In marine ecosystems in Slovenia, terns nest 
at artificially maintained Sečovlje saltpans and in 
carefully managed Škocjanski zatok Nature Reserve 
(Škornik 2012). 
Inland, the most often occupied natural 
breeding sites of Common Terns are gravel islands 
and banks in the areas of large rivers and lakes, where 
predators have limited access (Becker & Ludwigs 
2004). Nowadays, due to countless changes made in 
freshwater ecosystems by humans, these are among 
the rarest and most endangered habitats in Europe 
(EEA 2015). Hence terns are more and more diverted 
to nest in artificial areas and structures like gravel pits 
and nesting rafts. In Slovenia, the only permanent 
breeding site of Common Terns are artificial islands 
on Lake Ptuj on the Drava River (Denac et al. 2019), 
where they’ve nested since the reservoir was created 
in 1978 (Janžekovič  & Štumberger 1984). In 
2017, another breeding island was created on Brežice 
HPP reservoir on the Sava River. Common Terns 
bred there in 2018 and 2019, but whether this is to 
become another regular breeding site for Slovenia 
remains to be seen. In Croatia, regularly nesting 
terns can be found on two locations near the Sava 
River (Hrušćica and Rakitje) and two on the Drava 
(Lake Ormož and Drava River close to Repaš), while 
occasional breeding in smaller numbers also occurs 
at various fishponds of continental Croatia. The 
population of inland breeding Common Terns in 
both countries is estimated at 300–500 breeding 
pairs. Being so scarce and vulnerable, most breeding 
sites are strictly protected by law (Božič 2003, 
Radović et al. 2005) and well managed (Denac & 
Božič 2018, Martinović 2018). Still, to run an 
efficient protective breeding program for Common 
Terns, other factors important for suitability of the 
habitat should be controlled, too, in particular food. 
In Slovenia and Croatia, there have been no reports 
in this respect, beside notes on sporadic observations 
of terns foraging on different locations during the 
breeding season (e.g. Vogrin 2016). 
While breeding and when not on or near the 
nest, Common Terns spend a lot of their time 
flying over shallow waters, where they search for 
food, mainly fish up to 15 cm long (Becker  & 
Ludwigs 2004), which they catch by plunge-div-
ing from the air (Holbech et al. 2018). Beside for 
themselves, they use the catch for feeding their un-
fledged chicks. Since this means collecting food 
at some distant sources and carrying it to a nest 
(usually one by one), that makes them “central place 
foragers” (Wetter 1989, Dänhard et al. 2011). 
The optimal foraging theory predicts that the dis-
tance to which they fly during the hunt is somehow 
limited by economics (Pyke 1984)  – they can 
afford longer flights if food there is more abundant, 
easier to catch or energetically more profitable. But 
in general, the radius of activity of breeding terns is 
usually between 3 and 10 km, exceptionally up to 
30 km (Nisbet 2002) from the nest. 
Locations of nests, numbers and breeding 
success are relatively well known for inland popu-
lations of Common Tern in Slovenia and Croatia 
(Kralj 2018, Mihelič et al. 2019). On the 
other hand, food and feeding habits are virtual-
ly unknown, so we designed this study to comple-
ment our knowledge appropriately. Our aim was 
(1) to find the whole extent of areas adult terns use 
during the breeding season (area use) and (2) to 
find which are the most used ones (i.e. most impor-
tant). Since we did not observe the terns physical-
ly, we only presume that the majority of activities 
when not on or by the nest were to locate and catch 
the prey (feeding activity), although we realize that 
some were also due to other needs, like flying, pros-
pecting, etc. (Martinović et al. 2019). 
2. Study area
2.1. Slovenia
In Slovenia, the study was conducted on Lake Ptuj, 
where Common Terns nest, and in its surrounding 
areas (Figure 1). Lake Ptuj is an artificial water 
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Acrocephalus 40 (180/181): 55–67, 2019
Figure 2: Areas used by Common Terns in Croatia are presented with outlined polygons. Numbers correspond to 
numbers in Tables 3 and 4. Important foraging areas are filled in yellow, with crossbars indicating the colony for 
which these areas are deemed important (see Legend). Breeding colonies are filled in red. 
Slika 2: Raba območij navadnih čiger na Hrvaškem, označena s poligoni. Številke ustrezajo območjem v tabelah 3 
in 4. Rumena so območja, pomembna za prehranjevanje z različnimi šrafurami za obe koloniji (glej legendo). Rdeči 
poligoni označujejo položaj gnezdišč.
Figure 1: Areas used by Common Terns in Slovenia are presented with outlined polygons. Numbers correspond to 
numbers in Table 2. Filled in red important foraging areas, filled in pink important areas for foraging and as flying 
corridors. Yellow circle denotes position of breeding island. 
Slika 1: Raba območij navadnih čiger v Sloveniji, označena s poligoni. Številke ustrezajo območjem v tabeli 2. 
Rdeča so območja, pomembna za prehranjevanje, rožnata pa območja, pomembna za prehranjevanje in kot preletni 
koridorji. Rumeni krog označuje položaj gnezdišča.
58
body constructed in 1978 on the Pannonian stretch 
of the Drava River as reservoir for the channel-type 
hydropower plant Formin. It is 7.3 km long and 
1.2 km wide, with its surface area covering 346 ha. 
In recent years, terns have been nesting there on two 
gravel islands (both covering 1,100 m2) constructed 
in 2014, while other islands are occupied mainly 
by Black-headed Gulls Larus ridibundus (Božič & 
Denac 2014).
To the north of the lake spreads the town of Ptuj, 
while other parts of its surroundings are mostly in-
tensive cultural landscapes with many fields, some 
hedgerows and villages. About 19 km further north 
is a series of seven lakes in the Ščavnica and Pesnica 
valleys. About 18 km to the W is a series of fish-
ponds near the village of Rače and Medvedce res-
ervoir. In other directions within this perimeter are 
some other small fish ponds, mostly without official 
names. The Drava River represents both the inflow 
and the outflow to and from Lake Ptuj. The origi-
nal riverbed is in both sections complemented with 
an artificial, concrete channel, used to bring water 
to the power plant. Most of the water throughout 
the year is diverted to the channel, leaving the river-
bed with only a small volume of water flow (20 m3/s 
upstream and 10 m3/s downstream of Lake Ptuj 
during the Common Tern breeding season). 
2.2. Croatia
The Croatian part of the study was conducted 
along the Sava River in the surroundings of Zagreb 
(Figure 2). Terns were tagged on Lake Siromaja 2, a 
gravel pit with a floating breeding platform about 
15 km downstream (SE) from the city of Zagreb, and 
on Lake Rakitje just off the W edge of Zagreb with 
a fairly large colony on an artificial island. The Sava 
River is mostly regulated and canalized all the way 
through Zagreb, with semi-natural flows upstream 
from Rakitje, about 10 km to the Slovenian border, 
as well as downstream from Ivanja Reka (Slukan 
Altić 2010). The part downstream from Ivanja 
Reka is protected as a Natura 2000 site “Sava kod 
Hrušćice” (Radović et al. 2005.), and this area 
includes numerous small gravel pits, including 
Siromaja 2. There are numerous other gravel pits 
all along the Sava through and around Zagreb. The 
air distance between Siromaja 2 and Rakitje lakes is 
27 km and 31.5 km along the river.
3. Material and methods
3.1. Field work
We used the GPS-UHF solar-powered Nano-tags 
produced by Milsar, attached to birds with Teflon 
chest harness, to collect data on the movements by 
Common Terns. The device with harness weighed 
about 4 g, representing about 3.5% of body mass of 
the tern. While fitting the tags, we also marked the 
birds with stainless steel on one and plastic ring on 
the other leg. 
Adults were captured on the nest using nest 
traps as late as possible during the incubation 
period. According to observations from a dis-
tance and some visits to the breeding colony, not 
all terns with tags successfully fledged their chicks. 
If a tagged bird deserted the nest before chick was 
hatched or in case of malfunction of the tag, the 
collected data was not included in the analysis.
Loggers collected GPS fixes every 20 min 
during daylight (4 am to 10 pm) and every 4 hrs 
during nighttime. GPS location accuracy was 10 m 
for 90% of the records (manufacturer specifica-
tions). We regularly downloaded data to a comput-
er using an omnidirectional antenna connected to 
a base station. 
3.2. Data analysis
We define a location as a single geographic 
coordinate recorded by a tag. We used Google 
Earth and QGIS (QGIS Development Team 
2016) to graphically present distribution of all 
locations of all terns within the study site. As 
expected, locations were clustered around different 
water bodies ( rivers, lakes, submerged gravel pits, 
fish ponds, etc.). We visually defined each cluster 
within separate water body as a separate area. 
In Slovenia, large areas (in particular long water 
courses) were further separated into two or three 
smaller sub-areas (in the text referred to as “the 
area” for sub-areas too). In Croatia, the Sava River 
was divided into 14 segments, each of roughly 5 km 
in length. Additionally, as a nesting area, we defined 
islands or rafts where Common Terns nested, even 
if they were located within one of the larger areas. 
We used MS Excel to analyze distribution of 
locations according to the defined areas. Each area 
D. Tome et al.: Area use and important areas for Common Tern Sterna hirundo inland populations breeding in Slovenia and 
Croatia
59
Acrocephalus 40 (180/181): 55–67, 2019
was geo-fenced with a four corner polygon. For 
each location and each line of the polygon, we com-
puted the value 
 d=(x–x1)(y2–y1)–(y–y1)(x2–x1)
 (x, y = coordinates of location; x1,y1 = coordi-
nates of one end of the line; x2,y2 coordinates 
of the other end of the line)
to determine on which side of the line is the 
given location. If d > 0 the location was on one side, 
if d < 0 the location was on the other side, and if d = 0 
the location was exactly on the line. The final result 
after testing for all four lines of a particular polygon 
was information whether the location was inside or 
outside the polygon surrounding a specific area. 
To find which areas within home range were 
most often used, hence important for Common 
Terns (beside nesting area which was regarded as 
important by default), we first removed all loca-
tions from within the nesting area. For remaining 
locations, we used three aspects to highlight the 
level of importance of particular area: 
(1): Cumulative number of locations of all terns in 
a specific area. Subjectively we considered the 
area as an important site if it contained more 
than 10% of all locations. 
(2): The number of terns detected within a specif-
ic area. Subjectively we considered an area as a 
candidate for an important site if more than 
half of terns (4 for Slovenia, 2 for Croatia-Rak-
itje and 6 for Croatia-Siromaja) had at least one 
location within a specific area. 
(3): Number of terns commonly present in a spe-
cific area. As commonly present we considered 
birds with at least 10% of locations in the par-
ticular area. With this aspect also birds with 
short monitoring period influence a selection 
of important areas. A candidate for an impor-
tant site should have at least one regular visitor.
In the end we defined an area as an important 
site for Common Terns during the breeding season 
if it corresponded with the first aspect or if it was a 
candidate according to both other aspects. 
4. Results
We analyzed data from 23 loggers (7 in Slovenia 
and 16 in Croatia) which were collecting locations 
of terns for 5 or more consecutive days (Table 1). 
In Slovenia, loggers attempted to record location 
10,781 times. In 631 of these attempts (5.8%), 
GPS unit failed to connect to satellites within 
the pre-programmed time and record coordinates, 
or the registered location was erroneous. The rest 
were valid locations. 701 valid locations (6.9%) 
were disregarded due to being outside the selected 
areas mostly from birds recorded while flying 
from breeding areas over land to foraging areas or 
CN CO FD LD NR
961002 SI 5/27/2018 7/21/2018 2019
961003 SI 5/19/2018 7/11/2018 2313
961004 SI 5/19/2018 7/2/2018 1809
961007 SI 5/19/2018 5/23/2018 184
961009 SI 5/23/2018 5/27/2018 239
961014 SI 5/23/2018 7/2/2018 1897
961020 SI 5/20/2019 6/15/2019 988
961020 CRO 6/20/2018 7/19/2018 1535
961021 CRO 6/5/2018 7/20/2018 2634
961022 CRO 6/6/2018 8/1/2018 2470
961023 CRO 6/5/2018 7/12/2018 1082
961024 CRO 6/5/2018 7/26/2018 2894
961025 CRO 6/17/2018 8/1/2018 1758
961026 CRO 6/11/2018 7/20/2018 1061
961028 CRO 6/26/2018 8/12/2018 2315
961029 CRO 6/5/2018 8/1/2018 2720
961030 CRO 6/8/2018 7/12/2018 2085
961031 CRO 6/8/2018 7/20/2018 2387
961032 CRO 6/8/2018 7/19/2018 2372
961033 CRO 6/8/2018 8/1/2018 2905
961034 CRO 6/11/2018 7/26/2018 2308
961035 CRO 6/11/2018 7/20/2018 1490
961036 CRO 6/11/2018 7/14/2018 1690
Table 1: Statistics of tracking effort of Common Terns 
with GPS tags; CN = individual code number of the 
tern, CO = country, FD = date of first record; LD = date 
of last record; NR = Number of readings
Tabela 1: Statistika zbiranja GPS podatkov na navadni 
čigri; CN = individualna koda osebka, CO = država, 
FD = datum prvega podatka; LD = datum zadnjega 
podatka; NR = število podatkov
60
D. Tome et al.: Area use and important areas for Common Tern Sterna hirundo inland populations breeding in Slovenia and 
Croatia
Area Code number of individual tern statistics
di
st
an
ce
 (m
)
96
10
02
96
10
03
96
10
04
96
10
07
96
10
09
96
10
14
96
10
20
al
l t
er
ns
%
 lo
ca
tio
ns
N
 >
=1
0%
N C
um
m
ul
at
iv
e %
breeding islands 0 1035 654 1132 87 111 261 700 3980 100,0
1 lake Ptuj - N 800 258 273 55 45 30 114 41 816 14.9 5 7 14.9
2 lake Ptuj - S 1700 59 544 29 9 0 3 21 665 12.2 1 6 27.1
3 fish pond near Ptuj 2000 57 2 0 0 0 0 0 59 1.1 0 2 28.2
4 Drava at Ptuj 2100 187 129 5 4 34 94 49 502 9.2 3 7 37.3
5 RD Markovci to Ormož - E 5500 1 31 0 1 0 1 0 34 0.6 0 4 38.0
6 RD Ptuj - Rošnja 7600 269 378 5 13 59 1147 176 2047 37.4 6 7 75.4
7 RD Markovci to Ormož - C 9500 0 1 0 0 0 2 0 3 0.1 0 2 75.4
8 fish ponds at Bodkovci 13200 1 89 0 0 0 0 0 90 1.6 0 2 77.1
9 lake Savci 13500 1 36 0 0 0 0 0 37 0.7 0 2 77.8
10 RD Rošnja - Maribor 15700 1 2 0 0 5 11 0 19 0.3 0 4 78.1
11 fish ponds at Rače 16500 1 103 0 0 0 0 0 104 1.9 0 2 80.0
12 RD Markovci to Ormož - W 16500 0 4 0 3 0 13 0 20 0.4 0 3 80.4
13 lake Radehova 19000 129 0 406 10 0 198 0 743 13.6 4 4 94.0
14 lake Gradiško 19500 10 0 87 5 0 46 1 149 2.7 1 5 96.7
15 lake Pristava 21000 1 0 6 0 0 6 0 13 0.2 0 3 96.9
16 lake Komarnik 21200 0 0 3 0 0 0 0 3 0.1 0 1 97.0
17 lake Ormož 22000 0 32 0 7 0 1 0 40 0.7 0 3 97.7
18 Gajševsko lake 23000 0 22 0 0 0 0 0 22 0.4 0 1 98.1
19 Drava at Maribor 25000 0 13 0 0 0 0 0 13 0.2 0 1 98.4
20 lake Pernica 25000 9 0 81 0 0 0 0 90 1.6 1 2 100.0
all areas w/o breeding 
islands 984 1659 677 97 128 1636 288 5469
Table 2: Number of locations in particular areas for Common Terns from Lake Ptuj. Area – geographic name of 
the area (RD – former Drava river-bed); distance – from breeding island/raft to centroid of the area; % locations – 
percentage of locations (according to column “all terns”); N >= 10% – number of terns with >= 10% of locations in 
particular area (common visitors of the area); N – number of terns recorded in the area. Shaded are important areas 
and numbers reaching the threshold for selection of important area.
Tabela 2: Število lokacij v posameznih območjih za čigre s Ptujskega jezera. Area – ime območja (RD – stara struga 
Drave); distance – razdalja med gnezdiščem in centroidom območja; % locations - odstotek lokacij (iz kolone “all 
terns”); N = 10 % . število čiger, ki so imele >= 10 % lokacij v izbranem območju (redne obiskovalke); N – število 
čiger, zabeleženih v območju. Osenčena so pomembna območja za čigre in vrednosti, ki dosegajo mejo za določanje 
pomembnih območij.
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Acrocephalus 40 (180/181): 55–67, 2019
vice versa. In Croatia there were 38,172 recording 
attempts, 1,186 (3.1%) of them failed to record 
valid coordinates, 3,421 (9.2%) valid records were 
disregarded for the same reason as in Slovenia. After 
exclusion of erroneous and disregarded readings, 
we were left with 43,105 locations used for analysis 
(9,449 in Slovenia and 33,565 in Croatia).
4.1. Slovenia
In Slovenia, we determined 21 areas of activity 
(Figure 1, Table 2). One area was composed of 
two breeding islands, located within the larger 
area of Lake Ptuj. The most distant areas from 
breeding islands were Lake Pernica (20 in Table 2) 
and the Drava River at Maribor (19), which both 
have centroids about 25 km of straight line from 
breeding islands. 
A little more than 42% of analyzed locations 
were from breeding islands (Table 2). More than 
one third of the remaining recordings were from 
the Drava Riverbed between Ptuj and Rošnja (6 in 
Table 2; 2,047 locations or 37%) which was, beside 
breeding islands, the most used area. From the 
section of the Drava river-bed between Rošnja and 
Maribor (10) there were only 19 recorded locations. 
Within Lake Ptuj (without breeding islands), 
terns were documented 1,481 times (27% of loca-
tions), with the N part of the lake (1) having only mar-
ginally more locations than the S part (2; 815 or 15% 
compared to 665 or 12%). The only other area with 
over 10% of locations was Lake Radehova (13; 743 lo-
cations or 14%), while the Drava at Ptuj (4) came very 
close to 10% threshold (502 locations or 9%). 
Of the remaining areas, Lake Gradiško (14), 
fish ponds at Rače (11), small fishponds at Bod-
kovci (8), Lake Pernica (20) and a small fish pond 
near Ptuj (3) each had more than 1% of locations. 
The remaining ten areas combined had 3.6% loca-
tions. The entire Stara struga Drave between Mark-
ovci and Ormož, although more than 20 km in 
length, had only 57 locations.
The part of the Drava river-bed between Ptuj 
and Rošnja (6), the Drava at Ptuj (4) and N part 
of Lake Ptuj (1) were used by all 7 terns, although 
the percentage of locations was rather unequal-
ly distributed among individual birds (from 1% to 
70%). Five or six birds used the S part of Lake Ptuj 
(2), the part of Stara struga Drave between Rošnja 
and Maribor (10) and Lake Gradiško (14). Four 
birds used the E part of Stara struga Drave between 
Markovci and Ormož (5), as well as Lake Radeho-
va (13). Two areas with a relatively high number of 
locations, a small fish pond at Bodkovci (8) and fish 
ponds at Rače (11), were used mostly by one bird 
only (individual code number 961003). This bird 
was responsible for 99% of all locations in these 
two areas. 
The areas where at least one tern was commonly 
present were: Lake Ptuj (1,2), the Drava at Ptuj (4), 
part of Stara struga Drave between Ptuj and Rošnja 
(6), Lake Gradiško (14), Lake Radehova (13) and 
Lake Pernica (20).
4.2. Croatia
Results from colonies around Zagreb are shown in 
Tables 3 and 4 and Figure 2. We identified 28 areas 
where terns were recorded during the study (20 
for birds breeding on Siromaja 2 and 24 for birds 
breeding on Rakitje). Two were the breeding colonies 
themselves, although each also served as a visiting 
area for birds from the other colony. The Rakitje 
colony was itself situated within the larger Lake 
Rakitje (20 in Tables 3 and 4). The most distant areas 
from breeding islands were Sišćani (26) and Garešni-
ca-Poljana fishponds (27), the former with a centroid 
of about 60 km of straight line from Rakitje, and the 
latter about 65 km from Siromaja 2.
For birds breeding on Siromaja, 70.9% of all 
GPS fixes were within boundaries of the colony 
lake. Almost half of all other fixes were on the Sava 
at Hrušćica (9). The next most popular sites were 
Abesinija Lake (16), the Sava at Siromaja (10) and 
the Sava at Rugvica (11). The remaining 23 areas 
accounted for only 6.4% of locations outside the 
colonies. 
Birds breeding at the Rakitje colony had 57.0% 
of all GPS locations at the colony itself. They also 
had the highest share of their non-colony fixes, 
one quarter of them, on the Sava at Hrušćica (9). 
Rakitje Lake (20) was the next most popular site, 
while the rest of the locations were slightly more 
evenly scattered along the Sava River, most notably 
the Sava near Savica (6) and the Sava at Blato (4). 
More than a quarter of non-colony locations were 
spread out along the seven other areas of the Sava 
between Strmec and Rugvica, as well as on Lakes 
62
Abesinija (16) and Jarun (18). The remaining 15 
areas made up the remaining 3.8% of non-colony 
locations.
The Sava at Siromaja (10), Rugvica lakes (17), 
Abesinija Lake (16), the Sava at Rugvica (11) and 
the Sava at Hrušćica (9), all within 5 km of the 
colony on Siromaja 2, were used by all 12 birds 
breeding there. They spent a vast majority of their 
time (94.1% of all locations) in these areas closest 
to their colony. All five remaining parts of the Sava 
River between Savica and Prevlaka, within 15 km 
of the colony, were used by half or more of the 
tagged birds, although far less regularly (3.7% of 
all locations). The Sava at Hrušćica (9), the Sava at 
Siromaja (10), the Sava at Rugvica (11) and Abesin-
ija Lake (16) were the only areas commonly used by 
birds breeding on Siromaja 2.
A much larger number of areas, 12 of them, were 
used by all four birds breeding at Rakitje. A further 
7 were used by two or more birds, completing the 
entire stretch of the river from the Slovenian border 
to Prevlaka. Apart from the most popular areas the 
Sava at Hrušćica (9), Rakitje Lake (20), the Sava 
near Savica (6) and the Sava at Blato (4), one bird 
was present commonly on the Sava at Rakitje (3), 
and another at Abesinija Lake (16).
Table 3: Number of locations in particular areas for Common Terns from Siromaja 2. Abbreviations the same as in 
Table 2.
Tabela 3: Število lokacij v posameznih območjih za čigre s Siromaje 2. Okrajšave enake kot v tabeli 2.
D. Tome et al.: Area use and important areas for Common Tern Sterna hirundo inland populations breeding in Slovenia and 
Croatia
Area Code number of individual tern statistics
distance (m) 961021 961023 961024 961026 961029 961030 961031 961032 961033 961034 961035 961036 all terns % locations N >=10% Cummulative %
Siromaja colony 2160 667 1794 898 1506 1769 2134 1377 2050 1529 932 1316 18132 100.0
10 Sava at Siromaja 700 125 34 234 35 152 85 58 82 107 178 181 37 1308 17.6 12 10 17.6
17 Rugvica lakes 1000 3 2 27 4 5 3 2 14 11 9 3 1 84 1.1 12 0 18.7
16 Abesinija lake 2700 5 268 326 22 220 9 7 499 53 116 7 7 1539 20.7 12 6 39.5
11 Sava at Rugvica 3100 48 18 4 32 45 4 67 178 128 28 66 48 666 9.0 12 7 48.4
9 Sava at Hrušćica 3900 274 35 509 68 750 211 92 161 442 419 287 142 3390 45.6 12 11 94.1
12 Sava Novaki-Oborovo 5500 3 6 0 1 1 0 0 8 11 6 2 4 42 0.6 9 0 94.6
8 Sava at Ščitarjevo 7200 4 2 0 1 4 1 0 1 12 4 0 11 40 0.5 9 0 95.2
15 Čiče lake 8200 0 0 0 0 37 0 0 0 0 13 11 5 66 0.9 4 0 96.0
13 Sava Oborovo-Prevlaka 9400 1 0 0 0 0 0 0 8 25 2 1 1 38 0.5 6 0 96.6
7 Sava at Petruševec 9700 3 3 0 0 0 2 3 4 32 2 0 20 69 0.9 8 0 97.5
14 Sava Prevlaka-Prerovec 13500 0 1 0 0 0 0 0 3 2 1 0 0 7 0.1 4 0 97.6
6 Sava near Savica 14500 6 3 0 0 0 1 1 3 1 1 0 7 23 0.3 8 0 97.9
5 Sava Jarun-Bundek 18000 0 1 0 0 0 0 2 2 0 0 0 3 8 0.1 4 0 98.0
4 Sava at Blato 23100 0 3 0 0 0 0 11 3 0 0 0 2 19 0.3 4 0 98.3
3 Sava at Rakitje 26500 0 2 0 0 0 0 0 4 0 0 0 8 14 0.2 3 0 98.4
Rakitje colony 27100 0 17 0 0 0 0 5 11 0 0 0 29 62 0.8 4 0 99.3
20 Rakitje lake 27500 0 3 0 0 0 0 0 3 0 0 0 15 21 0.3 3 0 99.6
21 Kerestinec 28800 0 0 0 0 0 0 0 0 0 0 0 2 2 0.0 1 0 99.6
27 Garešnica-Poljana fishponds 65200 0 0 0 0 0 0 0 0 31 0 0 0 31 0.4 1 0 100.0
all areas w/o colony 472 398 1100 163 1214 316 248 984 855 779 558 342 7429
63
Acrocephalus 40 (180/181): 55–67, 2019
Area Code number of individual tern statistics
distance (m) 961021 961023 961024 961026 961029 961030 961031 961032 961033 961034 961035 961036 all terns % locations N >=10% Cummulative %
Siromaja colony 2160 667 1794 898 1506 1769 2134 1377 2050 1529 932 1316 18132 100.0
10 Sava at Siromaja 700 125 34 234 35 152 85 58 82 107 178 181 37 1308 17.6 12 10 17.6
17 Rugvica lakes 1000 3 2 27 4 5 3 2 14 11 9 3 1 84 1.1 12 0 18.7
16 Abesinija lake 2700 5 268 326 22 220 9 7 499 53 116 7 7 1539 20.7 12 6 39.5
11 Sava at Rugvica 3100 48 18 4 32 45 4 67 178 128 28 66 48 666 9.0 12 7 48.4
9 Sava at Hrušćica 3900 274 35 509 68 750 211 92 161 442 419 287 142 3390 45.6 12 11 94.1
12 Sava Novaki-Oborovo 5500 3 6 0 1 1 0 0 8 11 6 2 4 42 0.6 9 0 94.6
8 Sava at Ščitarjevo 7200 4 2 0 1 4 1 0 1 12 4 0 11 40 0.5 9 0 95.2
15 Čiče lake 8200 0 0 0 0 37 0 0 0 0 13 11 5 66 0.9 4 0 96.0
13 Sava Oborovo-Prevlaka 9400 1 0 0 0 0 0 0 8 25 2 1 1 38 0.5 6 0 96.6
7 Sava at Petruševec 9700 3 3 0 0 0 2 3 4 32 2 0 20 69 0.9 8 0 97.5
14 Sava Prevlaka-Prerovec 13500 0 1 0 0 0 0 0 3 2 1 0 0 7 0.1 4 0 97.6
6 Sava near Savica 14500 6 3 0 0 0 1 1 3 1 1 0 7 23 0.3 8 0 97.9
5 Sava Jarun-Bundek 18000 0 1 0 0 0 0 2 2 0 0 0 3 8 0.1 4 0 98.0
4 Sava at Blato 23100 0 3 0 0 0 0 11 3 0 0 0 2 19 0.3 4 0 98.3
3 Sava at Rakitje 26500 0 2 0 0 0 0 0 4 0 0 0 8 14 0.2 3 0 98.4
Rakitje colony 27100 0 17 0 0 0 0 5 11 0 0 0 29 62 0.8 4 0 99.3
20 Rakitje lake 27500 0 3 0 0 0 0 0 3 0 0 0 15 21 0.3 3 0 99.6
21 Kerestinec 28800 0 0 0 0 0 0 0 0 0 0 0 2 2 0.0 1 0 99.6
27 Garešnica-Poljana fishponds 65200 0 0 0 0 0 0 0 0 31 0 0 0 31 0.4 1 0 100.0
all areas w/o colony 472 398 1100 163 1214 316 248 984 855 779 558 342 7429
5. Discussion
A little more that 42% of locations obtained by 
tags in Slovenia were from breeding islands, while 
terns spent more time in Croatian colonies (70.9% 
on Siromaja 2 and 57% on Rakitje). To some point 
the number of locations on breeding islands is a 
consequence of the time when terns were caught 
and fitted with the tag. Our aim was to do this as 
late during incubation period as possible, preferably 
in the last two days of incubation. To minimize dis-
turbance, we did not monitor the colony on a daily 
basis, so we were not able to determine hatching 
day/last days of incubation period with great pre-
cision. The earlier we caught the terns during incu-
bation period, the more days it spent incubating the 
eggs, which increased the number of locations from 
this area. This rationale might seem to clash with 
data from Croatia: birds from the Rakitje colony 
were tracked from the first day of nesting because 
they had been equipped with devices beforehand 
on Lake Siromaja 2. Birds breeding at Siroma-
ja 2, on the other hand, were tagged in mid-incu-
bation and yet they had more colony points than 
birds at Rakitje. A possible cause for that might be 
polygon outlining – the Siromaja 2 colony polygon 
64
Table 4: Number of locations in particular areas for Common Terns from Rakitje. Abbreviations the same as in Table 2.
Tabela 4: Število lokacij v posameznih območjih za čigre z Rakitja. Okrajšave enake kot v tabeli 2.
Area Code number of individual tern statistics
dis-
tance 
(m) 961020 961022 961025 961028
all 
terns
% loca-
tions N >=10%
Cum-
mula-
tive % 
Rakitje colony 772 999 1299 1492 4562 100.0
20 Rakitje lake 500 74 399 23 59 555 16.1 4 2 16.1
3 Sava at Rakitje 700 21 61 95 38 215 6.2 4 1 22.4
21 Kerestinec 2900 0 6 0 0 6 0.2 1 0 22.5
2 Sava at Strmec & Orešje 4000 3 16 25 2 46 1.3 4 0 23.9
4 Sava at Blato 4700 34 127 51 136 348 10.1 4 2 34.0
18 Jarun lake 6800 3 14 7 17 41 1.2 4 0 35.2
5 Sava Jarun-Bundek 9100 23 65 26 40 154 4.5 4 0 39.7
1 Sava 1 9700 0 3 7 0 10 0.3 2 0 39.9
5 Bundek 11300 0 7 1 3 11 0.3 3 0 40.3
6 Sava near Savica 13100 28 210 65 104 407 11.8 4 3 52.1
29 Savica reserve 14700 0 2 0 4 6 0.2 2 0 52.3
7 Sava at Petruševec 17400 51 99 24 77 251 7.3 4 0 59.6
8 Sava at Ščitarjevo 20500 12 32 14 13 71 2.1 4 0 61.6
22 Crna Mlaka fishponds 22700 0 3 3 0 6 0.2 2 0 61.8
9 Sava at Hrušćica 23700 186 375 76 215 852 24.8 4 4 86.5
10 Sava at Siromaja 23700 37 42 18 32 129 3.7 4 0 90.3
16 Abesinija lake 25200 89 0 0 0 89 2.6 1 1 92.9
Siromaja colony 27100 25 2 0 54 81 2.4 3 0 95.2
11 Sava at Rugvica 27500 42 8 9 20 79 2.3 4 0 97.5
12 Sava Novaki-Oborovo 30200 3 0 4 8 15 0.4 3 0 98.0
24 Puhovec lakes 30900 36 0 0 0 36 1.0 1 0 99.0
13 Sava Oborovo-Prevlaka 31100 0 0 9 1 10 0.3 2 0 99.3
26 Sišćani fishponds 66400 24 0 0 0 24 0.7 1 0 100.0
all areas w/o colony 691 1471 457 823 3442
includes the entire lake since smaller platform size, 
GPS error and lake size led to “colony” points being 
scattered across almost half of the lake, while the 
Rakitje and Ptuj colony polygons included only the 
breeding islands. It is thus inevitable to surmise 
that Siromaja 2 points include foraging activity as 
well, while Rakitje and Ptuj colony points include 
only breeding activity and resting. 
At any rate, breeding islands that were de-
termined by us as separate areas within Lakes 
D. Tome et al.: Area use and important areas for Common Tern Sterna hirundo inland populations breeding in Slovenia and 
Croatia
65
Ptuj and Rakitje and the entire Siromaja 2 Lake, 
are, for obvious reasons, regarded as important 
for Common Terns by default, regardless of the 
number of recorded locations. We excluded those 
data for the analysis of other areas, so that a differ-
ence in the number of days terns spent incubating 
eggs did not influence further results.
As expected, area use of terns during breed-
ing season relates closely to large water bodies like 
rivers, lakes and fishponds. 
In Slovenia, some areas were dislocated from 
Lake Ptuj. Terns reached them only after flying 
up to 26 km over cultural landscape, where they 
crossed villages, highways and small forest patches. 
Sequences of locations showed that they were fol-
lowing the route in a rather straight line, indicat-
ing that they must have had previous experience 
about the location. The largest and continuous 
part of used area stretched along the Drava from 
Lake Ormož to Maribor. This is about 60 km long 
and very narrow part which terns seem to exploit 
simply by following the course of the Drava River. 
The most remote location from nesting ground was 
the Drava close to the village of Bresternica, NW of 
Maribor. It was about 30 km of straight line from 
nesting ground and was visited once by one tern 
only. 
The most used areas (beside nesting islands) 
were the Drava river-bed between Ptuj and Rošnja, 
the Drava at Ptuj and Lake Ptuj. These are all neigh-
bouring areas, together forming a large important 
area, which also includes breeding islands. Two 
areas that were almost 20 km of straight line away 
from the Drava, Gradiško Lake and in particu-
lar Lake Radehova, turned out to be important as 
well. The latter was used by 5 terns, four of which 
were commonly present in the area . As expected, 
the artificial channel with fast and deep water flow, 
which runs almost parallel to the Drava river-bed, 
turned out to be a totally inappropriate habitat for 
terns.
Compared to birds from Ptuj, terns in Croatia 
usually stayed much closer to the Sava River, very 
rarely straying more than 2 km from the river. The 
single most important area for birds from both 
Croatian colonies was part of the Sava River at 
Hrušćica, which is not surprisingly a Natura 2000 
protected area (Radović et al. 2005.). Apart from 
breeding within the boundaries of this Important 
Bird Area, 85% of localities of birds from the Siro-
maja colony were there as well, aligned along the 
river or at Lake Abesinija, and another 9% imme-
diately downstream. Even birds from the Rakitje 
colony regularly travelled 25 km to have a third of 
their locations there. Other areas identified as im-
portant for birds breeding on Rakitje were the Sava 
near Savica and, closer to their own colony, the Sava 
at Blato and the Sava at Rakitje, as well as Lake 
Rakitje itself.
There is a catch in our data which should be 
considered when interpreting the results. While 
tags gave us relatively exact locations of terns in 
exact time, there was no way we could read from 
them what they were doing while on the location. 
For locations within nesting area we can more or 
less accurately say that birds were involved in breed-
ing routine, whether incubating the eggs or caring 
for their chicks. A certain proportion of readings 
from this area was possibly also recorded during 
resting time. On the other hand, it is impossible to 
interpret activities of terns in other locations with 
such ease. But we believe that the majority of them 
belong to two types of activity: hunting (catching 
and searching for prey) and flying. 
Several locations outside the nesting site were 
definitively recorded during hunting activity, but 
some were almost certainly recorded also during 
flights, when terns were on their way to reach more 
distant hunting areas. In practice this means that 
some locations on Lake Ptuj and on the Drava 
at Ptuj were possibly obtained during flights to 
a hunting place on the Drava river-bed between 
Ptuj and Maribor and not during a hunt. The ratio 
between flying and hunting activity is impossible to 
quantify, but we presume that a higher proportion 
of flying activity was recorded closest to the nesting 
area than in more distant areas. It is thus possible 
that importance of some areas near the nesting 
place is both as a flying corridor toward hunting lo-
cations as well as a hunting area itself, while more 
distant ones probably had greater proportion of 
hunting activities as opposed to flying.
With this in mind, we believe that the most im-
portant hunting areas for Common Terns in Slove-
nia were the Drava river-bed from Ptuj to Rošnja, 
Gradiško Lake and Lake Radehova. Lake Ptuj 
and the Drava at Ptuj were definitely important as 
flying corridors and, to a certain level, as hunting 
Acrocephalus 40 (180/181): 55–67, 2019
66
areas too. The latter is confirmed by regular obser-
vations of terns foraging in these areas.
Taking this into account with Croatian colo-
nies, the importance for terns breeding at Rakitje 
of the far-away Sava at Hrušćica becomes even more 
obvious, and to a lesser extent that of the Sava near 
Savica. On the other hand, the Sava at Rakitje and 
the Sava at Blato both probably serve a dual purpose 
of feeding area and flight corridor.
Common Terns mostly fish within 3 to 10 km 
of the breeding grounds, with longest foraging 
distances being over 30 km (Becker & Ludwigs 
2004). The longest distances to the most frequently 
visited areas in this research were in this range, 
although rather different between locations: from 
4 km at Siromaja, to 20 and 24 km in Ptuj and 
Rakitje, respectively. The overall longest distance 
(it was made by two terns) was stunning 65 km. 
This distance corresponds roughly to the distance 
between Slovenian and Croatian breeding colonies 
at Ptuj and near Zagreb. Nevertheless, we did not 
find any evidence that terns from these colonies 
were in any kind of contact. On the other hand, 
results showed possible interactions between 
terns from the colony at Ptuj with the ones on 
Croatian side of Lake Ormož. Also, although not 
a single location of terns from Croatia was found 
on Slovenian side of the Sava, recoveries of ringed 
birds (Archives of Ringing Centers Zagreb and 
Ljubljana) showed that terns from Rakitje and 
Blato colonies do disperse to the colony on the 
Sava near Brežice.
We recommend conducting further studies of 
how environmental factors such as hydrology and 
productivity influence area use. These are neces-
sary in order to explain why areas like the Sava at 
Hrušćica and the Sava near Savica in Croatia, and 
Gradiško and Radehova Lakes in Slovenia are so fa-
vourable that birds are willing to fly 20–25 km to 
reach them instead of staying closer to their colony. 
More detailed analyses of home range are also 
necessary.
Acknowledgement: Research funding was 
provided by the Cooperation Programme Interreg 
V-A Slovenia  – Croatia (grant SLO-HR347). 
Permits for tagging and ringing of the birds were 
issued by relevant authorities (MOP in Slovenia 
and MZOE in Croatia). 
6. Povzetek
V sezonah 2018 in 2019 smo raziskovali območja, 
ki jih navadne čigre (Sterna hirundo) obiskujejo 
med gnezdenjem. Z GPS-UHF-oddajniki smo 
sledili 23 čiger iz Slovenije (7 osebkov) in Hrvaške 
(16 osebkov). Ptice smo ujeli na treh gnezdiščih: 
Ptujsko jezero, Siromaja 2 in Rakitje. Zaključki so 
narejeni na osnovi 43.105 podatkov o lokacijah, ki 
smo jih zbirali s frekvenco en odčitek na vsakih 20 
minut podnevi oz. en odčitek na vsake 4 ure ponoči.
V Sloveniji so čigre letale nad 60 km dolgim, 
ozkim območjem Drave od Ormoža do Maribora 
in nad enajstimi ribniki / jezeri v okolici, večino-
ma v dolini Pesnice. Najbolj oddaljena lokacija od 
gnezdišča je bila 30 km v ravni črti, a tako daleč 
se je odpravila le ena čigra, in to enkrat samkrat. 
Najpogosteje so se čigre zadrževale na stari strugi 
Drave med Ptujem in Rošnjo, na Ptujskem jezeru, 
na Dravi pri Ptuju ter na jezerih Radehova in 
Gradiško. Slednji sta oddaljeni okoli 20 km od gn-
ezdišča. To so bila v celinskem delu Slovenije verjet-
no najpomembnejša območja, na katerih so čigre v 
času gnezdenja iskale hrano.
Na Hrvaškem so čigre večinoma letale nad Savo, 
le izjemoma so se oddaljile tudi več kot 2 km stran 
od struge. Najdaljša razdalja od gnezdišča je bila več 
kot 60 km, a tam smo zabeležili zelo majhno število 
obiskov. Ptice iz kolonije Siromaja 2 so se večinoma 
zadrževale v radiju do 5 km od gnezdišča, medtem 
ko so ptice iz kolonije Rakitje redno letale do 
23 km oddaljenih območij. Najpogosteje obiskano 
območje za čigre iz Siromaje in Rakitja je bila Sava 
pri Hrušćici. Ptice iz kolonije v Rakitju so redno 
obiskovale tudi Savo pri Savici in nekatere vodne 
površine v radiju 5 km. To so bila v okolici Zagreba 
verjetno najpomembnejša območja, na katerih so 
čigre v času gnezdenja iskale hrano. 
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Prispelo / Arrived: 3.10.2019
Sprejeto / Accepted: 25.11.2019
Acrocephalus 40 (180/181): 55–67, 2019