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First data on breeding success of Croatian inland colonies of Common Tern Sterna hirundo

Authors:
  • JU Zeleni prsten Zagrebačke županije

Abstract and Figures

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 productivity 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 statistically significant. Our study provided baseline data for future monitoring of phenology and breeding success with regard to the management of breeding colonies.
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A  40 (180/181): 9 7–103, 2019
10.1515/-2 019- 0007
F    
 C  
 C T Sternahirundo
Prvi podatki o gnezditvenem uspehu
celinske populacije navadne čigre
Sternahirundo na Hrvaškem
M M1, J K1,
TR2, L J3,
AP4, I S5
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 Zag reb, Faculty of Science, Depa rtment
of Biology, Rooseveltov trg 6 10000 Zagreb, Croatia,
e-mai l: ida.svetlicic@biol.pmf.hr
Abstract
In 2018 and 2019, the breeding success of two
Common Tern colonies on articial lakes near the
River Sava in Zagreb, Croatia, was studied. e
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 edging success
was within previously observed ranges, apart from
a low hatching success on Siromaja in 2019. e
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 dierences were not statisti-
cally signicant. Our study provided baseline data
for future monitoring of phenology and breeding
success with regard to the management of breeding
colonies.
1. Introduction
e Common Tern Sterna hirundo breeds,
oen colonially, throughout the Holarctic and
winters along seacoasts of South America, Africa,
Australia, as well as southern and south-eastern
Asia. It inhabits dierent coastal habitats, but
also breeds in freshwater habitats, where it readily
accepts articial sites (B& L 2004).
e Common Tern is a relatively long-lived species,
with the oldest ringed bird reported to be 33 years
old (F et al. 2010). ey generally lay 1–3
eggs which they incubate 20–21 days if undis-
turbed, or 27–28 days in areas with disturbance
(N & C 1975). Chicks wander from
the nest aer 3 days and edge 22–28 days aer
hatching (N& D 1972). In the case of
early nest failure, Common Terns usually have re-
placement clutches (N& C 1975), oen
at the same colony (G-S et al. 1999,
B& Z 2011).
Common Terns breed along the Sava and
Drava Rivers in continental Croatia, and on
islets in the Adriatic Sea (K et al. 2013).
eir breeding population of 400–700 estimated
breeding pairs (K et al. 2013) is classied
as near threatened (T et al. 2013). e
Common Tern is a target breeding species for 13
Natura 2000 sites in Croatia (N N
2019). Breeding population census eorts are,
however, reduced to occasional nest counts for
inland colonies (M et al. 2017, K et
al. in print) and rare adult or nest counts along
the coast (L et al. 2012). Some studies
did mention reasons for unsuccessful breeding
(F 2005), but no investigations of hatching
success, edging 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 (S& B 2012).
We set out to establish breeding success mon-
itoring of our largest inland colonies. e aims of

this study were to test breeding success monitoring
methodology and to provide rst 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 oating
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 articial 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 (K et
al. 2013). We also regularly check other potential
breeding sites in the area and count breeding pairs
if present (K 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 H & LC
(1971). Aer conrming the start of incubation,
we did not disturb the colony for two weeks due
to high risk of nest desertion (K 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
K et al. (2011), while in 2019 we used
acrylic paint and permanent markers on pebbles.
e 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 K 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 (W et al.
1995, M et al. 2007), including non-marked
nests on Rakitje. We revisited the colonies each
subsequent week and repeated the process, mark ing
any new nest observed on the Siromaja colony.
We ringed any chicks observed to be at least two
weeks old (N& D 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 edged.
2.3. Data analysis
e 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
(W et al. 1995). e 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 edged 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 edged chicks with the total
estimated number of hatched chicks, we calculated
edging success. Colony productivity was calculat-
ed as the number of edged 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. M et al.: First data on breeding success of Croatian inla nd colonies of Common Tern Sternahirundo

breeders” were estimated to be nests rst observed
before “peak incubation”, i.e. the date of the highest
observed number of active nests–being 26 May
(W et al. 1995). “Late breeders” were nests
rst observed on or aer that date, thereby includ-
ing most of the current year’s renesting attempts.
is 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 rst and subsequent breeding
attempts. Yate’s corrected Chi square test was used
to test dierences in breeding success.
3. Results
In 2018, we observed Common Terns regularly on
or near their breeding sites from late April. ey
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
edged throug h 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 edged
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 edged chicks divided by total estimated number of hatched chicks.
Productivity – number of successfully edged 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%
edging success
uspeh speljave 45.45% 52.17% -41.60%
productivity (edglings/bp)
produktivnost (speljanih/gp) 1.0 0.92 0.57 0.78
A  40 (180/181): 9 7–103, 2019

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
ooded until mid-June, so terns started nesting
there when parts of the island started resurfacing.
We estimate that 61 chicks edged successfully. On
the last day of ringing we observed 21 dead chicks
and 3 dead adults on the colony.
e 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. e colony successfully edged 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. e total number of edged 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 dierence in hatching success was not signi-
cant2= 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 rst observed before the date when we counted the greatest number of
active nests on the colony (26 May), while “late breeders” were nests rst observed on or after that date. Difference
signicance 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 dierence signi cance
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. M et al.: First data on breeding success of Croatian inla nd colonies of Common Tern Sternahirundo

3 clutches of 3, 2 and 1 eggs respectively. e dif-
ference was not signicant (χ2= 6.81, df= 5, P =
0.23). e total number of edged chicks at the
colony was 36.
Monitoring methodology
Regarding nest marking, in 2018 terns used some
of the bamboo garden markers as nest material.
is disabled identication 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 (B & B 1982), as well
as in central Poland (M et al. 2015). ey
occurred about 10 days earlier than those observed
in populations breeding along the North Sea
(B& Z 2011, D et al. 2017) and
more than two weeks earlier than populations in
south-eastern Massachusetts (N & D
1972) and the Danish Wadden Sea (B
et al. 2015). In 2018, birds started incubating
between four and six weeks aer they had started
congregating near their breeding sites, while in
2019 incubation started two to three weeks aer
arrival. Time between arrival and laying for 2019
corresponds to that observed previously (B&
Z 2011), but in 2018 the pre-laying period
lasted longer. e most probable cause for this are
extremely high spring water levels that occurred
in 2018 in Croatia, ooding all potential breeding
sites (K et al. in print). Common Terns
therefore did not have any suitable breeding sites in
the area until we set up the oating 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 (L
1972, B & G 1981), western
Germany (W 1970), Denmark (B et
al. 2015) and Poland (M 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%, W
1970), southern Finland (80%, L
1973) and northern Germany (73%, B
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%, B et al. 1993), found in
1988 to have had a critical level of organochlorines
which reduce hatchability (F 1976). Since there
are many potential factors which can inuence
hatchability (B & G 2003), we
propose further monitoring of hatching success
on the colony, as well as a toxicological survey of
eggshells.
Considering early a nd late breeders on Siromaja
in 2019, there was no statistically signicant
dierence between early and late breeder clutch
size and hatching success. B & Z
(2011) did not nd any dierences in breeding
success between early and late breeders either,
while B 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 signicant dierences.
Fledging success and productivity were
smaller than observed in older studies (W
1970, L 1972, L 1973),
but productivity was in line with other studies
(N& D 19 72 , H 19 78, JNCC 2016 ).
Observed high mortality and consequential low
edging success on Rakitje in 2018 have later
been connected to an avian pox virus infection
diagnosed from dead birds (S pers. comm.).
M et al. (2015) observed higher productivity
in a large colony compared to smaller ones,
although chicks from the larger colony were in
poorer condition. ey attributed the dierence
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
A  40 (180/181): 9 7–103, 2019

be a result of habitat type dierences. Chicks are
protected from terrestrial and aquatic predators
on the fenced oating 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 edglings on the colony.
We recommend a study of edgling condition on
these colonies to further test the conclusions from
M et al. (2015).
According to breeding success data, Siroma-
ja and Rakitje seem like average Common Tern
breeding colonies. ey 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 eldwork 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, edging 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
A  40 (180/181): 9 7–103, 2019
... The colony at Siromaja has been established since 2018, whereas Rakitje had a continuous Common Tern colony on the same islet since 2009. This could explain some of the differences in behaviour for newer colonies such as Siromaja, which has already proven to have shorter foraging time and higher breeding success than Rakitje (Martinović et al. 2023, Martinović et al. 2019). On the other hand, Limmer & Becker (2009) have demonstrated that more experienced breeding pairs have a higher foraging success rate, enabling them to spend more time in their colony. ...
... Taking into account the results presented herein as well as previous findings of terns prioritising seasonally abundant food (Erwin 1977), foraging areas with high prey density such as commercial fishponds (Becker et al. 1997) and shallow, flowing water (Urmy & Warren 2018) we concluded that prey availability and predictability play a major role in Common Tern behaviour during incubation. The differences in nest attendance during the incubation period could affect reproductive success, which has already been proven for the two studied freshwater colonies (Martinović et al. 2019). ...
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