Impact of water level on spring bird assemblages in a natural river valley in central Europe

Water level appears to be crucial for waterbirds, especially in the changeable conditions prevailing in river valleys. One of the best preserved Central European rivers is the Biebrza in north-eastern Poland, which seasonally overflows in spring, creating optimal habitats for various waterbirds. We examined a number of environmental factors which could strongly influence different foraging groups and bird assemblages with respect to water surface area and depth. Number of waterbirds peaked during the second half of March, when a large wave of geese and ducks stopped over there. The species diversity was the greatest in the first half of April, when more species arrived from their wintering grounds. We did not find any relationship between water depth and total abundance, but the former did significantly influence the numbers of some ecological feeding groups of waterbirds, i.e. diving omnivores, diving carnivores and surface carnivores. We assume that higher water level in a river valley is favourable to birds during their spring passage, because it creates areas of shallow floodwater that are intensively used by many different aquatic species. Owing to the well-preserved hydrological conditions and its full protection as a national park, the Biebrza valley not only offers optimal conditions for migratory waterbirds, but is also a good research area for studies of the phenomena taking place in avian communities during their migratory staging.