Acrocephalus 2.6 (124): 37 — 40, 2005 Size and distribution of breeding colonies of Grey Heron Ardea cinerea in lowland Croatia Velikost in razporeditev kolonij sive ~aplje Ardea cinerea v ni`inskih delih Hrva{ke Jozsef Mikuska1, Tomislav Bogdanovi}1, Tibor Mikuska2, Alma Mikuska1, Vlatka [ali}1 1 Department of Biology, University of Osijek, Lj. Gaja 6, HR-31000 Osijek, Croatia 2 Kopa~ki rit Nature Park Management Office, Ul. Petefi Sandora 33, HR-31327 Bilje, Croatia, e-mail: tmikuska@kopacki-rit.com 1. Introduction Grey Heron Ardea cinerea is the most abundant of the nine species of the Heron family Ardeidae recorded in Croatia (Mikuska 1992, Kralj 1997, Luka~ 1998). The first national census was carried out in 1991, when 1529 pairs were counted in eight colonies (Mikuska 1992). This survey was continued throughout the decade and resulted in the discovery of new colonies, with consequent increase in the national population (Mikuska et al. 1994, Dolenec 1997, Mikuska & Mikuska unpubl.). At the beginning of the 21st century the population was estimated at 1500 - 3000 pairs, which was the second largest of the neighbouring central European countries (Hungary 2500 - 3500, Serbia & Montenegro 2200 - 2500, Slovenia 500 -600, Bosnia & Herzegovina 7 - 10 pairs; BirdLife International 2004). 2. Methods During 2004 we attempted to visit all the known colonies and count the breeding pairs, in order to establish the total national breeding population. Data on Grey Heron colony locations were obtained through communication with other ornithologists and our previous fieldwork. The research area was limited to the lowland areas (below 300 m a.s.l.) of Pannonian and continental Croatia bordered by the rivers Drava on the north, Danube on the east and Kupa and Sava on the south. Of the 22 recently identified Grey Heron breeding sites, we failed to census three smaller colonies that are distributed in the continental part of Croatia south of our research area: at Cvijanovi} Brdo near Slunj (with 15 pairs in 1998), near Mili}i village on the Kupa river (with 12 pairs during 1990) and one possible colony in Plitvi~ka jezera national park (Luka~ 2004). Most censuses were made during the early breeding season in late March and early April and the numbers of Apparently Occupied Nests were counted (Bibby et al. 1992). At that time, most herons are incubating or their chicks are just hatching, so that the number of nests correlates with the number of breeding pairs (Bibby et al. 1992). One or two observers entered each colony, and nests on the trees were counted from the ground with the help of binoculars. Since these colonies are built on hardwood trees (predominantly Quercus robur or Fraxinus sp.) and 20 - 30 metres high, the observer’s view of the nests was not obstructed by leaves. From our experience, carrying out a census later in the season would give ambiguous results due to leaf cover on trees that would obstruct the view of nests. Slow and quiet movement through the colony did not create much disturbance to breeding birds who returned to the nest immediately after the observer had left the vicinity of the occupied tree. It took approximately half an hour to count a site holding up to 200 nests, and, even in the case of the largest colonies, the whole census took less than a 1.5 hour per site. In a few cases, namely colonies at fishponds Donji Miholjac, Grudnjak and Na{i~ka Breznica, counts were made later in the season (June), during the execution of a ringing programme. These colonies are built on willow bushes Salix sp. and reedbeds Phragmites communis and several heron species breed there. For these reasons we avoided entering the colonies during the incubation period of late nesting species – such as Little Egret Egretta garzetta, Night Heron Nycticorax nycticorax, Great Egret Ardea alba and Purple Heron A. purpurea – in order to prevent unnecessary disturbance and possible nesting failure of breeding birds. During the census, observers walked through the colony and counted each nest with hatched chicks. At this time it was easy to distinguish between the Grey Heron nests with large or already fledged chicks and similar - sized nests of other heron species, namely Great Egrets and Purple Herons, which had eggs or up to two - week old chicks. The colony at Ko{utarica was also censused 37 J. Mikuska et al.: Size and distribution of breeding colonies of Grey Heron Ardea cinerea in lowland Croatia later in the season because it was not accessible, due to to be discovered along the Drava River, as well as exceptionally high flooding in March and April. along the Sava river floodplain east of Slavonski Brod. However, breeding of Grey Herons in the mountainous 3. Results and discussion or Mediterranean parts is unlikely. The only mixed species heron colony in the Mediterranean region During 2004 we recorded 3674 nests of Grey Herons is situated on Vransko lake near Biograd, but Grey in 19 colonies (Table 1). The number suggests that Herons are not breeding there (V. Dumbovi} - Ru`i} the Croatian national breeding population is currently pers. comm.). Further, heron colonies in the delta of higher than the latest estimates and that the breeding the Neretva River have not existed since the seventies population threshold should be set to 3000 - 4000 of the 20th century (Rucner (1970), D. Kitoni} pers. pairs. This result is in accordance with the overall comm.). trend of the European population, which is stable or The colonies are distributed primarily in lowland increasing (BirdLife International 2004). Despite Croatia bordered by the floodplains of large rivers - the fact that we lack recent population values for Danube, Drava, Sava and Kupa (Figure 1). Based on neighbouring countries, our data suggest that Croatia colony size we can distinguish three categories: has the largest proportion of breeding Grey Herons in the Pannonian plain. 1. Large colonies comprising over 200 pairs (Kopa~ki Given the lack of previous records, we cannot exclude rit - ]o{ak {ume, Mrsunjski lug, Berek, Piljenice the possibility that a few additional colonies remain and Ko{utarica) – these are situated along the large Table 1: Breeding of Grey Herons Ardea cinerea in lowland Croatia during 2004 Tabela 1: Gnezdenje sive ~aplje Ardea cinerea v ni`inski Hrva{ki leta 2004 Colonies on hardwood trees, nests 20 - 30 m above the ground/ Kolonije na listavcih, gnezda 20 - 30 m nad tlemi Locality / Lokaliteta UTM Date / Datum No. of nests / 1 Berek XL46 07.04. 412 2 ^epin CR14 05.04. 29 3 ^rnec WL91 01.04. 155 4 Jastrebarsko WL56 01.04. 168 5 Kopa~ki rit - ^o{ak {ume CR35 04.04. 885 6 Ko{utarica XL51 09.06. 243 7 Kravarsko WL84 01.04. 89 8 Mokrice WL69 07.04. 41 9 Mrsunjski Lug YL20 29.04. 479 10 Piljenice XL43 28.03. 383 11 Razljev XL25 01.04. 130 12 Slovinci XL21 01.04. 165 13 Topusko WL71 01.04. 26 14 Trebovec XL06 01.04. 157 15 Vrpolje BR91 07.04. 27 16 Veliki Strug 2 XL60 during April 2004 37 Colonies on willows Salix sp. and reedbeds, nests up to 5 m above the ground/ Kolonije na vrbah Salix sp. in trstju, gnezda do 5 m nad tlemi 17 Donji Miholjac fishpond BR77 16.06. 30 18 Grudnjak fishpond YL35 08.06. 83 19 Na{i~ka Breznica fishpond BR84 02.06. 135 Total / Skupno 3674 38 ACROCEPHALUS l6 (124): 37 - 4O, 2OO5 Figure 1: The location of the colonies of Grey Heron Ardea cinerea in lowland Croatia in 2004: 1) Berek, 2) ^epin, 3) ^rnec, 4) Donji Miholjac fishponds, 5) Grudnjak fishponds, 6) Jastrebarsko, 7) Kopa~ki rit – ]o{ak {ume, 8) Ko{utarica, 9) Kravarsko, 10) Mokrice, 11) Mrsunjski lug, 12) Na{i~ka Breznica fishponds, 13) Piljenice, 14) Razljev, 15) Slovinci, 16) Topusko, 17) Trebovec, 18) Vrpolje, 19) Veliki Strug 2 Slika 1: Kolonije sive ~aplje Ardea cinerea v ni`inski Hrva{ki leta 2004 areas of the remaining floodplains of the Danube and Sava rivers. Commercial fishponds that were built on former wetlands (Podunavlje fishponds at Kopa~ki rit, Jelas fishponds near Mrsunjski lug, and Lipovljani fishponds near Piljenice) have helped to sustain these colonies, or even increase the number of breeding pairs after part of the wetland was drained and converted to agricultural land. For example, the number of breeding pairs in Kopa~ki rit - ]o{ak {ume increased from 400 during 1991 to over 1000 pairs during 2002. This increase is probably related to the abandoning of fish production on a large 200 ha pond that resulted in decrease in the water level, making a larger area suitable for foraging. An exception to this rule is the colony at Berek, that is situated in a remote forested area, exactly halfway between two commercial fishponds – Narta and Kon~anica – from where herons make daily foraging flights. 2. Medium sized colonies comprising 50 - 200 pairs – these are situated along the remaining, but much smaller, floodplain areas along the Sava and Kupa rivers (e.g. ^rnec, Trebovec, Razljev and Slovinci along Sava, as well as Kravarsko along the Kupa) or are attached to one of the existing commercial fishponds (e.g. Grudnjak, Jastrebarsko or Na{i~ka Breznica). The size of the colonies on these fishponds is influenced by the disturbance accompanying the execution of Cormorant culling programmes (mainly by shooting throughout the year) or by water and vegetation management (e.g. the pond with a colony at Grudnjak fishponds was drained and burnt during 2004). 3. Small colonies comprising less than 50 pairs – these are situated at the edge of the Croatian breeding population and in less suitable habitats without larger wetlands (Mokrice in Zagorje or Topusko in the hilly part of the Croatia). As a curiosity, colonies 39 J. Mikuska et al.: Size and distribution of breeding colonies of Grey Heron Ardea cinerea in lowland Croatia at ^epin and Vrpolje are situated near large pig -farms. The wetland areas along these colonies are limited to several large melioration ditches, suggesting that the birds take advantage of the large rodent populations that are common at these sites rather than feeding on the aquatic food source. However, further studies are needed to explore this hypothesis. Summary Grey Heron Ardea cinerea is the most abundant heron species in Croatia, with an estimated population of 1500 - 3000 pairs. We conducted a survey of existing colonies during the 2004 breeding season and counted 3674 pairs in 19 colonies. Based on this data the Grey Heron breeding population thresholds should be increased and set to 3000 - 4000 pairs. The colonies are distributed primarily along the remaining floodplains of the Danube, Drava, Sava and Kupa rivers as well as on commercial cyprinid fishponds. Povzetek Siva ~aplja Ardea cinerea je naj{tevil~nej{a med vsemi vrstami ~apelj na Hrva{kem, saj je bila populacija ocenjena na 1500 do 3000 parov. Avtorji so napravili pregled obstoje~ih kolonij v gnezdilni sezoni leta 2004 in na{teli 3674 parov v 19 kolonijah. Na osnovi teh podatkov pa se je ocena populacije te vrste povi{ala na 3000 - 4000 parov. Kolonije so bile razporejene predvsem na poplavnih ravnicah Donave, Drave, Save in Kolpe ter na ribnikih s krapovci. References Bibby, C.J., Burgess, N.D. & Hill, D.A. (1992): Bird Census Techniques. – Academic Press Ltd., London. BirdLife International (2004): Birds in Europe: Population estimates, trends and conservation status. – BirdLife International, Cambridge. Dolenec, Z. (1997): Grey Heron (Ardea cinerea L.), new breeding species of Hrvatsko Zagorje. – Larus 46: 128. Kralj, J. (1997): Croatian ornithofauna in the last 200 years. – Larus 46: 1-112. Luka~, G. (1998): List of Croatian Birds. Spatial and temporal distribution. – Natura Croatica Vol. 7, Suppl. 3: 1- 60. Luka~, G. (2004): Ptice nacionalnog parka Plitvi~ka jezera. – Plitvi~ki bilten 6: 29-70. Mikuska, T. (1992): Gnije`|enje ~aplji (Ardeidae) u Hrvatskoj. - Diplomski rad – Prirodoslovno - matemati~ki fakultet Sveu~ili{ta u Zagrebu, Zagreb. Mikuska, T., Mikuska, J. & [etina, M. (1994): Stanje ~aplji (Ardeidae) u Hrvatskoj u periodu 1990 - 1993. – Zbornik sa`etaka priop}enja Petog kongresa biologa Hrvatske, pp. 379-380. Hrvatsko biolo{ko dru{tvo, Zagreb. Rucner, D. (1970): Nadopuna poznavanju ptica doline Neretve III. – Larus 21/22: 99-114. Prispelo / Arrived: 24.3.2005 Sprejeto / Accepted: 19.9.2005 4e