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Article

Biodiversity and Seasonal Dynamics of Waterbirds in the Danube Wetland North of Kopački Rit

by
Zlatko Nedić
1,*,
Raluca Nicolae
2,
Stefan Popescu
2,
Vlatko Rožac
3 and
Vera Nikolić
4
1
Competence Centre Ltd. for Research and Development, Glagoljaška 27, 32100 Vinkovci, Croatia
2
Geostud SRL, Sangerului Str., No. 11, 014617 Bucharest, Romania
3
Public Institution Nature Park Kopački Rit, Mali Sakadaš 1, 31327 Kopačevo, Croatia
4
Institute of Zoology, Faculty of Biology, University of Belgrade, Studentski trg 16, 11000 Belgrade, Serbia
*
Author to whom correspondence should be addressed.
Diversity 2025, 17(10), 669; https://doi.org/10.3390/d17100669
Submission received: 7 August 2025 / Revised: 23 September 2025 / Accepted: 24 September 2025 / Published: 25 September 2025
(This article belongs to the Special Issue Wetland Biodiversity and Ecosystem Conservation)
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Abstract

Wetlands play a vital role in conserving waterbirds, particularly along major European river systems such as the Danube River and its tributaries. To promote the importance of wetlands and address the lack of data on bird biodiversity in aquatic ecosystems, this study was conducted to assess the baseline status of a demonstration site within the European project DaWetRest. The research focused on a lesser-studied pilot area, the Danube North of Kopački rit, near the village of Draž, located in the tri-border region of Croatia, Hungary, and Serbia. Systematic ornithological monitoring was carried out monthly from January to December 2024 across three monitoring areas (total transect length: 4200 m). A total of 26 bird taxa were recorded, comprising 2148 individual observations. The most dominant species were the Mallard (Anas platyrhynchos), Grey Heron (Ardea cinerea), and Great Cormorant (Phalacrocorax carbo). Conversely, the species that were the least present were White Stork (Ciconia ciconia), Squacco Heron (Ardeola ralloides), Great Crested Grebe (Podiceps cristatus), Eurasian Teal (Anas crecca), and Black Stork (Ciconia nigra). This study also examines the seasonal dynamics of waterbirds, providing essential baseline data for evaluating the effectiveness of upcoming restoration measures planned for the area.

1. Introduction

Wetlands are among the most biologically diverse and ecologically significant ecosystems in Europe, serving as crucial habitats for a wide variety of species, particularly waterbirds [1,2]. They provide essential nesting, feeding, and migratory stopover sites, supporting both common and endangered avian species, and they play a central role in preserving biodiversity on a continental scale [3,4,5]. The avifauna of countries in Central and Southeastern Europe is strongly dependent on the wetlands of the Danube River Basin, which hold exceptional importance for regional bird populations. One such country is Croatia, a small EU member state [6] that hosts several wetlands critical for ornithological conservation [7]. Some of these wetlands, such as Kopački rit, are widely recognized and protected under the Ramsar Convention [8,9,10], while many others are part of the Natura 2000 ecological network. Ornithological studies of these wetlands are essential for their sustainable management, as they provide data on habitat conditions, using waterbirds as bioindicators of habitat quality, water quality, and human pressures on ecosystems [11,12].
Croatia’s long-standing tradition of ornithological studies, dating back to the late 19th century, has provided a solid foundation for avian research and conservation efforts [7,13,14]. However, smaller, isolated, and lesser-known wetlands have remained largely under-surveyed [15]. This highlights the importance of the DaWetRest project, which was developed to demonstrate that even small wetland areas can be ecologically significant—both at the local and transnational levels—particularly when their geographic location allows them to function as ecological corridors.
Although small and almost isolated, the protected wetland of Draž Municipality is closely connected to Kopački rit, the backbone of wetland biodiversity in Croatia [16]. Situated at the confluence of the Drava and Danube Rivers, Kopački rit is renowned for its rich biodiversity and expansive floodplains. Kopački rit is a hotspot for waterbirds, including herons, egrets, spoonbills, and various duck species [17,18,19,20]. Numerous ornithological surveys performed here have confirmed its importance as both a breeding and migratory habitat, underlining the site’s transboundary ecological significance [21]. North of Kopački rit, the area of Draž Municipality, with the Topoljski dunavac (“Dunavac” is a term commonly used in Croatia, Serbia, and other countries along the Danube River. It refers to a small side arm or branch of the Danube, i.e., a secondary watercourse that branches off from the main river, often meanders through lowlands, and may eventually rejoin the main channel or become isolated as an oxbow lake or cut-off meander.) oxbow lake and the surrounding Danube canals of Šarkanj, Lorencov dunavac, and Mačkalučki dunavac, further contribute to the regional ornithological richness. Comprehensive studies in the area of Draž Municipality, including all Danube side arms and Topoljski dunavac, have either not been performed or are not available in scientific databases. However, one study has been performed in the ecosystem of Topoljski dunavac that indicates the presence of 58 species (including waterbirds), highlighting the vital role that riverine and wetland ecosystems in the broader Middle Danube region play in sustaining bird populations, particularly water-dependent species [22]. This monitoring area has been chosen as we aimed to assess the baseline condition of waterbird diversity prior to the implementation of active restoration measures in terms of fish repopulation and hydrotechnical interventions that aim to improve the ecosystem of Danube side arms. The aims of this study are to (a) contribute to ornithological knowledge by providing data on waterbird biodiversity from a less-studied area; (b) generate data that can be used to update existing platforms and portals on bird biodiversity; and (c) create a baseline for assessing the long-term effectiveness of the implemented restoration measures on waterbird fauna after the DaWetRest project was completed.

2. Materials and Methods

2.1. Study Area

From June to December 2023, preparatory activities, including transect delineation and initial area assessment, were carried out. The main field study took place from January to December 2024 in the study area of Draž Municipality. This area lies within the boundaries of Podunavlje i donje Podravlje (HR1000016—Special Protection Area under the Birds Directive), which has been part of the Natura 2000 network since 2013 due to its importance for avian conservation [23] (Figure 1 and Figure 2), and it forms part of the protected area Danube North of Kopački rit (45°51′01″ N/18°48′11″ E).
In this area, four transects were delineated:
  • Topoljski dunavac (45°50′55″ N/18°47′45″ E) is a large hydro-accumulation, and it represents the biodiversity core of the entire study area. It was naturally created from an abandoned meander of the old Danube River. Through a system of canals, most notably the Šarkanj canal, it is irrigationally connected to the Danube. The dominant vegetation is reed (Phragmites australis), and the surrounding landscape primarily consists of arable land. The length of the transect is about 1400 m, and this is Monitoring Area 1 (Figure 2).
  • Šarkanj (45°50′52″ N/18°48′17″ E) is predominantly characterized by willow (Salix spp.) vegetation and connected via an artificial canal to Topoljski dunavac in the west and the Danube River in the east. The Šarkanj canal is about 4 km long. The transect measures around 700 m and falls within Monitoring Area 2 (Figure 2).
  • Šarkanj-Lorencov dunavac (45°51′11″ N 18°48′16″ E) includes mixed lowland forests, with the Lorencov dunavac canal playing an important ecological role as a biodiversity hotspot. Lorencov dunavac connects further with Mačkalučki dunavac, near the Danube River. This transect is approximately 700 m long and falls within Monitoring Area 2 (Figure 2).
  • Lorencov-Mačkalučki dunavac (45°52′57″ N/18°49′05″ E) is located on the Mačkalučki dunavac side arm, which is close to the Danube River but not connected to it. The surrounding area is dominated by mixed forests, particularly stands of poplar (Populus spp.) and willow (Salix spp.). This transect measures approximately 1400 m, and this is Monitoring Area 3 (Figure 2).
All of the aforementioned transects were selected in relation to the planned restoration activities and fish repopulation measures defined under the DaWetRest project [24]. The monitoring focused on water-dependent bird species, particularly waterfowl [25] and the Common Kingfisher (Alcedo atthis), due to their ecological dependence on aquatic habitats and the role that they play as indicators of fish availability and habitat quality. Using the methodological approach, two smaller and two larger transects were established. Each larger transect defines one monitoring area, while the two smaller transects belong to the same monitoring area (Figure 2). The data obtained through this study will serve as a baseline for evaluating the ecological impacts and effectiveness of the restoration actions proposed within the project framework.

2.2. Survey Implementation

A total of twenty-one field observations were conducted across four transects within three monitoring areas (Figure 2). Surveys took place from January to December 2024, on a monthly basis and covering all seasons, with the following distribution: winter—6 visits; spring—5; summer—4; and autumn—6. No survey was conducted in September, in accordance with the managing authority’s recommendation to avoid disturbing the deer during rutting season. Each survey was carried out between 08:30 and 12:30, with standardized observation times: Topoljski Dunavac and Mačkalučki Dunavac were surveyed for 60 min each, and Šarkanj and Lorencov Dunavac were surveyed for 30 min each.
All transects were surveyed on foot by two or three observers. One observer was responsible for direct bird counts, while the other team members collected photographic documentation along the transects to provide additional verification. Photographs were analyzed after each field visit to confirm identifications and detect any individuals that may have been overlooked during the live observation.
Auditory cues and indirect evidence (e.g., feathers, pellets, and eggshells) were not systematically used, as the monitoring focused on water-dependent species typically identified through morphology and behavior in open wetland habitats. Surveys were limited to daytime hours, and no call-playback stimulation was applied to elicit vocal responses. This may have reduced the detectability of cryptic or nocturnal species.
A Canon EOS R6 camera equipped with a Canon RF 600 f/4 I IS USM telephoto lens was used to take photographs during this study. All photographs were geo-tagged at the time of capture. Bird taxa were identified using a standard field identification key [26] and the mobile app “Ptice na dlanu” (from Croatian: “Birds on the palm of your hand”) directly on the field and during the analysis of the photographs.

2.3. Data Analysis

Data were initially collected in Microsoft Excel and subsequently transferred to STATISTICA 10 for statistical analysis. The total number of reported individuals per species was calculated from the raw data (Figure 3) as well as relative abundance for species that showed the highest and lowest presence in the entire study period. Frequency of occurrence was calculated based on the species presence per field visits (Table 1 and Table 2). To test the distribution of species richness across seasons and monitoring areas a one-way ANOVA was applied, considering the following variables: season—21 cases that represent field observations belonging to a certain season (Figure 4A); species richness per season—the total number of reported species for each of 21 cases (Figure 4A); monitoring areas—63 cases representing field observations assigned to each monitoring area (Figure 4B), and species richness per monitoring area—total number of species reported for each of 63 cases (Figure 4B). Moreover, Tukey’s post hoc test with automatically adjusted p-value was applied after conducting a one-way ANOVA to identify where significant differences occurred among seasons and monitoring areas. To evaluate bird species richness across the study areas, the exponential Shannon index (eH′ also known as Hill Number 1; Table 3) was used as a standard metric for quantifying community heterogeneity [27]. MANOVA (repeated measures) was used to test the statistical significance of species richness across the monitoring areas while also accounting for seasonal effects (Figure 5). The following variables were included: season—21 cases representing field observation per single monitoring area (the cases belong to a specific season according to the number of field visits) and variables Monitoring area 1, Monitoring area 2, and Monitoring area 3 that represent the total number of reported species per each case, separately for each monitoring area.

3. Results

During the monitoring activities, a total of 2148 individuals were registered, encompassing 26 taxa from 9 families (Table 1 and Table 2, Figure 3). The number of individuals varied widely among species, with some species being highly present while others were relatively rare. The results revealed that two species, Grey Heron (Ardea cinerea) and Mallard (Anas platyrhynchos), were the most dominant, each accounting for approximately 17.09% of all reported individuals. Following closely was Great Cormorant (Phalacrocorax carbo), which made up around 16.48% of the total observations. Together, these three species comprised over 50% of all individuals observed, highlighting their ecological dominance in the study area. In contrast, other species such as Greylag Goose (Anser anser) and Mute Swan (Cygnus olor) showed notable but lower relative abundances of 10.61% and 7.40%, respectively. At the opposite end of the spectrum, several species demonstrated minimal dominance. For example, Eurasian Teal (Anas crecca) and Black Stork (Ciconia nigra) each accounted for only 0.05% of all observations, making them the least frequently recorded taxa. They were followed by Great Crested Grebe (Podiceps cristatus) with 0.09% and both White Stork (Ciconia ciconia) and Squacco Heron (Ardeola ralloides), each representing 0.14% of the total abundance. These exceptionally low percentages indicate that these species were rarely encountered and had minimal influence on overall species composition during the monitoring period.
The maximum number of species recorded per season varied significantly (F = 4.2743; p < 0.05), and more species were reported during the summer compared to the winter (MS = 2.1843; df = 17.000; p < 0.05; Figure 4A, Table 1). We identified four species that were present across all seasons: Ardea cinerea, Anas platyrhynchos, Phalacrocorax carbo, and Cygnus olor (Table 1).
Statistical significance was detected in terms of species richness per monitoring area (F = 35.667; p < 0.05). The results revealed that Monitoring Area 1 showed a significantly higher mean number of species compared to Monitoring Areas 2 and 3 (MS = 2.7317; df = 60; p < 0.05). There was no identified statistical significance between Monitoring Areas 2 and 3 (p > 0.3012). These findings suggest that Monitoring Area 1 serves as a biodiversity hotspot, whereas Areas 2 and 3 support fewer species and do not differ significantly from each other in terms of richness (Table 2, Figure 4B).
Statistical significance was not detected considering species richness per monitoring area per season (F = 2.0311; p = 0.06348), but the test results were close to the threshold, highlighting higher species richness in Monitoring Area 1 (Figure 5).
Monitoring Area 1 showed the highest biodiversity, with index values ranging from H = 1.606 to H = 7.365. Monitoring Area 2 indicated lower values (from H = 1.419 to H = 5.044), as did Monitoring Area 3 (from H = 1.583 to H = 4.696). Our analysis clearly identified Monitoring Area 1 as a backbone of waterbird biodiversity for the whole area, underscoring its ecological significance in the context of the broader wetland ecosystem (Table 3).

4. Discussion

Although Croatian ornithology has a long-standing tradition, studies focusing on small biodiversity hotspots remain spatially uneven and fragmented, and they are often concentrated on specific taxa or specific ecological aspects [31]. This study provides the current status of waterbird fauna in Draž (Figure 1 and Figure 2), within the protected Natura 2000 site Danube North of Kopački rit (code HR2001309). One of the most important resources for comparing these findings with previous/ongoing monitoring activities is the platform that aggregates expert-verified data [32]. According to this portal, approximately 45 waterbird taxa have been recorded in the wider Danube North of Kopački rit region. Our study documented 26 taxa, representing over 57% of the area’s total recorded waterbird diversity [32]. The presence of these taxa is further corroborated by the eBird platform, which documents observations for many of the focal species over a 40-year period (1985–2024) [33]. The high prevalence of piscivorous species such as the Grey Heron (Ardea cinerea) across all seasons and monitoring areas is consistent with previous studies and should be characterized as a common occurrence [33,34,35]. The favorable habitat conditions of Topoljski dunavac, characterized by heterogeneous feeding grounds and diverse fish availability, further explain the abundance of this species [36]. The Great Cormorant (Phalacrocorax carbo) also showed high presence throughout the year in all three areas; however, no nesting activity was recorded for cormorants, which aligns with existing studies identifying Lonjsko polje and Kopački rit as the primary breeding sites for this species [37]. This pattern is consistent with other studies in Croatia, which documented frequent occurrence of non-breeding individuals foraging at fishponds [38].
The high presence of the Anatidae family was largely due to the presence of Greylag Goose (Anser anser) and Mallard (Anas platyrhynchos). The Croatian Red Book of Birds states that Topoljski Dunavac was considered one of the key nesting sites for Greylag Goose, with up to 30 pairs recorded since 2009 [35,39]. This species was most frequently observed in summer and autumn in Monitoring Area 1, which is surrounded by arable and grassy land that likely satisfies their feeding requirements. Although specific monitoring data for Anas platyrhynchos in the study area are lacking, over 170 observations with more than 3500 individuals have been recorded since 2000 within the broader Danube North of Kopački rit [32]. According to open-access biodiversity platforms, Mallards (Anas platyrhynchos) were previously recorded in Monitoring Area 3 but not in Areas 1 or 2. This highlights the need for systematic, holistic, and spatially comprehensive monitoring efforts that enable contributions from all interested stakeholders [33,40]. Mallards heavily rely on aquatic invertebrates during brood-rearing, while fish are only a minor dietary component. Studies show that fish-rich waters can reduce duckling survival by depleting invertebrate prey [41,42]. Other studies [43,44] further confirm that fish presence alters invertebrate availability and affects duckling foraging behavior, making fish-dense habitats less suitable. For this reason, it is necessary to determine and recommend future restoration measures that ensure that the habitat remains favorable for all species inhabiting the area.
The number of certain species, such as Common Teal (Anas crecca), was lower than in previous studies in areas along the Drava River [45]. The presence of species such as the Black-crowned Night Heron (Nycticorax nycticorax), Great White Egret (Ardea alba), Mute Swan (Cygnus olor), and Goosander (Mergus merganser) supports the assumption of generally favorable environmental conditions in the study area, as these species typically require suitable water levels, diverse aquatic vegetation, and sufficient fish resources to thrive [35,39]. Conversely, certain members of the Ardeidae family were less present, such as Purple Heron (Ardea purpurea), Little Egret (Egretta garzetta), and Squacco Heron (Ardeola ralloides). This pattern could be attributed to environmental pressures, including habitat degradation, particularly of reed communities, caused by intensive agriculture, as well as reduced fish availability [39]. Agricultural activities contribute to the reduction in and fragmentation of reedbeds (Phragmites australis), which are essential for the foraging and breeding of A. purpurea [46,47]. Unlike the Grey Heron (Ardea cinerea), an ecological generalist capable of exploiting a wide range of habitats, including open wetlands, fishponds, rivers, and anthropogenically altered landscapes [48,49], A. purpurea is strongly dependent on well-preserved reed-dominated habitats for both nesting and food supply [46,47]. This specialization makes the species particularly vulnerable to reedbed degradation. Therefore, while both species may face pressures from reduced fish availability, the added constraint of reedbed loss disproportionately affects A. purpurea.
According to data from open-access platforms, the presence of Ardea purpurea, Egretta garzetta, and Ardeola ralloides was lower compared to species from other families, which aligns with our recent findings [32,33]. Similar patterns were observed for Common Goldeneye (Bucephala clangula), Ferruginous duck (Aythya nyroca), and Common Teal (Anas crecca), with the latter two recorded only during winter, consistent with earlier studies [32,45]. Artificially created ecological conditions (caused by dredging and cleaning of Topoljski dunavac) likely contributed to the presence of these species, creating relatively shallow wetlands with well-structured, mosaic-like vegetation and a diversity of microhabitats [39,50].
Among Laridae, the Black-headed Gull (Larus ridibundus) was the most commonly recorded species, consistent with studies from similar habitats [22,33]. The Common Tern (Sterna hirundo) was observed only once (19 April 2024), likely during the spring migration route to its typical breeding habitats of the Adriatic island region [51,52]. The Black Tern (Chlidonias niger) was also detected and, as with S. hirundo, its presence is probably linked to seasonal migration [53].
The Common Kingfisher (Alcedo atthis) was observed across all monitoring areas. This is likely due to the availability of suitable nesting substrates, such as sandy or loamy embankments formed during canal maintenance [54]. The estimated Croatian population ranges from 700 to 1000 breeding pairs, with over 240 recorded within ecological network sites [35]. Its consistent presence is further supported by recent records, which confirm the stable occurrence of the species in the Draž region over the past two years [33].
The presence of the Common Sandpiper (Actitis hypoleucos) in April and May confirms its status as a regular passage migrant from inland Croatia [35]. Its presence was likely facilitated by the artificially constructed sandbanks in Topoljski dunavac. There is no previous evidence of the presence of the mentioned species in the Draž area [32]. In contrast, the Green Sandpiper (Tringa ochropus) was reported only twice in Draž in 2023, highlighting the value of continuous monitoring to capture such variations [32].
Two Rallidae species were reported: Common Coot (Fulica atra) and Moorhen (Gallinula chloropus). Common Coot (Fulica atra) is very common in the Draž area, and the available sources suggest that its actual prevalence may be higher than what was observed in this study [32]. The most recent update from 2025 reported more than 600 individuals during feeding events [32]. In our monitoring, the species was observed only during winter, which corresponds with studies indicating its seasonal presence at fishponds during both winter and summer periods [55]. Gallinula chloropus was reported only in the summer and autumn, exclusively in the Topoljski dunavac transect. Its presence may be linked to the creation of shallow water habitats through dredging and regular maintenance activities, which led to the formation of artificial accumulations suitable for this species [56].
From Ciconiidae, two species with low abundance were reported: White Stork (Ciconia ciconia), with three individuals from May to July in flight, and Black Stork (Ciconia nigra), with one individual in April in flight. The available data suggest that breeding populations of Ciconia ciconia in Croatia are unknown [39], and past sightings in the area are typically of birds in flight [32]. Although fish represent a substantial component of storks’ diet [57], this species was less frequently observed in the monitoring areas. The reasons for this most likely result from environmental pressures and threats, such as pesticide use [39]. The other reason is that breeding pairs nest within Draž village, close to the monitoring areas, and may forage at different times compared with those covered by the survey, resulting in their absence during fieldwork [58]. The presence of Ciconia nigra was also low. According to existing data, the breeding population in nearby regions (Gornje Podunavlje and Donje Podravlje) ranges from 35 to 55 pairs [39]. Since 2014, this species has been reported only 19 times in the Danube North of Kopački rit, based on observations by experts and citizen scientists, confirming its rarity in the area [32].
Finally, the Great Crested Grebe (Podiceps cristatus), the only representative of the Podicipedidae family observed, showed the lowest abundance, consistent with previous observations indicating just six sightings between 2017 and 2022 [32].
Our results stress the need to view such wetland habitats not in isolation and rather part of a larger ecological network essential to the lifecycles of water-dependent birds. Monitoring Area 1 demonstrated significantly greater species richness compared to Monitoring Areas 2 and 3, likely due to Area 1’s optimal microhabitat configuration and permanent hydrological connectivity to the Danube River, which make this area particularly valuable from a biodiversity perspective [12,35,39].
Seasonal fluctuations in the biodiversity index indicate that species diversity is not uniform throughout the year. This is probably due to the large number of taxa that occur across all seasons and species-specific biological characteristics, especially in relation to their feeding habits and migratory patterns [59]. Considering the relatively high number of species registered in the study area, and taking into consideration the Shannon index values alongside the findings from comparable wetland studies, our results reveal a similar trend, namely Draž’s slightly higher Shannon index. This is likely attributable to the habitat complexity and ecological richness of Topoljski dunavac, which supported a greater number of reported taxa and higher diversity overall [60].
Our monitoring results indicate that the Draž wetland currently supports fewer waterbird species than expected given its habitat potential. For instance, comparable European wetlands of similar habitat complexity, such as Kopački Rit in Croatia and Neusiedler See—Seewinkel on the Austrian-Hungarian border, regularly support much richer assemblages: Kopački Rit harbors nearly 300 bird species and more than 20,000 waterbirds in winter [9,16], while Neusiedler See—Seewinkel sustains tens of thousands of migratory waterbirds annually, including herons, egrets, and large flocks of geese and ducks [61,62]. Compared to these sites, the Draž wetland appears underutilized by waterbirds despite offering suitable habitat conditions.
This limitation arises primarily from the reduced hydrological connectivity of the lateral canals with the Danube River, which restricts water exchange and fish migration. Hydrological connectivity and flood pulses are widely recognized as central processes supporting biodiversity, nutrient exchange, and fish recruitment in rivers–floodplain systems [63,64,65]. Planned restoration measures are therefore essential to re-establish stable water levels and enhance habitat suitability for water-dependent birds [66,67]. Beyond biodiversity gains, such interventions would strengthen ecosystem services, including nutrient cycling, carbon sequestration, and flood regulation [68,69], while also supporting local socioeconomic development through ecotourism and nature-based innovation [70]. By restoring ecological processes and improving habitat quality, the site can contribute more effectively to the broader wetland network of the Danube region [71].
This study has some limitations. Transect-based monitoring may under-detect cryptic, nocturnal, or highly mobile species, and the single-year dataset restricts the ability to capture interannual variability. In addition, surveys were limited to daytime hours and did not employ call–playback techniques, which likely reduced the detectability of secretive or nocturnal taxa such as rallids, Ixobrychus minutus, Nycticorax nycticorax, and Botaurus stellaris. Therefore, in future research, monitoring should be extended across multiple years, hydrological and fish population data should be integrated, and expert-led surveys with citizen-science platforms should be combined. Such approaches will ensure a more robust evaluation of restoration outcomes and strengthen adaptive management for wetlands across the wider Danube basin.

5. Conclusions

This study provides the first year-round assessment of waterbird diversity in the Draž Municipality, within the Natura 2000 site “Danube North of Kopački rit”. By documenting 26 taxa and over 2100 individuals, we establish a crucial baseline ahead of planned restoration efforts under the DaWetRest project. Importantly, Topoljski dunavac (Monitoring Area 1) was identified as a biodiversity hotspot, underscoring the role that this area plays in terms of hydrological connectivity and habitat diversity to sustain high species richness and community stability.
Our findings contribute to current knowledge by demonstrating the ecological value of small, semi-isolated wetlands, which function as key nodes in regional ecological networks. They also highlight management priorities: restoration measures, particularly hydrotechnical and habitat interventions that should be ecologically calibrated to preserve trophic balance and support sensitive life stages such as breeding and brood-rearing. Moreover, the presence of habitat specialists such as Ardea purpurea emphasizes the need to safeguard reedbeds and shallow-water mosaics as critical microhabitats.
Although this study covers only a single year, the results may provide valuable baseline for future restoration and monitoring at Draž. Building on this foundation, subsequent efforts should integrate longer-term ecological observations and expanded methodological approaches. In this way, restoration actions would not only safeguard local biodiversity but also strengthen the role of Draž within the wider Danube wetland network, enhancing both ecological resilience and conservation outcomes.

Author Contributions

Conceptualization Z.N. and V.N.; methodology, Z.N. and R.N.; software, ZN.; validation, V.R. and S.P.; formal analysis, Z.N.; investigation, Z.N.; resources, V.R.; data curation, V.R.; writing—original draft preparation, Z.N. and V.N.; writing—review and editing, V.R. and R.N.; visualization, V.N. and S.P.; supervision, V.R.; project administration, Z.N.; funding acquisition, R.N. All authors have read and agreed to the published version of the manuscript.

Funding

This work is a part of the Danube Wetlands and flood plains Restoration through systemic, community engaged and sustainable innovative actions (DaWetRest) project, which is co-funded by the European Union Horizon Europe programme (HORIZON-MISS-2022-OCEAN-O1-02), under Grant Agreement no. 101113015. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or CINEA. Neither the European Union nor the granting authority can be held responsible for them.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

Authors express their gratitude to Alma Mikuška from Josip Juraj Strossmayer University of Osijek, Department of Biology, for her support during the preparation of this paper and to Filip Županić from the Competence Centre Ltd. for research and development for technical support during the field observation. This research was also supported by Ministry of Science, Technological Development and Innovation of the Republic of Serbia (Contract No. 451-03-137/2025-03/200178).

Conflicts of Interest

Authors declare that the research was conducted in the absence of any commercial or financial relationships that could be constructed as potential conflicts of interest.

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Figure 1. Danube N of Kopački rit—geographical location: (a) Europe; (b) regional level. Green color marks areas that belong to the ecological network Natura 2000 (modified by author; source: [16]).
Figure 1. Danube N of Kopački rit—geographical location: (a) Europe; (b) regional level. Green color marks areas that belong to the ecological network Natura 2000 (modified by author; source: [16]).
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Figure 2. Monitoring areas and overlapping Ramsar and Natura 2000 protected areas in the Danube wetlands near Kopački Rit.
Figure 2. Monitoring areas and overlapping Ramsar and Natura 2000 protected areas in the Danube wetlands near Kopački Rit.
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Figure 3. Total number of reported individuals per species and families counted in the entire study area over all monitoring rounds.
Figure 3. Total number of reported individuals per species and families counted in the entire study area over all monitoring rounds.
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Figure 4. Distribution of the total number of reported species per (A) season and (B) monitoring area.
Figure 4. Distribution of the total number of reported species per (A) season and (B) monitoring area.
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Figure 5. Distribution of the total number of reported species per monitoring area and seasons.
Figure 5. Distribution of the total number of reported species per monitoring area and seasons.
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Table 1. List of reported waterbirds, their conservation status, and seasonal frequencies: W—winter; SP—spring; SU—summer; A—autumn; Mean—average of reported species per season; SD—standard deviation for seasonal frequency.
Table 1. List of reported waterbirds, their conservation status, and seasonal frequencies: W—winter; SP—spring; SU—summer; A—autumn; Mean—average of reported species per season; SD—standard deviation for seasonal frequency.
TaxaIUCN [28]
/EU Bird Directive [29]
/(Bern Convention) [30]		Frequency of Occurence Throughout Season (n = Number of Field Observation Conducted in Certain Season)
W
(n = 6)		SP
(n = 5)		S
(n = 4)		S
(n = 6)		MeanSD
ANSERIFORMES
Anatidae
Anas creccaLC/II/(III)0.170.000.000.000.040.09
Anas platyrhynchosLC/II/(III)22.6722.4021.508.2518.716.99
Aythya nyrocaLC/I/(III)0.000.003.250.000.811.63
Bucephala clangulaLC/II/(III)3.830.000.000.000.961.92
Cygnus olorLC/II/(III)5.003.808.7518.759.086.79
Mergus merganserLC/II/(III)18.003.800.005.256.767.81
Anser anserLC/II/(III)0.000.002.2554.7514.2527.02
PODICIPEDIFORMES
Podicipedidae
Podiceps cristatusLC(III)0.000.000.000.500.130.25
CHARADRIIFORMES
Scolopacidae
Actitis hypoleucosLC(II)0.004.200.000.001.052.10
Tringa ochropusLC(II)2.170.200.250.000.661.02
Sternidae
Chlidonias nigerLC/I/(II)0.000.004.000.001.002.00
Sterna hirundoLC/I/(II)0.006.600.000.001.653.30
Laridae
Chroicocephalus ridibundusLC/I/(III)0.000.000.0019.004.759.50
Larus sp. 0.170.000.505.001.422.4
CICONIIFORMES
Ardeidae
Ardea albaLC/I/(III)1.675.0012.251.755.174.97
Ardea cinereaLC(III)7.1716.2039.7521.0021.0313.73
Ardea purpureaLC/I/(II)0.000.601.751.500.960.81
Ardeola ralloidesLC/I/(II)0.000.400.000.250.160.20
Egretta garzettaLC/I/(II)0.002.600.000.000.651.30
Nycticorax nycticoraxLC/I/(II)0.006.0011.759.006.695.04
Ciconiidae
Ciconia ciconiaLC/I/(II)0.000.200.500.000.180.24
Ciconia nigraLC/I/(II)0.000.200.000.000.050.10
Rallidae
Fulica atraNT/II/(III)2.330.000.000.000.581.17
Gallinula chloropusLC/II/(III)0.000.003.751.501.311.77
PELECANIFORMES
Phalacrocoracidae
Phalacrocorax carboLC(III)18.335.0010.5044.2519.5217.37
CORACIIFORMES
Alcedinidae
Alcedo atthisVU/I/(II)0.170.203.003.251.661.70
Table 2. Locational frequencies: M1—Monitoring Area 1; M2—Monitoring Area 2; and M3—Monitoring Area 3; Mean—average of reported species per monitoring area; SD—standard deviation for frequency per monitoring areas.
Table 2. Locational frequencies: M1—Monitoring Area 1; M2—Monitoring Area 2; and M3—Monitoring Area 3; Mean—average of reported species per monitoring area; SD—standard deviation for frequency per monitoring areas.
Frequency of Occurrence per Monitoring Areas (n = Number of Field Observation Conducted in Certain Monitoring Area)
TaxaM1 (n = 21)M2 (n = 21)M3 (n = 21)MeanSD
ANSERIFORMES
Anatidae
Anas crecca0.000.050.000.020.03
Anas platyrhynchos10.763.483.245.834.27
Aythya nyroca0.430.000.190.210.22
Bucephala clangula1.100.000.000.370.64
Cygnus olor5.950.710.902.522.97
Mergus merganser2.951.812.292.350.57
Anser anser10.810.050.003.626.23
PODICIPEDIFORMES
Podicipedidae
Podiceps cristatus0.000.000.100.030.06
CHARADRIIFORMES
Scolopacidae
Actitis hypoleucos1.000.000.000.330.58
Tringa ochropus0.620.000.100.240.33
Sternidae
Chlidonias niger0.760.000.000.250.44
Sterna hirundo1.570.000.000.520.91
Laridae
Chroicocephalus ridibundus3.570.050.001.212.05
Larus sp.1.100.000.000.370.64
CICONIIFORMES
Ardeidae
Ardea alba2.051.570.711.440.68
Ardea cinerea9.765.142.575.823.64
Ardea purpurea0.760.000.000.250.44
Ardeola ralloides0.140.000.000.050.08
Egretta garzetta0.570.050.000.210.32
Nycticorax nycticorax5.330.050.001.793.06
Ciconiidae
Ciconia ciconia0.100.050.000.050.05
Ciconia nigra0.000.000.050.020.03
Rallidae
Fulica atra0.670.000.000.220.39
Gallinula chloropus1.000.000.000.330.58
PELECANIFORMES
Phalacrocoracidae
Phalacrocorax carbo12.191.713.435.785.62
CORACIIFORMES
Alcedinidae
Alcedo atthis0.670.570.050.430.33
Table 3. The exponential Shannon index eH′, also known as Hill Number 1 (H): n = number of field observations per season; Min = lowest value of eH′ reported for a given season and monitoring area; Max = highest value of eH′ reported for a given season and monitoring area; Mean = average value of eH′ for a given season and monitoring area; SD = standard deviation.
Table 3. The exponential Shannon index eH′, also known as Hill Number 1 (H): n = number of field observations per season; Min = lowest value of eH′ reported for a given season and monitoring area; Max = highest value of eH′ reported for a given season and monitoring area; Mean = average value of eH′ for a given season and monitoring area; SD = standard deviation.
SeasonnMonitoring Area 1Monitoring Area 2Monitoring Area 3
MinMaxMeanSDMinMaxMeanSDMinMaxMeanSD
Winter63.5896.3764.5211.1011.7382.4561.9760.3261.5834.6962.7351.355
Spring53.3356.8234.5571.5252.0904.2093.2390.9471.9613.5972.8450.599
Summer44.0987.3656.2701.4721.4195.0442.8161.9502.3242.8142.5470.248
Autumn61.6064.7243.0701.0411.7553.6522.7660.7841.7552.2191.9020.213
Total211.6067.3654.4481.6131.4195.0442.7301.0341.5834.6962.5260.793
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MDPI and ACS Style

Nedić, Z.; Nicolae, R.; Popescu, S.; Rožac, V.; Nikolić, V. Biodiversity and Seasonal Dynamics of Waterbirds in the Danube Wetland North of Kopački Rit. Diversity 2025, 17, 669. https://doi.org/10.3390/d17100669

AMA Style

Nedić Z, Nicolae R, Popescu S, Rožac V, Nikolić V. Biodiversity and Seasonal Dynamics of Waterbirds in the Danube Wetland North of Kopački Rit. Diversity. 2025; 17(10):669. https://doi.org/10.3390/d17100669

Chicago/Turabian Style

Nedić, Zlatko, Raluca Nicolae, Stefan Popescu, Vlatko Rožac, and Vera Nikolić. 2025. "Biodiversity and Seasonal Dynamics of Waterbirds in the Danube Wetland North of Kopački Rit" Diversity 17, no. 10: 669. https://doi.org/10.3390/d17100669

APA Style

Nedić, Z., Nicolae, R., Popescu, S., Rožac, V., & Nikolić, V. (2025). Biodiversity and Seasonal Dynamics of Waterbirds in the Danube Wetland North of Kopački Rit. Diversity, 17(10), 669. https://doi.org/10.3390/d17100669

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