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Acrocephalus 40 (180/181): 97–103, 2019
10.1515/acro-2019-0007
First data on breeding success 
of Croatian inland colonies 
of Common Tern Sterna hirundo
Prvi podatki o gnezditvenem uspehu 
celinske populacije navadne čigre 
Sterna hirundo na Hrvaškem
Miloš Martinović1, Jelena Kralj1, 
Tomica Rubinić2, Luka Jurinović3, 
Ana Petrović4, Ida Svetličić5
1 Institute of Ornithology, Croatian Academy of 
Sciences and Arts, Gundulićeva 24, 10000 Zagreb, 
Croatia, e-mail: martinovic@hazu.hr, jkralj@hazu.hr
2 Public Institution Green Ring, 151. samoborske 
brigade HV 1, 10430 Samobor, Croatia, e-mail: 
projekt-cigra@zeleni-prsten.hr
3 Croatian Veterinary Institute, Poultry Centre, 
Heinzelova 55, 10000 Zagreb, Croatia, e-mail: luka.
jurinovic@gmail.com
4 Pleternička 26, 31000 Osijek, Croatia, e-mail: 
petroana@gmail.com
5 University of Zagreb, Faculty of Science, Department 
of Biology, Rooseveltov trg 6 10000 Zagreb, Croatia, 
e-mail: ida.svetlicic@biol.pmf.hr
Abstract
In 2018 and 2019, the breeding success of two 
Common Tern colonies on artificial lakes near the 
River Sava in Zagreb, Croatia, was studied. The 
colonies were visited weekly from May to July and 
we collected data on phenology, number of breeding 
pairs, clutch size as well as egg and chick survival. 
We also conducted a comparison between early 
and late breeders. Hatching and fledging success 
was within previously observed ranges, apart from 
a low hatching success on Siromaja in 2019. The 
smaller colony on Siromaja had a higher produc-
tivity in both years than the colony on Rakitje, 
although in 2018 an avian pox virus killed much 
of the late chicks on Rakitje. Early breeders seem 
to have had higher hatching success and average 
clutch size. Furthermore, a greater proportion of 
them managed to hatch all their eggs compared to 
late breeders, but the differences were not statisti-
cally significant. Our study provided baseline data 
for future monitoring of phenology and breeding 
success with regard to the management of breeding 
colonies.
1. Introduction
The Common Tern Sterna hirundo breeds, 
often colonially, throughout the Holarctic and 
winters along seacoasts of South America, Africa, 
Australia, as well as southern and south-eastern 
Asia. It inhabits different coastal habitats, but 
also breeds in freshwater habitats, where it readily 
accepts artificial sites (Becker & Ludwigs 2004). 
The Common Tern is a relatively long-lived species, 
with the oldest ringed bird reported to be 33 years 
old (Fransson et al. 2010). They generally lay 1–3 
eggs which they incubate 20–21 days if undis-
turbed, or 27–28 days in areas with disturbance 
(Nisbet  & Cohen 1975). Chicks wander from 
the nest after 3 days and fledge 22–28 days after 
hatching (Nisbet & Drury 1972). In the case of 
early nest failure, Common Terns usually have re-
placement clutches (Nisbet & Cohen 1975), often 
at the same colony (González-Solís et al. 1999, 
Becker & Zhang 2011). 
Common Terns breed along the Sava and 
Drava Rivers in continental Croatia, and on 
islets in the Adriatic Sea (Kralj et al. 2013). 
Their breeding population of 400–700 estimated 
breeding pairs (Kralj et al. 2013) is classified 
as near threatened (Tutiš et al. 2013). The 
Common Tern is a target breeding species for 13 
Natura 2000 sites in Croatia (Narodne Novine 
2019). Breeding population census efforts are, 
however, reduced to occasional nest counts for 
inland colonies (Mikuska et al. 2017, Kralj et 
al. in print) and rare adult or nest counts along 
the coast (Lucić et al. 2012). Some studies 
did mention reasons for unsuccessful breeding 
(Fenyosi 2005), but no investigations of hatching 
success, fledging success or productivity have thus 
far been conducted in Croatia. Breeding success 
monitoring and other demographic indicators 
in addition to population size can give valuable 
information for timely problem diagnosis and 
conservation actions (Szostek & Becker 2012). 
We set out to establish breeding success mon-
itoring of our largest inland colonies. The aims of 
98
this study were to test breeding success monitoring 
methodology and to provide first data on Common 
Tern breeding success and phenology in Croatia. 
2. Methods
2.1. Study location
During the 2018 and 2019 breeding seasons, we 
surveyed breeding success on two Common Tern 
colonies near Zagreb, Croatia. One was a floating 
platform set up in mid-May 2018 in Siromaja 
gravel pit (45°45'23.9"N, 16°11'05.0"E), while the 
other is an artificial island located in Rakitje gravel 
pit (45°47'49.8"N, 15°50'24.9"E). Both lakes are 
located c. 500  m from the Sava River. Common 
Terns usually arrive to these sites during April and 
depart through August and September (Kralj et 
al. 2013). We also regularly check other potential 
breeding sites in the area and count breeding pairs 
if present (Kralj et al. in print).
2.2. Field work
Colonies were either visited or observed weekly 
from mid-April in order to determine the start 
of incubation. In case we missed observing this 
milestone, we estimated it by using the egg-im-
mersion method as described by Hays & LeCroy 
(1971). After confirming the start of incubation, 
we did not disturb the colony for two weeks due 
to high risk of nest desertion (Koffijberg et al. 
2011). During the third week, we visited the colony 
and counted the number of nests and eggs per nest. 
Where following hatching success, we marked as 
many nests as possible. In 2018 we used bamboo 
garden markers numbered with a permanent 
marker to mark the nests, as recommended by 
Koffijberg et al. (2011), while in 2019 we used 
acrylic paint and permanent markers on pebbles. 
The reasons for this change are described in results 
under Monitoring methodology. In 2018 we only 
marked nests on the platform at Siromaja, whereas 
in 2019 we marked nests on both colonies. On 
Siromaja we attempted to mark all observed nests, 
while on the larger colony at Rakitje we marked 
a representative proportion following guidelines 
by Koffijberg et al. (2011). We recorded the 
number of eggs and/or chicks in each nest and their 
status (observed state, incubation stage), as well as 
counted all apparently active nests (Walsh et al. 
1995, Mikuska et al. 2007), including non-marked 
nests on Rakitje. We revisited the colonies each 
subsequent week and repeated the process, marking 
any new nest observed on the Siromaja colony. 
We ringed any chicks observed to be at least two 
weeks old (Nisbet & Drury 1972) with metal and 
coloured plastic rings. We also noted the number 
of dead chicks, documenting those that had been 
ringed earlier. We stopped visiting a colony when 
all chicks had fledged.
2.3. Data analysis
The highest number of observed active nests during 
a single visit was taken as the number of breeding 
pairs on that colony for the respective season 
(Walsh et al. 1995). The maximum number of 
eggs per nest was noted as clutch size for that nest 
for the season, while the number of eggs present in 
a nest on the last visit before hatching was taken 
as an estimate of number of hatched chicks. We 
omitted a nest from the calculations if for any 
reason we could not estimate the fate of its clutch 
(e.g. loss or mixing of markings, documented brood 
size inconsistency, etc.). Hatching success was thus 
calculated as the estimated number of hatched 
chicks in monitored nests divided by the total 
number of monitored eggs. Since it turned out to 
be impossible to monitor all nests on either colony, 
we estimated the total number of hatched chicks on 
a colony with the formula:
Total number of hatched chicks =
breeding pairs × average clutch size × 
monitored nests hatching success
Number of fledged chicks was calculated as all 
ringed chicks plus any small chicks present during 
the last visit, subtracted by the number of dead 
chicks observed with the current year’s rings. Di-
viding the number of fledged chicks with the total 
estimated number of hatched chicks, we calculated 
fledging success. Colony productivity was calculat-
ed as the number of fledged chicks divided by the 
estimated number of breeding pairs.
For the colony on Siromaja in 2019 we also com-
pared “early breeders” and “late breeders”. “Early 
M. Martinović et al.: First data on breeding success of Croatian inland colonies of Common Tern Sterna hirundo
99
breeders” were estimated to be nests first observed 
before “peak incubation”, i.e. the date of the highest 
observed number of active nests–being 26 May 
(Walsh et al. 1995). “Late breeders” were nests 
first observed on or after that date, thereby includ-
ing most of the current year’s renesting attempts. 
This analysis was not performed for either colony 
in 2018 or for Rakitje in 2019 because the relative-
ly late starting dates for those colonies blurred any 
distinction between first and subsequent breeding 
attempts. Yate’s corrected Chi square test was used 
to test differences in breeding success.
3. Results
In 2018, we observed Common Terns regularly on 
or near their breeding sites from late April. They 
started incubating on the platform on Siromaja 
lake in late May, while incubation on the island on 
Rakitje lake started in mid-June (Table 1). Young 
fledged through July on Siromaja and from mid-July 
to late August on Rakitje. In 2019, we regularly 
observed Common Terns on or near breeding sites 
from mid-April. On both Siromaja and Rakitje, 
incubation started in early May. Young fledged 
Table 1: Common Tern breeding success across two years on Siromaja and Rakitje, two colonies near Zagreb. 
Hatching success – estimated number of hatched chicks in monitored nests divided by total number of monitored 
eggs. Fledging success – number of fledged chicks divided by total estimated number of hatched chicks. 
Productivity – number of successfully fledged chicks per breeding pair.
Tabela 1: Gnezditveni uspeh navadne čigre v dveh letih na kolonijah Siromaja in Rakitje. Uspeh izvalitve – ocenjeno 
število izvaljenih mladičev, deljeno s številom gnezd (le gnezda, ki so bila spremljana redno). Uspeh speljave – število 
speljanih mladičev, deljeno z oceno izvaljenih mladičev. Produktivnost – število speljanih mladičev na gnezdeči par.
Siromaja Rakitje
2018 2019 2018 2019
start of incubation
začetek valjenja 23 May 02 May mid-June 08 May
last chick ringed
obročkan zadnji mladič 26 July 16 July 23 August 05 August
breeding pairs
gnezdečih parov 30 39 106 134
monitored nests
število nadzorovanih gnezd 23 43 - 43
hatching success
uspeh izvalitve 85.29% 69.16% - 79.41%
mean clutch size
povprečna velikost legla 2.77 2.43 - 2.37
% pairs with clutch of 1
% parov z enim jajcem 8.57% 10.00% - 6.98%
% pairs with clutch of 2
% parov z dvema jajcema 5.71% 36.67% - 51.16%
% pairs with clutch of 3
% parov s tremi jajci 85.71% 53.33% - 39.53%
% pairs with clutch of 4
% parov s štirimi jajci 0% 0% - 2.33%
fledging success
uspeh speljave 45.45% 52.17% - 41.60%
productivity (fledglings/bp)
produktivnost (speljanih/gp) 1.0 0.92 0.57 0.78
Acrocephalus 40 (180/181): 97–103, 2019
100
from early June to mid-July on Siromaja, and from 
late June to early August on Rakitje.
Lake Rakitje had a colony of an estimated 106 
breeding pairs in 2018. Most of the island was 
flooded until mid-June, so terns started nesting 
there when parts of the island started resurfacing. 
We estimate that 61 chicks fledged successfully. On 
the last day of ringing we observed 21 dead chicks 
and 3 dead adults on the colony.
The colony on lake Rakitje had an estimat-
ed 134 breeding pairs in 2019. Based on 43 moni-
tored nests, 81 out of 102 monitored eggs hatched. 
From this we extrapolated an estimated 293 eggs 
that successfully hatched on the whole colony that 
year. Of the 43 monitored breeding pairs, one had 
a clutch of four eggs, 17 had clutches of three, 22 
were clutches of two and 3 had clutches of one egg. 
30 of the pairs (69.8%) managed to hatch all their 
eggs. The colony successfully fledged 105 chicks.
On the platform at Siromaja lake we estimated 
30 breeding pairs of Common Terns in 2018. 
Based on 23 successfully monitored nests, 58 out 
of 68 monitored eggs hatched successfully. We 
estimate that 8 more unmonitored eggs hatched. 
Of the 23 monitored breeding pairs, 15 (65.22%) 
managed to hatch all their laid eggs. During the 
season we observed 30 clutches of 3 eggs, two 
clutches of 2 eggs and three clutches with only 
one egg. The total number of fledged chicks at the 
colony was 30.
In 2019 we estimated 39 pairs of Common 
Terns to be breeding on the Siromaja colony. Based 
on 43 monitored nests (some obviously being re-
nesting attempts), 74 out of 109 monitored eggs 
hatched. We estimate an additional 32 eggs to have 
hatched. Of the 43 monitored breeding pairs, 22 
(51.63%) managed to hatch all their laid eggs, 19 
of them being early breeders and 3 late. Consider-
ing early versus late breeder performance (Table 2), 
the difference in hatching success was not signifi-
cant (χ2 = 0.77, df = 1, P = 0.380) and neither was 
the proportion of pairs with all hatched eggs (χ2 = 
1.36, df = 1, P = 0.244). Considering all nests ob-
served on the colony, they had an average clutch size 
of 2.43 eggs, with 32 clutches of 3 eggs, 22 clutches 
of 2 eggs and six with only one egg. Among those, 
early breeders had 23, 8 and 3 clutches of 3, 2 and 1 
eggs respectively, while late breeders had 9, 14 and 
Table 2: Comparison of early versus late breeder breeding success on a breeding platform near Zagreb in 2019. 
“Early breeders” were estimated to be nests first observed before the date when we counted the greatest number of 
active nests on the colony (26 May), while “late breeders” were nests first observed on or after that date. Difference 
significance is given as p-value of Yate’s corrected Chi square test.
Tabela 2: Primerjava gnezditvenega uspeha med zgodnjimi in poznimi gnezdilci na gnezdilnem splavu pri Zagrebu 
v letu 2019. Pri zgodnjih gnezdilcih so bila gnezda najdena pred datumom, ko je bilo na gnezdišču največ gnezd 
(26. maj), pri poznih so bila gnezda najdena prvič na ta dan ali kasneje. Značilnost razlike je prikazana s Hi-kvadrat 
testom in Yeatsovim popravkom.
Siromaja 2019
early breeders
zgodnji gnezdilci
late breeders
pozni gnezdilci
difference significance
značilnost razlike
hatching success
uspeh izvalitve 72.78% 53.85% p = 0.38
nests with 100% hatched eggs
gnezda s 100% izvaljenimi jajci 63.33% 23.08% p = 0.24
mean clutch size
povprečna velikost legla 2.59 2.23
% pairs with clutch of 1
% parov z enim jajcem 8.82% 11.54%
p = 0.23% pairs with clutch of 2% parov z dvema jajcema 23.53% 53.85%
% pairs with clutch of 3
% parov s tremi jajci 67.65% 34.62%
M. Martinović et al.: First data on breeding success of Croatian inland colonies of Common Tern Sterna hirundo
101
3 clutches of 3, 2 and 1 eggs respectively. The dif-
ference was not significant (χ2 = 6.81, df = 5, P = 
0.23). The total number of fledged chicks at the 
colony was 36.
Monitoring methodology
Regarding nest marking, in 2018 terns used some 
of the bamboo garden markers as nest material. 
This disabled identification of nests, the markers of 
which had been taken, as well as of nests containing 
multiple markers. Additionally, towards the end of 
the breeding season, some markings faded away and 
became impossible to read. For these reasons we 
were unable to follow 7 out of 34 (20.59%) marked 
nests until the end of the study. In 2019, when using 
pebbles with acrylic paint, we only lost track of 2 
out of 42 (4.76%) marked nests.
4. Discussion
Arrival of Common Terns to the colonies around 
Zagreb in both years and the start of incubation 
in 2019 coincide with those observed along the 
Po River (Bogliani  & Barbieri 1982), as well 
as in central Poland (Minias et al. 2015). They 
occurred about 10 days earlier than those observed 
in populations breeding along the North Sea 
(Becker & Zhang 2011, Dobson et al. 2017) and 
more than two weeks earlier than populations in 
south-eastern Massachusetts (Nisbet  & Drury 
1972) and the Danish Wadden Sea (Bregnballe 
et al. 2015). In 2018, birds started incubating 
between four and six weeks after they had started 
congregating near their breeding sites, while in 
2019 incubation started two to three weeks after 
arrival. Time between arrival and laying for 2019 
corresponds to that observed previously (Becker & 
Zhang 2011), but in 2018 the pre-laying period 
lasted longer. The most probable cause for this are 
extremely high spring water levels that occurred 
in 2018 in Croatia, flooding all potential breeding 
sites (Kralj et al. in print). Common Terns 
therefore did not have any suitable breeding sites in 
the area until we set up the floating platform at lake 
Siromaja in mid-May, while their traditional colony 
resurfaced only in mid-June.
All mean clutch sizes are within the range de-
scribed by previous studies in England (Langham 
1972, Bullock  & Gomersall 1981), western 
Germany (Witt 1970), Denmark (Bregnballe et 
al. 2015) and Poland (Minias et al. 2015).
Hatching success on Siromaja in 2018 and 
Rakitje in 2019 was within the range described by 
previous studies in western Germany (77%, Witt 
1970), southern Finland (80%, Lemmetyinen 
1973) and northern Germany (73%, Becker 
et al. 1993). However, hatching success in the 
Siromaja colony in 2019 was smaller than that 
of a “highly contaminated” colony in northern 
Germany (68%, Becker et al. 1993), found in 
1988 to have had a critical level of organochlorines 
which reduce hatchability (Fox 1976). Since there 
are many potential factors which can influence 
hatchability (Burger  & Gochfeld 2003), we 
propose further monitoring of hatching success 
on the colony, as well as a toxicological survey of 
eggshells.
Considering early and late breeders on Siromaja 
in 2019, there was no statistically significant 
difference between early and late breeder clutch 
size and hatching success. Becker  & Zhang 
(2011) did not find any differences in breeding 
success between early and late breeders either, 
while Bregnballe et al. (2015) found that 
early breeders had larger clutches. Continued 
monitoring of early and late breeder breeding 
success is required to elucidate whether there are 
any significant differences.
Fledging success and productivity were 
smaller than observed in older studies (Witt 
1970, Langham 1972, Lemmetyinen 1973), 
but productivity was in line with other studies 
(Nisbet & Drury 1972, Hays 1978, JNCC 2016). 
Observed high mortality and consequential low 
fledging success on Rakitje in 2018 have later 
been connected to an avian pox virus infection 
diagnosed from dead birds (Savić pers. comm.). 
Minias et al. (2015) observed higher productivity 
in a large colony compared to smaller ones, 
although chicks from the larger colony were in 
poorer condition. They attributed the difference 
to opposing selective pressures – higher predation 
on smaller colonies versus higher parasitism/social 
stress on the larger ones. However, we observed 
an opposite situation regarding productivity. 
Lower apparent productivity on Rakitje (the 
larger colony) in comparison to Siromaja might 
Acrocephalus 40 (180/181): 97–103, 2019
102
be a result of habitat type differences. Chicks are 
protected from terrestrial and aquatic predators 
on the fenced floating platform on lake Siromaja. 
On the other hand, the island on lake Rakitje has 
dense patches of reeds, willow and poplar around 
its edges. While these patches provide chicks with 
cover from predators, they also enable them to 
hide from researchers, thus potentially lowering 
the surveyed number of fledglings on the colony. 
We recommend a study of fledgling condition on 
these colonies to further test the conclusions from 
Minias et al. (2015).
According to breeding success data, Siroma-
ja and Rakitje seem like average Common Tern 
breeding colonies. They thus represent a valuable 
opportunity to monitor and research Common 
Tern population dynamics, ecology and ethology.
Acknowledgements: Funding for this study was 
provided by the Cooperation Programme Interreg 
V-A Slovenia  – Croatia (grant SLO-HR347). 
We thank Niko Brajdić, Luka Basrek and Sven 
Kapelj for fieldwork assistance in counting nests 
and ringing tern chicks. We are also grateful to 
anonymous reviewers for their valuable comments.
5. Povzetek
V letih 2018 in 2019 smo raziskovali gnezditveni 
uspeh na dveh kolonijah navadne čigre v bližini 
Save pri Zagrebu. Kolonije smo obiskovali tedensko 
od maja do julija. Zbirali smo podatke o fenologiji, 
številu gnezdečih parov, velikosti legla in preživetju 
jajc ter mladičev. Primerjali smo podatke z zgodnjih 
in poznih gnezd. Uspešnost izvalitve in speljave sta 
bili v rangu prejšnjih opazovanj z izjemo nizkega 
uspeha speljave na Siromaji leta 2019. Manjša kolonija 
na Siromaji je imela v obeh letih višjo produkcijo 
mladičev kot kolonija na Rakitju. Leta 2018 je zaradi 
virusa na Rakitju poginilo veliko mladičev. Zgodnji 
gnezdilci so imeli večji uspeh izvalitve z večjim 
leglom, večji delež jih je izvalil vsa jajca v primerjavi 
s poznimi gnezdilci, a razlike statistično niso bile 
značilne. Naša raziskava je osnova za nadaljnje 
preučevanje fenologije in gnezditvenega uspeha 
predvsem pod vplivom naravovarstvenih ukrepov.
Keywords: phenology, clutch size, hatching 
success, fledging success, productivity
Ključne besede: fenologija, velikost legla, 
izvalitveni uspeh, uspeh speljave, produkcija
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Prispelo / Arrived: 5.9.2019
Sprejeto / Accepted: 10.1.2020
Acrocephalus 40 (180/181): 97–103, 2019