Acrocephalus 23 (115): 163 – 168, 2002 Common Tern Sterna hirundo breeding population: development and nature conservation management results at the Ormo` wastewater basins between 1992 and 2002 (NE Slovenia) Razvoj kolonije navadnih ~iger Sterna hirundo in rezultati naravovarstvenega upravljanja v bazenih za odpadne vode pri Ormo`u v obdobju 1992-2002 (SV Slovenija) Damijan Denac Gorki~eva 14, SI -1000 Ljubljana, Slovenia, e-mail: katarina.senegacnik@guest.arnes.si A colony of Common Terns Sterna hirundo and Black-headed Gulls Larus ridibundus formed in 1981 in the wastewater basins of the sugar factory in Ormo`. From than on, the colony has been monitored regularly by direct counting. The site was flooded in 1994 and, in 1995 and 1996, Common Terns and Black-headed Gulls did not breed. In 1997, the first artificial breeding raft (surface area 12.5 m2) was placed, followed in 1998 and 2001 by additional rafts (14 m2 and 96 m2, respectively). All rafts were placed with the intention of preserving the Common Tern, since this species is highly endangered in Slovenia. As a result, the biggest mixed Common Tern and Black-headed Gull colony in Slovenia has been established on the artificial breeding rafts. In 2002, 64 pairs of Common Tern and 113 pairs of Black-headed Gull bred on rafts, amounting to 45% and 50% respectively of the Slovene breeding population. Black-headed Gulls started to breed in higher numbers on the largest rafts, placed in 2001, and, for that reason, the percentage of Terns breeding on the rafts decreased. In 2000 all the Terns, but in 2001 only 80% of those in the basins were breeding on rafts. It is most probable that the bigger rafts, with special breeding structures for Common Terns, were more attractive for Black-headed Gulls. Obviously they liked the heterogeneous raft surface, because they built their nests on the breeding structures planned to increase the breeding success of Common Terns. Key words: Sterna hirundo, Common Tern, Larus ridibundus, Black-headed Gull, breeding, density, artificial rafts, management, sugar factory, Slovenia Klju~ne besede: Sterna hirundo, navadna ~igra, Larus ridibundus, re~ni galeb, gnezdenje, gostota, umetni splavi, upravljanje, tovarna sladkorja, Slovenija 1. Introduction The earliest breeding records for the Common Tern Sterna hirundo in Slovenia date from 1921 (Reiser 1925). In that year, the species was breeding in the furcation zone of the river Drava. The next reported breeding was in 1977 on a natural gravelly islet on the Drava ([tumberger in: Geister 1995). These were the last Common Terns reported to breed in Slovenia on the species’ natural breeding habitat. No further breeding on natural riverbanks or islands was confirmed. The main cause is the large-scale river canalisation and regulation for hydroelectric power stations. The absence of natural river dynamics (annual floods) resulted in dried river channels and disappearance of gravelly habitats ([tumberger 1995). All further colonies were found on artificial, more or less man-made structures – salt pans ([kornik 1983), artificial islets (Jan`ekovi~ & [tumberger 1984), gravel pits (Vogrin 1991), concrete objects (Bra~ko 1999), a sandy islet in an artificial accumulation lake ([alamun 2001) and in the basins of the sugar factory ([tumberger 1982). At the latter site, a mixed Common Tern and Black-headed Gull Larus ridibundus colony was established in 1981, and has 163 D. Denac: Common Tern Sterna hirundo breeding population: development and nature conservation management results at the Ormo` wastewater basins between 1992 and 2002 (NE Slovenia) been monitored annually from that time on. After the complete breeding failure in 1993 and 1994, members of DOPPS – BirdLife Slovenia started to implement a Common Tern conservation programme in that area. In this paper, the population dynamics of breeding Common Terns and Black-headed Gulls in the period between 1992 and 2002, together with the results of Common Tern conservation programme, are presented. 2. Study area and methods 2.1. Study area The wastewater basins are situated near the Ormo` accumulation lake on the river Drava. The river was recognised as an Important Bird Area - IBA (Polak 2000). The whole IBA area covers 8300 ha and was identified as the proposed Special Protected Area -SPA (Bo`i~ in press). The area also includes basins which are sugar factory’s cleaning device for wastewater. Water is cleaned in 6 rectangular sedimentation basins. Two of them (111 × 411 m) are for collecting mud and four (160 × 418 m) for collecting water. They were built in 1980 when the very first campaign for sugar production from sugar beet took place. The total surface area of the water basins’ is 0.27 km2 and the length of their banks is 6.7 km (E. [krinjar pers. comm.). They are situated in the former flooded area of the Drava. Basins are very important nationally from the ornithological point of view. They are the most important resting areas in Slovenia for migrating shorebirds and the only recently known breeding places for certain waterbird species, such as Black-necked Grebe Podiceps nigricollis, Pintail Anas acuta, and Common Redshank Tringa totanus ([tumberger 2001a & b, in press). 2.2. Common Tern conservation programme and description of rafts The main goal of our conservation programme was to preserve the Common Tern as a breeding species in the basins. For that purpose we decided to make and place the very first artificial breeding raft (raft 1) in Slovenia. Encouraged by the first year’s results we placed an additional raft in the basins in the following year (raft 2). Based on observation of the rafts Table 1: Raft characteristics Tabela 1: Podatki o splavih Raft 1 / splav 1 Raft 2 / splav 2 Rafts 3 / splav 3 Surface area / povr{ina Surface material/ material na povr{ini Height of edges (above gravel level)/ vi{ina robov (nad prodom) Structures on the surface/ strukture na povr{ini 2.5 m2 gravel (5 cm layer)/ prod (5 cm sloj) 7 cm 14.0 m2 gravel (5 cm layer)/ prod (5 cm sloj) 1998-1999: 7 cm 2000-2002: 20 cm none / brez 1998-1999: none / brez 2000-2002: 7 "chickshelters"a/ 7 kritij za mladi~ea Duration / trajanje Other / drugo 1997-2000 one 1 m wide plank from the raft to the water/ meter {iroka deska iz splava v vodo 1998-2002 96.0 m2 gravel (5 cm layer)/ prod (5 cm sloj) 35 cm 5 "chick shelters", old branches and stumps on each raft / na vsakem splavu 5 kritij za mladi~e, veje, {tori 2001- in function/ {e v uporabi one i m wide plank 3 rafts 8x4m, firmly from the raft to the connected; without planks water / meter {iroka deska to the water / 3 ~vrsto povezani iz splava v vodo splavi; brez desk do vode Remark: amodified after Burness & Morris (1991) Opomba: aprirejeno po Burness & Morris (1991) 164 Acrocephalus 23 (115): 163 – 168, 2002 Table 2: Positions of Common Tern Sterna hirundo nests at the wastewater basins of the Ormo` sugar factory Tabela 2: Mesta gnezdenja navadne ~igre Sterna hirundo v bazenih za odpadne vode Tovarne sladkorja v Ormo`u Year / leto Elsewhere / drugje Raft 1/ splav 1 Raft 2/ splav 2 Rafts 3/ splavi 3 Elsewhere % / drugje % Rafts % / splavi % 1997 7 5 - - 58 42 1998 5 13 20 - 13 87 1999 3 19 22 - 7 93 2000 0 30 35 - 0 100 2001 5 - 14 45 8 92 2002 16 - 5 59 20 80 between 1997 and 2000 and on findings about the most appropriate construction to meet the demands of the Common Tern (Becker & Sudmann 1998), we constructed and placed the last, most sophisticated rafts (rafts 3; Table 1). 2.3. Monitoring methods We used the direct count method for estimating the number of breeding Common Terns and Black-headed Gulls. The counting unit was an apparently occupied nest-site, defined as those birds sitting 150 i I 100 ,3 50 S o d 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 year / leto Figure 1: Population development of the Common Tern Sterna hirundo (black) and Black-headed Gull Larus ridibundus (grey) breeding colony at the basins of the Ormo` sugar factory Slika 1: Razvoj populacije navadne ~igre Sterna hirundo (~rno) in re~nega galeba Larus ridibundus (sivo) v bazenih Tovarne sladkorja v Ormo`u tight and apparently incubating eggs or brooding chicks (Bibby & Burgess 1993). Counts were made with a telescope from different positions, so that the whole colony could be counted. A minimum of 4 counts were carried out each breeding season. We considered the highest counted number of breeding birds. In 1999, counts were done more thoroughly, so we could count fledged young, too. From 1997-1998 and from 2000-2002 we could not accurately count fledged terns. 3. Results In 1992, Common Terns and Black-headed Gulls were breeding in a mixed colony within stands of aquatic vegetation Carex sp. in the basins of the sugar factory. In 1994, due to the high water level, the colony was flooded. Consequently, in the same year, Common Terns attempted to lay their replacement clutches on a gravel road by the basins, but without success. Only one pair laid an egg, but the young did not hatch. The high water level was the reason for an absence of breeding in 1995 and 1996. The increase in population after 1997 (Figure 1) was the result of the artificial breeding rafts placed for Common Terns. In 1997, when the first raft was placed, the majority of Common Terns were breeding on accumulated material that had appeared that year on the water surface (branches, pieces of wood, plants, etc., deposited by water). In the following years, the percentage of Common Terns breeding on rafts increased dramatically but started to decrease again after 2001, when three new rafts were placed (Table 2). Black-headed Gulls did not breed on artificial rafts until 1999. Since then the percentage 165 D. Denac: Common Tern Sterna hirundo breeding population: development and nature conservation management results at the Ormo` wastewater basins between 1992 and 2002 (NE Slovenia) Table 3: Positions of Black-headed Gull Larus ridibundus nests at the wastewater basins of the Ormo` sugar factory Tabela 3: Mesta gnezdenja re~nega galeba Larus ridibundus v bazenih za odpadne vode Tovarne sladkorja v Ormo`u Year / leto Elsewhere / drugje Raft 1/ Raft 2/ Rafts 3/ splav 1 splav 2 splavi 3 Elsewhere % / drugje % Rafts % / splavi % 1997 1998 1999 2000 2001 2002 6 0 - - 100 0 35 0 0 - 100 0 49 3 3 - 89 11 22 3 3 - 79 21 4 - 4 100 4 96 27 - 3 110 19 81 breeding on rafts has increased, most drastically in 2001 when they almost completely occupied the three new rafts (Table 3). We have data on the breeding success of the Common Tern on rafts only for 1999. From 11 and 16 fledged Common Terns counted on rafts 1 and 2, respectively, in 1999, we calculated breeding success as 0.58 fledgling / pair for raft 1 and 0.72 fledgling / pair for raft 2. The highest joint breeding density of Common Terns and Black-headed Gulls (2.71 nests / m2) was observed on raft 2 in the year 2000 (Figure 2). 1999 2000 year / leto 2002 ? raft 1 / splav 1 o raft 2 / splav 2 û rafts 3 / splavi 3 Figure 2: Joint densities of nests of Common Tern Sterna hirundo, and Black-headed Gull Larus ridibundus, on rafts at the wastewater basins of the Ormo` sugar factory Slika 2: Skupna gnezditvena gostota navadne ~igre Sterna hirundo in re~nega galeba Larus ridibundus na gnezditvenih splavih v bazenih za odpadne vode Tovarne sladkorja v Ormo`u 4. Discussion This Common Tern and Black-headed Gull colony is the very first on artificial breeding rafts in Slovenia. From a nature conservation point of view the colony is very important for both species, as it is larger than before its collapse in 1994. According to the number of breeding pairs it is the biggest mixed Common Tern and Black-headed Gull colony in Slovenia and the fourth known Common Tern breeding site after Se~ovlje salt pans (Makovec et al. 1998), Ptuj accumulation lake (Geister 1995) and Gaj{evsko lake ([alamun 2001). 45% of the Slovene Common Tern population breeds in these basins. The basins are the second known breeding place of Black-headed Gull in Slovenia, after the Ptuj accumulation lake. In 2002, 50% of the total population of Black-headed Gull bred on rafts. Thus, by placing rafts, we have contributed positively to the conservation of two highly endangered species that are on the Red list of breeding birds in Slovenia. Management of Common Tern breeding sites and placing artificial rafts are effective methods for conserving the Common Tern population in areas of middle Europe where their natural breeding sites have been destroyed by river canalisation and regulation (e.g. Boschert & Dronneau 1998, Raab 1998, Stark 1998, Zintl 1998). The immediate occupation of rafts by Common Terns in 1997 can be explained as a sign of the absence of appropriate natural breeding sites. After the colony collapsed in 1994, the Common Terns remained in the basins, probably because of their nest site fidelity (Cramp 1994, Wendeln & Becker 1998), however they did not breed because of the absence of breeding structures, which were flooded. Breeding rafts and water deposits, both of which appeared in 1997, offered appropriate breeding sites, and Common Terns 166 Acrocephalus 23 (115): 163 – 168, 2002 occupied both. The number breeding on deposits gradually decreased and in 2000 all the Terns were breeding on rafts. The most probable reason for their population increase was immigration from other areas, because breeding success was too small to account for it. Actual breeding success was established only for 1999, when the values 0.58 and 0.72 fledged young / pair were low. Only the latter is close to the value at which a population is self-sustainable (Wendeln & Becker 1998). Breeding success is an important population parameter and is indispensable for evaluating the efficiency of placing rafts. In 1997-1998 and after 1999 we established breeding success indirectly, as it correlates negatively with breeding density (Sudmann 1998). According to this relation, we found that breeding success was highest in 1997, low in 1998 and lowest in 2000. It is typical of the Common Tern that, in cases of high breeding densities, negative intraspecific interactions like throwing young into the water increase and breeding success decreases (Sudmann 1998). Nest density at the basins was comparable to that on artificial rafts abroad (between 0.28 and 2.13 nests / m2 on the Lower Rhine; Sudmann 1998). Until 2000, no special structures (e.g. chick shelters) for increasing breeding success were placed on rafts. Such structures can decrease predation of young (Burness & Morris 1991) and heat shock, and provide shelter from the rain. After placement of the three rafts in 2001, the trend in selecting Common Tern breeding sites changed. They again started to breed on natural structures in the basins because Black-headed Gulls massively occupied the rafts. Black-headed Gulls start to breed earlier (Cramp 1994), so they can occupy the majority of places on rafts before the arrival of Terns from migration. Terns bred elsewhere in the basins, most probably due to the competition with Black-headed Gulls for nest space on rafts. Black-headed Gulls started to breed on rafts in 1999, and until 2001 they were breeding there in smaller numbers (maximum six pairs). Most of them were breeding on water deposits and on basin edges overgrown with vegetation. They prefer a heterogeneous breeding place with higher vegetation. The height of the vegetation is the factor which most characterizes the difference in breeding niche of the two species (Fasola & Canova 1992). It is very likely that addition of special breeding structures for Common Terns on the new rafts made them more attractive for Gulls. They even built some of their nests on the branches. Very probably it was the larger size of the rafts placed in 2001 that stimulated Gulls to occupy them massively, because on smaller rafts their numbers were much lower. Breeding success of Common Terns in relation to interspecific competition should be researched, to establish whether it is high enough for the Terns to maintain a stable population or not. In the latter case, additional measures should be undertaken to improve their breeding conditions (e.g. smaller rafts with breeding structures or an artificial island with simulated natural breeding conditions). Acknowledgements: We would like to thank all the volunteers who helped to build and place the rafts: Dominik Bombek, Branko Bo`i~, Luka Bo`i~, Franc Bra~ko, Cecilija Denac, Katarina Denac, Zoran Denac, Danilo Ker~ek, Matja` Ker~ek, Luka Koro{ec, Miran Koro{ec, Miha Kova~i~, Tine Kova~i~, Tina † Lon~ar, Dejan Muhi~ , Borut Pittner, Davor Po~iva{ek, Jakob Smole, Jo{t Stergar{ek, @eljko [alamun, Borut [tumberger, Karmen [pilek-[tumberger, Tadej Trstenjak, Peter Uratnik, Toma` Urban~i~. Also we would like to thank the director of the Ormo` sugar factory, Mr. Jurij Dog{a, their technician Mr. Emil [krinjar, Mr. Rudi Habjani~, Mr. Marjan Ivanu{a and Mr. [tefan Malec for their understanding and help. Special thanks go to the mayor of Ormo` municipality, Mr. Vili Trofenik, and to Mr. Branko @alik and Mr. Janez Kolenko. Last but not least thanks to Katarina for her immense support. 5. Povzetek Leta 1981 so navadne ~igre Sterna hirundo in re~ni galebi Larus ridibundus oblikovali novo kolonijo v bazenih za odpadne vode Tovarne sladkorja v Ormo`u. [tevilo gnezde~ih re~nih galebov in navadnih ~iger redno spremljamo z metodo ve~kratnega {tetja s teleskopom. Leta 1994 je zaradi dviga vodne gladine v bazenih kolonijo preplavilo. V letih 1995 in 1996 navadne ~igre in re~ni galebi v bazenih niso gnezdili. Leta 1997 je bil v bazenih postavljen prvi gnezditveni splav povr{ine 12,5 m2, leta 1998 drugi s povr{ino 14 m2 in leta 2001 trije med seboj povezani enaki splavi s skupno povr{ino 96 m2. Postavili smo jih z namenom, da bi se oblikovala nova kolonija navadne ~igre in da bi se ta ptica ohranila kot gnezdilka na tem obmo~ju. Na gnezditvenih splavih je tako nastala najve~ja me{ana kolonija navadnih ~iger in re~nih galebov v Sloveniji. Leta 2002 je na splavih gnezdilo 64 parov navadnih ~iger in 113 parov re~nih galebov. To je 45% slovenske populacije navadnih ~iger in 50% re~nih galebov. Re~ni galebi so za~eli mno`i~no gnezditi na splavih {ele leta 2001, ko smo namestili tri najve~je 167 D. Denac: Common Tern Sterna hirundo breeding population: development and nature conservation management results at the Ormo` wastewater basins between 1992 and 2002 (NE Slovenia) splave. Odstotek gnezde~ih ~iger na splavih se je zmanj{al: leta 2002 jih je 20% gnezdilo na naplavinah in blatnih polojih, leta 2000 pa so vse gnezdile na splavih. Menim, da so re~ni galebi mno`i~no zasedli nove splave zaradi njihove velikosti in heterogene povr{ine. Na splave smo namestili ve~je {tevilo gnezditvenih struktur (stre{nike, veje, {tore - za kritje mladi~em pred de`jem, vro~ino in plenilci) za navadno ~igro. 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(1991): Nova kolonija re~nega galeba Larus ridibundus in navadne ~igre Sterna hirundo v Ho~ah pri Mariboru. Acrocephalus 12 (49): 121-122. Wendeln, H. & P.H. Becker (1998): Populationbiologische Untersuchungen an einer Kolonie der Flußseeschwalbe Sterna hirundo. Vogelwelt 119 (3-5): 209-213. Zintl, H. (1998): Bestandsentwicklung der Flußsee-schwalbe Sterna hirundo in Bayern. Vogelwelt 119 (3-5): 123-132. Arrived / Prispelo: 2.12.2002 Accepted / Sprejeto: 6.3.2003