Factors influencing structural characteristics of wetland bird communities in the middle Uzh River flow

Abstract

Background. Long-term surveys of wetland bird communities of the Uzh mountain river in Zakarpattia region, along with an analysis of scientific data published over the past century enabled us to identify a set of anthropogenic factors that significantly influence the species and spatial structures of these communities. Two of the factors are specifically related to human disturbance of the riverbed and the floodplain. Namely, installation of hydraulic engineering structures aimed at water regulation and flood protection, as well as direct interventions in the river channel for smoothing the natural bed of the river bottom, removing small sedimentary islands and clearing the vegetation on the river banks. The factor of urbanization is favorable for species that tend to be synanthropic. For other wetland bird species, urbanization plays a positive role in winter. Materials and Methods. The studies were conducted in the middle Uzh River flow, which has a mountainous character. As the highest diversity of the wetland birds in the middle flow of the river is observed within the city of Uzhhorod, the main surveys were conducted right there. Birds were monitored all year round during the years 1994–2002 and 2020–2021. From 2003 to 2019, the studies were conducted sporadically. An annual cycle was subdivided into 6 periods: winter (November 20 – February 20), early spring (February 21 – April 10), breeding (April 11 – June 30), post-breeding (July), early autumn (August–September) and autumn (October – November 19). Results and Discussion. In the course of the monitoring activities on the Uzh River within the city of Uzhhorod, we identified 30 species of wetland birds belonging to 24 genera, 14 families and 10 orders. According to the pattern of their occurrence on the river throughout the year, they include resident nesting (3 species), wintering (3), migratory (4), wandering (4), summering (3) and occasional (13) bird species. Several of these species are new to the river in urban areas, while some others were not spotted there for the past 10–20 years due to a dramatic change in the habitat conditions. Numbers of certain species have also undergone significant changes due to the effects of various anthropogenic factors. The analysis of the results obtained over 30 years of studies allows us to identify 3 factors influencing the species and spatial structure of the wetland birds on the Uzh River. The first is urbanization, attracting numerous bird species. The second is water regulation, i.e. the construction of dams and water storage facilities. This factor causes a negative impact on wetland birds by changing the river water dynamics, slowing the natural flow down, and raising the water level by retaining dams. The last one is a direct intervention into the river bed, which alters the river bottom, destroys riparian vegetation and alluvial islands that form additional micromeanders in the riverbed, as well as shallow and deep water sections of the river. Conclusion. For the wetland bird species of the mountain rivers, it is crucial to keep the mosaic nature of their habitat. Interventions in the morphology of the riverbed, as well as alterations in the water dynamics of the river due to water management measures, adversely affect the species structure of wetland bird communities and the nature of their presence on the river. The cascades of retaining dams along the mountain rivers can substantially decrease the diversity of typical mountain river species and lead to a decline of these species’ population in the Carpathians. Cities, on the contrary, can serve as refuges for birds in critical winter periods.