Številčnost in razširjenost izbranih gnezdilk struge reke Drave med Mariborom in Središčem ob Dravi (SV Slovenija) v letih 2006 in 2009 ter vzroki za zmanjšanje njihovih populacij
Between mid-April ...and mid-June of 2006 and 2009, breeding Little Ringed Plovers Charadrius dubius, Common Sandpipers Actitis hypoleucos and Kingfishers Alcedo atthis were surveyed along the bed of the Drava River between Maribor and Središče ob Dravi. The entire bed (56.1 km) was surveyed in this area in 2006, and the part between Maribor and the state border at Zavrč (38.8 km) in 2009. Birds were mostly counted from a moving boat and during stops on the river's gravel bars. The area was divided into five sections, and birds counted twice in each. Special criteria for breeding pair definition were applied during interpretation of the results. In 2006, the size of the Little Ringed Plover's breeding population was estimated at 75-87 pairs, the Common Sandpiper's at 56-65 pairs, and the Kingfisher's at 17-24 pairs. The part of the Drava River surveyed in 2009 was used as a breeding ground by 16-22 Little Ringed Plover pairs, 22-24 Common Sandpiper pairs, and 14 Kingfisher pairs. In 2006, the linear density in the entire study area was 1.3-1.6 Little Ringed Plover pairs/km, 1.0-1.2 Common Sandpiper pairs/km and 0.3-0.4 Kingfisher pairs/km of the river stream, while in 2009 their densities were somewhat lower. The differences in densities between survey sections were similar in both years. In both 2006 and 2009, 1-2 Little Ringed Plover pairs bred on most of the occupied gravel bars. In 2006, this species bred on 33 of a total of 92 gravel bars (35.9%) in the part of the river between Maribor and Zavrč, and in 2009, on 15 bars (16.3%). In the upper part of the Drava River, the Common Sandpiper was a relatively sparse breeding bird during the two surveys, but much more numerous in the middle and lower parts of the river. With an average of one pair per 2-3 km of the river stream, the Kingfisher was fairly evenly distributed along the entire bed of the Drava River during both surveys. In 2009, the number of Little Ringed Plover breeding pairs in the part of the river between Maribor and Zavrč was by 62% lower than in 2006. Consideration of all possible causes for these results leads the authors to conclude that the major cause of the birds' reduced abundance over this period lies in the increased overgrowing of gravel bars with woody plants and herbs and thus reduced surface area of bare shingle that is, apart from the height of gravel bars, the most significant factor for the settling of this species. The increased overgrowing of gravel bars in 2009 can be explained by the difference in the number of days with large discharges (≥ 400 m3/s) in the years prior to the two surveys, given that during the three years before the 2009 survey there were considerably fewer (3/1) than in the same period before the 2006 survey (6/6). The condition in 2009 was thus the direct result of the absence of floods that maintain the surfaces of bare shingle on gravel bars and reduce their overspreading with herbage. In the study area, the linear densities of Little Ringed Plover and Common Sandpiper are among the highest in Central Europe, while those of the Kingfisher are approximately the same as on other rivers in this geographical region. The large Little Ringed Plover population that breeds here in its natural habitat is of special significance for conservation. In fact, the Drava River is of a great national significance for all three species, it being the breeding ground of more than 10% or 15% of the national population of Little Ringed Plover and Common Sandpiper, and 8% of the Kingfisher's national population.
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