09_RaJkovic.indd UDC 598.244.2(497.11) DISTRIBUTION PATTERN, NEST-TREE FEATURES AND BREEDING PERFORMANCE OF POPULATION OF THE BLACK STORK, CICONIA NIGRA (CICONIIFORMES, CICONIIDAE), IN NORTHWESTERN SERBIA D. Z. Rajković Šumadijska 18, Novi Sad, 21000 Republic of Serbia E-mail: strix.draze@gmail.com https://orcid.org/0000-0002-2626-0076 Distribution Pattern, Nest-Tree Features and Breeding Performance of Population of the Black Stork, Ciconia nigra (Ciconiiformes, Ciconiidae), in Northwestern Serbia. Rajković, D. Z. — Distribution pat- tern, nest-tree characteristics and reproductive features of Black Stork’s population were evaluated in the fl oodplains of Gornje Podunavlje in Northwestern Serbia over fi ve consecutive years. In total, 44 diff erent nests were discovered and monitored. Nests were mostly placed on wide-diameter European Oaks and White Poplars between 5.5 and 18 m above ground level. Annually, the number of occupied nests varied between 26 and 16 with the declining trend over study years. Th e mean nesting density was 9.8 ± 2 occu- pied nests per 100 km2. Th rough study area, occupied nests were uniformly distributed and very isolated concerning the conspecifi c pairs. Predominantly, Black Stork preferred to nest on the lateral branches of the autochthonous mature trees with straight trunks and extensive crowns. Th ese results suggest that the existence of old native forest stands represent crucial microhabitat criteria in the choice of the particular nesting site. In almost half of successful nesting cases, the most frequent number of fl edglings was three. Overall productivity was relatively low with some variations between years implying poor nesting success. K e y w o r d s : Black Stork, Ciconia nigra, breeding, Gornje Podunavlje, productivity, nesting success. Introduction Th ere is a sizeable and hardly conceivable quantity of wildlife surveys and various monitoring schemes across the world. Th ese programmes are principally designed to provide data sets to help to understand the fundamental parameters in population ecology such as abundance, spatial distribution, dispersion and population trend (Kleewein, 1999; Krebs, 2014). Mentioned parameters directly depend on the balance between fecundity, mortality, immigration and emigration (Siriwardena et al., 2000). Among them, breeding performance (fecundity) is a supremely important process underlying individual fi tness and population tenacity (Etterson et al., 2011). Th us, storing, analysing and interpreting collected information represents one of the inevitable questions in population ecology and reproductive biology. In threatened, keystone and umbrella species, possession of the mentioned information are especially valuable because they may be utilised for conservation and management purposes (Witmer, 2005), or to determine the composition of communities, their structure, and ecosystem processes (Roberge & Angelstam, 2004). Th e Black Stork Ciconia nigra (L., 1758) is widely recognised as an imperilled, umbrella species due to large foraging range, and its specifi c requirements for food and nesting sites (Roberge & Angelstam, 2004; Moreno-Opo et al., 2011). Zoodiversity, 55(2): 175–184, 2021 DOI 10.15407/zoo2021.02.175 Ornithology 176 D. Z. Rajković In general, the Black Stork population has been well studied, at least in most European countries especially in the last two decades. In particular, aspects of the species habitat selection and use (Augutis & Sinkevičius, 2005; Lõhmus et al., 2005), nesting sites (Treinys et al., 2008; Vlachos et al., 2008) and breeding performances (Strazds, 2011; Tamás, 2012; Alexandrou et al., 2016; Fraissinet et al., 2018) has gained noticeable attention. Diet, foraging strategies and migration pattern have also been the focus of a few studies (Hampl et al., 2005; Tamás & Kalocsa, 2006; Bobek et al., 2008). Unfortunately, peer-reviewed papers relating to this species from Western Balkan countries, including Serbia, are scarce or completely lacking. To date, seven articles have been published respectively (Marčetić, 1957; Popović, 1960; Puzović et al.,1988/89; Schneider-Jacoby, 1999; Erg, 2002; Tucakov et al., 2006; Velevski et al., 2008). All of them are considerably varied in the quality of data and analyses mostly turned to general information and preliminary studies while more specifi c and comprehensive data are missing. In that context, the principal goal of this study is to increase general knowledge and obtain meaningful data on the reproductive biology and ecological requirements of the poorly studied Black Stork population in Serbia and adjacent areas (except Hungary). Hence, the aims of this study were (i) to estimate abundance, den- sity and short-term trend, (ii) to describe the distribution pattern and characteristics of nests, and (iii) to evalu- ate the variation in the breeding performance of Black Stork population over fi ve research years (2011–2015). Material and methods S t u d y a r e a Alluvial fl ood plains represent one of the most remarkable habitats for the Black Stork population survival in Europe (Sackl & Strazds, 1997; Kalocsa & Tamás, 2002). In whole Europe, one of the most outstanding area for this sensitive species lies along the intermediate course of the Danube River, from the mouth of the Sió chan- nel in Hungary downstream to the mouth of the Drava in Croatia covering approximately 750 km2 of wetlands (Tamás, 2012). Th is vast periodically fl ooded landscape is separated into three administrative parts by country borders — Croatia, Hungary, and Serbia. Th e research took place on the Serbian side (left Danube bank) in Vojvodina Province where lies “Gornje Podunavlje” Special Nature Reserve. It has an irregular and elongated shape stretched along 1.367–1.433 Danube river km and consists of two swampy areas — Monoštor and Apa- tin. Together with several detached forest fragments outside of natural reserve borders study area in total cover 201 km2, respectively. Deciduous forest stands cover at least 50 % of the study area. Th e White Willow (Salix alba L., 1758), the White and the Black poplar (Populus alba L., 1758 and P. nigra L., 1758) and European Oak (Quercus robur L., 1758), are the most prevalent tree species (Stojanović et al., 2014). Besides natural and semi- natural forest stands, substantial parts are covered with plantations of allochthonous tree species, mainly hybrids of rapid-growing poplars (Tucakov et al., 2006). In general, the study area is fl at, 80–90 m above sea level and has a temperate continental climate with warm summers and icy winters. Consequently, the warmest month is July (average 21.9 ºC) while the coldest month of the year is January (–0.1 ºC). Mean annual precipitation is 612.4 mm with the smallest sum in February (29.9 mm) and the greatest in June (81.5 mm; Stojanović, 2018). F i e l d w o r k a n d d a t a c o l l e c t i o n Th e data referred to this article have been collected through extensive fi eld surveys carried out in fi ve breeding seasons, from January of 2011 to May of 2015 in the alluvial forests of Northwestern Serbia. In the whole territory of the research area, data collection includes abundance, distribution, characteristics of the nest- ing tree, breeding habits, reproductive success, and when it is possible nest failures. Yet, due to lack of funding, research coverage during 2015 was highly reduced to data on controlled and occupied nests. Th e description of terms regarded to reproduction follows the terminology recommended by Steenhof et al., (2017). Initially, the Black Stork territories and nests were located and mapped through inquiries address to fi eld- oriented professions in the area: foresters and reserve rangers. Th e author with colleagues also took part in the active search for nests. Th ese fi eld excursions were primarily conducted in February, March and April. Surveys were undertaken by off -road vehicles and on foot. Surveys during a winter-spring period are more appropriate than those in other seasons because large Black Stork’s stick nests are oft en remarkably exposed in a deciduous forest without leaves and also because there is no risk of disturbance (Puzović et al., 1988/89; Tamás, 2012). Ad- ditionally, in late March and through April, the study area was scanned by binoculars from prominent points. Th e beginning of spring is a time when Black Storks are building or repairing their nest and when they display territorial behaviour. Territorial birds were distinguished from fl oaters by performing a conspicuous aerial court- ship display close to nesting eyries mostly around the midday (Schneider-Jacoby, 1999; Sackl, 2000). Aft er locat- ing territorial birds, they were followed from a distance in hope to reveal breeding eyrie or exact location of the nest. Both ways of searches for nests were conducted every winter/spring period and occasionally throughout the whole year to supplement targeted search. If formerly known nests are unoccupied, but Black Storks are observed in close vicinity, a search for an alternative nest(s) was conducted. During the mapping process, details of each discovered nest were written in fi eld protocol and georeferenced by a hand-held GPS device (Garmin GPSmap 62s). For every discovered nest, the following data were recorded: the tree species, placement position, and height of the nest base measured from the ground point. Classifi cation of nest placement was according to their position on the nesting tree (Tamás, 2012). Type 1: nest is in the main pitchfork of the tree; type 2: nest is on the horizontal branches, leaned or near (˂ 1.5 m) to the tree trunk; type 3: nest is on the horizontal branches more than 1.5 m 177Distribution Pattern, Nest-Tree Features and Breeding Performance of Black Stork, Ciconia nigra… distant from the tree trunk; and type 4: nest is on the horizontal tree trunk bent by a wind storm. Measurements of nest heights were performed with a ribbon aft er climbing on nest platform during mid June-early July. In order to describe each nest-tree characteristics, the following information were recorded: general position (interior, edge ≤ 40 m from clearing or road, or solitary), trunk shape (straight, slightly crooked, crooked, or forked), diameter at breast height (DBH) and crown class (dominant, codominant, intermediate, or suppressed). Nest-tree character- istics and associated measurements were performed according to the instructions of USDA Forest Service (2006). Aft er the nest was found, it checked at least once, usually during the April or rarely on the beginning of May to determine it is occupied or not. Th is early check was always carried out from a safe distance to avoid disturbance and during favourable weather conditions. Th e nest is considered as occupied if fresh building material was added (e. g. new twigs) or at least one adult bird was observed perching/incubating on the nest platform or in the proximity (Lõhmus et al., 2005; Tamás, 2012). All occupied nests were re-visited at least once in June–early July, to determine the nesting success in the pre-fl edgling period. In cases when it was possible nest failure causes were recorded. Nesting failures were determined by obvious clues that indicate human pres- ence or predator activity, such as footprints of feathers recorded on nest substratum. Finally, the distribution pattern of the Black Stork nests was expressed through the nearest neighbour distance index (Clark & Evans, 1954) and isolation index (Carrete et al., 2006). Th e nearest neighbour distance index (R) has a limited range with the value indicative of perfectly uniform (R ˃ 1), random (R = 1) and wholly aggregated (R = 0) pattern of distribution (Clark & Evans, 1954). Isolation index (Si) ranged from 0 to 1; from more isolated to more connected (Carrete et al., 2006). Both indices were calculated with a set of linear distances measured to nearest 5 m with a combination of Google Earth Pro and QGIS version 2.18. (QGIS Development Team, 2016). Nesting density was calculated as the number of occupied nests per 100 km2. Productivity was de- fi ned as the number of well-feathered nestlings divided by the number of successful pairs. Term well-feathered nestling refers to juvenile aged about 55–70 days, respectively (Lõhmus et al., 2005; Tamás, 2012). Nesting suc- cess was determined as the mean number of well-feathered juveniles per occupied nest. S t a t i s t i c a l a n a l y s i s Prior to data analyses, all sample variables were examined for normality by the use of the Kolmogorov- Smirnov test (sample size ˃ 50), Shapiro-Wilk test (sample size < 50) and normal probability plots. In all tested cases, data were not normally distributed; thus, the non-parametric techniques were applied to hypothesis testing. Mann-Whitney (U) and Kruskal-Wallis test (H) applied to verify whether the samples originate from the same distribution and G-test for homogeneity with Williams’ correction factor (Gadj) was used for analysing frequencies (Sokal & Rohlf, 1995). Spearman rank correlation coeffi cient (rs) applied to check the signifi cance of the relationship between two measured variables (Sokal & Rohlf, 1995). All performed tests were two-tailed. Values of p < 0.05 were accepted as signifi cant. Th e measurement values are presented as arithmetic mean ± one standard deviation. Statistical processing was performed using the IBM SPSS soft ware version 23.0 for Windows (IBM Corporation, 2015). Results D i s t r i b u t i o n p a t t e r n , p o p u l a t i o n t r e n d a n d d e n s i t y A total of 44 diff erent nests of Black Stork were detected through fi ve research years. Th e number of controlled nests was stable during the study period. However, only 6.81 % (n = 3) of nests were continuously occupied during 2011–2015, while 2.3 % (n = 1) of nests during four consecutive years, 15.9 % (n = 7) of nests during three, 34.1 % (n = 15) of nests during two and 40.9 % (n = 18) of nests were occupied during one breeding season. On average, 19.8 ± 4 (± 95 % CI) pairs of the Black Stork tried to breed on the year base ranged between 16 and 26 pairs (table 1). Nevertheless, the Black Stork population moderately decreased by 2.3 occupied nests per year suggests a precipitous declining trend in the study area (rs = –0.9, p = 0.037). Th e mean nesting density was 9.8 ± 2 (± 95 % CI) occupied nests per 100 km2 of the study area. Th e highest concentration of occupied nests was situated in central parts of Apatin swamp (locality Jelensko ostrvo) where six diff erent occupied nests were recorded at the surface of circa 4.3 km2 during 2011. Th e spatial distribution of the occupied nests displays a uniform distribution pattern (R = 1.73). Furthermore, the occupied nests were extremely isolated concerning the conspecifi c pairs (Si < 0.01). Altogether, Black Stork pairs nested an average of 2,201.7 m ± 2,020.2 m from conspecifi cs (range 150–9,460 m). Despite year- to-year variations, no signifi cant diff erence was noted through years between a distance of the nearest neighbour (H82 = 4.18, p = 0.242, df = 3). 178 D. Z. Rajković N e s t t r e e f e a t u r e s In the study area, nests of Black Storks were placed on fi ve tree species. Majority of the nests are constructed on the European Oak and the White Poplar (84 %), indicating considerable preferences in the selection of tree species for nest placement by Black Stork pairs (fi g. 1). Consequently, comparison among tree species on which Black Storks built their nests shows the statistically signifi cant diff erence (Gadj = 39.71, df = 4). Additionally, Black Storks avoided to nesting on solitary trees (2.27 %) and forest edges (27.27 %) and more favoured interior parts of forest stand for nest placement (70.45 %). Nests were situated on trees between 5.5 and 18 m above ground level (11.2 ± 3.3 m). All four types of nests placement were recorded and Black Storks tended to build nests on a horizontal branch of old, mature trees (DBH = 85.7 ± 15.95 cm). Indeed, 84.1 % of nests were constructed on a lateral branch close to the tree trunk (type 2; fi g. 2). Furthermore, trees with straight trunk shape were highly signifi cantly selected for nest placement than the other three types tested (Gadj = 27.91, df = 3). Following trunk shape, there is a signifi cant tree selection heterogeneity between measured four crown classes (Gadj = 26.06, df = 3) where dominant and codominant tree crowns are preferred (together 84.09 %). T a b l e 1 . Breeding parameters of a Black Stork (Ciconia nigra) population in Northwestern Serbia monitored from 2011 to 2015 Year Controlled nests Occupied nests Successful pairs Fledglings Productivity Nesting success 2011 31 26 11 31 2.81 1.19 2012 32 21 4 8 2 0.38 2013 36 19 9 24 2.66 1.26 2014 32 16 10 29 2.9 1.81 2015 33 17 – – – – Total – 99 34 92 – – Mean ± SD 32.8 ± 1.9 19.8 ± 4.0 8.5 ± 3.1 23 ± 10.4 2.58 ± 0.4 1.16 ± 0.6 Fig. 1. Th e proportion of tree species picked for nest placement by the Black Stork (Ciconia nigra) in Northwestern Serbia (n = 44). 179Distribution Pattern, Nest-Tree Features and Breeding Performance of Black Stork, Ciconia nigra… B r e e d i n g p e r f o r m a n c e Depending on the year, the number of successful nesting attempts and where at least one nestling was fl edged range from four to 11 (8.5 ± 3.1). It means that between 24 and 39 % of occupied nests did not pro- duce any fl edglings (table 1). In almost half of successful nesting cases, the most frequent number of fl edglings was three (47.06 %). However, within the study area, productivity did not vary between years (H34 = 1.48, p = 0.69, df = 3). Th e share of successful pairs which build new nests was a slightly higher than for those who reused last year’s nest (55.6 vs. 44.4 %). Th ere was no statistically signifi cant distinction in nesting success between reused and newly built nests (U56 = 205, z = –0.97, p = 0.33). Average annual productivity of Black Storks was moderately negatively, but not signifi cantly correlated to the annual number of occupied nests (rs = –0.4, p = 0.6). Altogether, the overall productivity does not correlate with distance to the nearest occupied nest (rs ˂ –0.01, p = 0.97). Principally, it means that the number of fl edglings per occupied nest was stable regardless of whether the nearest neighbour is relatively close or distant. Out of 48 unsuccessful attempts, in only seven cases (14.6 %) the cause of nest failures was undoubtedly known. In three cases, the nest failed due to direct human disturbance dur- ing the incubation stage, and four nests were placed under predation. Discussion Woodlands along Upper Danube fl ow has always been considered as the most impor- tant breeding ground of Black Stork in Serbia. Indeed, the presented study confi rmed this general statement. Depending on the year, between 10.5 and 17.1 % of the national popula- tion of Black Storks bred in Gornje Podunavlje area. Taking into account the possibility of a few overlooked eyries in remote fl ooded parts, the results indicate existence between 19 and 27 occupied eyries per year during the study period. According to the previously done Important Bird Area (IBA) assessments, the nesting population of Black Storks of Gornje Podunavlje was estimated on 35–45 and 30–40 pairs (Puzović & Grubač, 2000). At least during the study period, it has been shown that these optimistic estimations are not valid. In contrast to IBA assessment, Tucakov et al., (2006) found 15 nesting pairs in the Gornje Podunavlje for the equivalent period. Th e data concerning relatively poor re-occupancy of nests can be explained by high mortality of adults, low nest site fi delity or infl uence some factor that forces birds to shift location like regular predation or human disturbance. Th e fi rst two reasons seem unrealistic, especially keeping in mind that adult individuals of Black Storks have relatively high survival rate and strong site fi delity (Tamás, 2012). Also, adult Black Storks have almost no natural enemies; so, this factor can be rejected too. At the same time, the study area is subject to very intensive forest management with a strong network of forest roads. Forestry activities begin growing aft er the snow melts (beginning of March) and reach signifi cant intensity during April and May. Hence, the time frame of forestry operations coincides with the period of courtship, nest reparation and incubation of Black Storks. Th us, forestry and other human activities might partly explain the poor re-occupancy rate of the nest and consequently may have implications on productivity and nesting success. Several other studies have confi rmed that Black Stork is disturbance- sensitive species especially regarding forestry operations near nesting site (Puzović et al., 1988/89; Lõhmus et al., 2005; Strazds, 2011). Fig. 2. Th e ratio of Black Stork’s (Ciconia nigra) nest placement types in the Northwestern Serbia (n = 44). 180 D. Z. Rajković Many large birds, including Black Stork, are typical K-selected species, which have a large body, long lifespan, produce fewer off spring that requires prolonging parental care and reach maturity relatively late (MacArthur & Wilson, 2001). K-selected species are usually constant in numbers and close to the maximum that the environment can carry. However, the result obtained in this study shows the declining short-term trend of Black Stork breeding population in the investigated area. Th e apparent reasons for the reduction in the number of mature individuals in Gornje Podunavlje are uncertain. Several published studies have addressed that breeding population decline or signifi cant annual fl uctuations might be linked to the lack of mating partners, especially females (Konovalov et al., 2019), poaching during major migration routes (Campbell & Veríssimo, 2015) or unfavourable ecological conditions on winter grounds (Saino et al., 2004). Furthermore, electrocution may also infl uence fl uctuations and decline in numbers of sexually mature individuals at least on the local level (Kalocsa & Tamás, 2018). Also, a factor that may results population decline is decreasing density of breeding pairs as a consequence of poor reproduction in the past (Lõhmus et al., 2005), which in the end can lead to genetic deterioration and reduced fertility in adults (Burgman et al., 1988). Extensive changing of the breeding regions or adult starvation is unlikely because populations in neighbouring Hungary and Croatia are stable and no signifi cant reduction in food resources was recorded (Tamás, 2012). Despite annual diff erences and some local nest aggregations, it seems that spatial dis- tribution of the Black Stork in Gornje Podunavlje was continual throughout the entire area. Th e moderately high mean nesting density of ~ 10 per 100 km2 is twice lower than recorded just across the border in Gemenc area in Hungary (Kalocsa & Tamás, 2006), but in the end typically for this trans-boundary alluvial area and much higher than in rest of Europe (Tamás, 2012). On the local level, it is interesting that the high density of occupied nests on Apatin swamp in this study was also recorded during 1996 when seven pairs of Black Storks bred on the surface of 1.53 km2 (S. Puzović personal communication in Tamás, 2012). Th us, native forest stands in the heart of Apatinski rit provides optimal conditions for nesting, that is not only temporarily, but also probably is constant, at least for the last two decades. Earlier results from Serbia, Hungary, and Croatia are also pointing the im- portance of old native forest stands of European Oaks and White Poplars for nest place- ment (Puzović et al., 1988/89; Tucakov et al., 2006; Tamás, 2012). In mentioned studies and this study as well, over 80 % of nests were built on these two tree species highlights the affi nity towards autochthonous stands with well-developed crowns. In opposite, just a few nests were situated on the hybrid poplars indicating that the Black Storks avoids planta- tions and has the low ability of adaptation to intensively managed forests. Th e reasons for avoiding plantations of fast-growing Poplars during breeding could be numerous. Yet, they are probably connected with homogeneity of plantation structure like spatial layout, tree density and branch-foliage characteristics followed by intensive management and frequent human presence in comparison with natural stands (Calladine et al., 2018). Other possibili- ties including nest visibility, which could be essential in protection from avian predators and enough free space around the nest as an important factor during copulation and land- ing (Strazds, 2011). However, the obtained results once again confi rm that breeding pairs of Black Storks are an excellent indicator of the existence of mature forest stands (Treinys et al., 2008; Tamás, 2012). Strong preference towards the European Oak has also been re- corded in higher geographical latitudes of the European continent, although in those areas mixed and coniferous forests were dominated forest types (Czuchnowski & Profus, 2008; Treinys et al., 2009; Strazds, 2011). Nest heights and DBH found in this study are largely co- inciding to those found previously in Serbia (Puzović et al., 1988/89; Tucakov et al., 2006) and other European countries (Lõhmus & Sellis, 2003; Strazds, 2011; Tamás, 2012). Con- versely, nest position on the tree is signifi cantly diff erent in comparison to those analysed in the Hungarian part of Danube (Tamás, 2012). Th is could be simply due to diff erent supply of forest stands in the fi eld and their general habitus. Another possibility is a kind 181Distribution Pattern, Nest-Tree Features and Breeding Performance of Black Stork, Ciconia nigra… of trade between favourable tree features and safety of nesting location in terms of human disturbance (Strazds, 2011). In addition to the importance of Oaks and White Poplar trees, in microhabitat scale, a preference to interior forest stands is another considerable feature of Black Stork nest stands in the study area. Th is fi nding is consistent with previously ob- tained studies (Treinys et al., 2009; Tamás, 2012). Th e Black Stork’s population in the study area exhibit similar symptoms as many other declining bird populations, including lowered or fl uctuated productivity and poor nesting success (Newton, 2004). Various abiotic and biotic factors such as weather conditions or human disturbance are strongly infl uencing the productivity and nesting success of the Black Stork (Kalocsa & Tamás, 2002; Tamás, 2012). Th e obtained negative correlation be- tween annual productivity and abundance in occupied nests could be assumed. Th e roots of this assumption lie in carrying capacity of a natural environment and quality of individual breeding territory (Newton, 1998). Hence, an increase in abundance and density of Black Stork pairs lead to the occupation of territories with poorer general conditions and conse- quently to the lower productivity and nesting success. Th e latter parameter will frequently be the fi rst one to signalise the growing impact of threats and failures (Väli, 2015). In the presented study, the nesting success of the Black Stork is rather low compared to the other European studies (table 2) and together with declining population trend calls for concern. In neighbouring Hungary, water level and the existence of shallow water bodies as foraging grounds dictate the amount of food availability, which is one of the main determinants of nesting success (Newton, 1998; Tamás, 2012). Th us, quality of foraging habitat, food availability and prey dynamics may adequately explain poor nesting success in the studied area, at least to some extent. Besides, understanding the reasons that aff ect the nest success may help identify factors contributing to obtained population declines in general. In the present study, it was impossible to determine the reasons for nest failures in more than 85 % of cases. Consequently, it is impossible to draw any conclusions or regularity about failures; thus, key drivers that infl uenced low productivity and nesting success remain rather unclear. Th erefore, further research is needed to understand better these ecological processes and fi ll many gaps in our understanding of the ecology of the Black Stork population in the Gornje Podunavlje fl oodplains. Lastly, because of the high population ratio at the country level and its rarity, conservation measures based on scientifi c evidence and well-designed strategies are highly recommended, in order to protect one of the strongholds of Black Stork population in Serbia and this part of Europe. Conclusions Th e Black Stork is a regular and relatively common breeding species in the fl oodplains along the Danube in Northwestern Serbia. Th is extensive study demonstrates once again that Black Stork is a good indicator species of old, mature trees inside native forest stands. T a b l e 2 . Productivity and nesting success of the Black Stork (Ciconia nigra) published in diff erent European studies Country Productivity Nesting success Source Estonia 2.40 1.09 Sellis, 2000 Spain 2.53 2.33 Cano Alonso & Fernandez, 2003 Cze. Republic 3.29 – Pojer, 2003 Latvia 2.66 1.81 Strazds, 2011 Hungary 2.63 – Tamás, 2012 Greece 3.26 – Alexandrou et al., 2016 Italy 3.06 2.31 Fraissinet et al., 2018 Serbia 2.58 1.16 this study 182 D. Z. Rajković Presented data showed a strong preference to mature, large-diameter European Oak and White Poplar as host tree species for nest placement and avoidance of a plantation of hybrid Poplars. Despite its high breeding density, the obtained data suggest declining short-term trend and very likely long-term trend based on published estimations in the last decades. Th e pattern of population decline runs closely parallel to the productivity fl uctuations and low nesting success, but the reasons remain unclear and speculative. Th e study of breeding Black Stork’s population in Gornje Podunavlje would not have been possible without the fi nancial and logistic support of Public Enterprises “Vojvodinašume” (Forest holding “Som- bor” from Sombor) and their dedicated employees Milan Rajić, Radmila Šakić Peurača, Biljana Latić, Ivan Blažev, Đuro Ratković, Željko Krtinić which support and contribution were irreplaceable and essential. Be- sides, the author owes gratitude to Milan Ružić and Draško Grujić for assistance during fi eldwork. Dimitrije Radišić and Saša Rajkov have done GIS data processing. Kristina Floigl, Anna Enikő Tamás and Th omas Oli- ver Mérő reviewed the initial manuscript and improved the English. During 2014, the survey and monitor- ing activities were supported by the grant of European Union through a project “Wildlife health and con- servation of selected NATURA 2000 species within the Danube Cross-border region in Serbia and Hungary (Wildcond) — HUSRB/1203/122/224–02”. 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