CONCLUSIONS In 1994 we were observing changes in the occurence of water-fowl and birds of prey representatives (Falconiformes) in the region of the hydroelectric power structures Gabcikovo, in the section of the Hrusov water reservoir and the adjacent part of the old river-bed of the Danube. In the period from March to August, 1994 fifteen field observations were made, with the use of ordinary audio-visual methods, and the occurence of 52 species of birds of the above groups was recorded.
The preliminary results suggest that the rise of a wide-spread water surface in the immediate neighbourhood of the old river-bed of the Danube gave rise to biotopes suitable for many species of birds. We observed the shifting of one part of the water-fowl from the old flow of the Danub to the water reservoir, with a considerable growth of numbers of individuals, while even the species not typical for the Danubian area before the construction of the hydroelectric power structures appear in abundant numbers. Besides, we observed the occurence of several spieces that are rare in our country. The overall numbers of birds found in the observed area amounted from approximately 1400 to 1800 pieces in March to approximately 17000 pieces in August. Further, it was observed that the changes in the water-level of the reservoir have a considerable impact on the dislocation of birds and the representation of individual species.
On the basis of the obtained results it is possible to state that the hydroelectric power structures Gabcikovo has become an important place of hibernation for the hibernating species, a place of nesting for the nesting species, and a place for the species finding food in the water reservoir. With a view to the above facts and the excellent opportunity to document the settlement of the newly built water reservoir by the water-fowl, we consider necessary to continue in the research and to concentrate on observation of behavioural changes of the most important species, due to adaptation to newly-formed life conditions.
Table 1: Survey of species found in individual parts of the observed territory in the period March - August 1994.
| SPECIES | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
|---|---|---|---|---|---|---|---|
| Gavia stellata | - | + | - | - | - | - | - |
| Gavia arctica | - | + | - | - | - | - | - |
| Podiceps cristatus | + | + | - | - | - | + | + |
| Podiceps nigricollis | - | + | - | - | - | - | - |
| Podiceps griseigena | - | + | - | - | - | - | - |
| Tachybaptus ruficollis | - | + | + | - | + | + | - |
| Phalacrocorax carbo | + | + | - | + | + | + | + |
| Ardea cinerea | + | + | + | + | + | + | + |
| Ardea purpurea | + | - | - | - | - | - | - |
| Egretta alba | + | - | - | + | - | - | + |
| Ciconia ciconia | - | - | - | + | - | - | - |
| Ciconia nigra | + | - | - | - | - | - | - |
| Cygnus olor | + | + | + | + | + | - | + |
| Cygnus atratus | + | - | - | - | - | - | - |
| Anser fabalis | + | (+) | - | (+) | - | - | - |
| Anas platyrhynchos | + | + | + | + | + | + | + |
| Anas strepera | - | + | - | - | - | - | - |
| Anas querquedula | + | + | + | - | - | - | - |
| Spatula clypeata | + | - | - | - | - | - | - |
| Netta rufina | + | - | - | - | - | - | - |
| Aythia ferina | + | + | - | - | - | - | - |
| Aythia fuligula | + | + | - | + | - | - | + |
| Aythia marila | + | - | - | - | - | - | - |
| Bucephala clangula | + | + | - | - | - | - | - |
| Clangula hyemalis | - | + | - | - | - | - | - |
| Mergus albellus | - | + | - | - | - | - | - |
| Mergus serrator | - | - | - | + | - | - | - |
| Milvus migrans | + | - | - | - | - | - | - |
| Haliaetus albicilla | - | - | - | + | - | - | - |
| Buteo buteo | + | - | + | + | - | - | + |
| Circus aeruginosus | + | - | - | - | - | - | - |
| Falco subbuteo | - | - | - | + | - | - | - |
| Falco tinnunculus | + | - | + | - | - | - | + |
| Pandion haliaetus | - | - | - | + | - | - | - |
| Gallinula chloropus | - | - | - | - | + | - | - |
| Fulica atra | + | + | + | - | + | - | + |
| Vanellus vanellus | + | + | - | + | - | - | - |
| Charadrius dubius | + | + | - | + | - | - | - |
| Phillomachus pugnax | + | + | - | - | - | - | - |
| Tringa nebularia | + | + | - | + | - | - | - |
| Tringa hypoleucos | - | + | - | - | - | - | - |
| Tringa totanus | + | + | - | - | - | - | - |
| Numenius arquatus | + | - | - | - | - | - | - |
| Capella gallinago | - | + | - | - | - | - | - |
| Recurvirostra avosetta | + | - | - | + | - | - | - |
| Larus argentatus/cachinnans | + | + | + | + | - | - | - |
| Larus ridibundus | + | + | + | + | + | + | + |
| Chlidonias nigra | + | + | - | - | - | - | - |
| Sterna hirundo | + | + | - | + | - | - | - |
| Riparia riparia | + | + | + | + | - | - | - |
| Motacilla alba | + | + | - | + | - | - | - |
| Motacilla flava | - | + | - | - | - | - | - |
Table legeng:
The minimum numbers of birds found in the whole investigated territory during one observation fluctuated around 1400-1800 individuals in March and the maximum state has so far been found in August, when the total numbers amounted to approximately 17000 individuals.
In the area of the old river-bed of the Danube general decrease in numbers of individuals may be observed in recent years. This phenomenon may be documented e.g. on changes in the numbers of the wild duck (Anas platyrhynchos), found during regular January international counting of the Danubian water-fowl (Table 2).
Table 2: Results of January counting of the wild duck (Anas platyrhynchos)
| Year | 1991 | 1992 | 1994 |
|---|---|---|---|
| Numbers of wild ducks in the section of Danube from rkm 1842 to rkm 1871,5 | 653 | 539 | 168 |
The high numbers of birds found in the reservoir area (maximum around 14000 pieces) suggest that this newly built water surface is very attractive for the water-fowl, since it provides relatively suitable conditions for hibernation, rest taking, finding food, and with many species also for nesting. Even in the present stage it is evident that the Hrusov reservoir has become an important gathering - place of migrating and hibernating birds.
The results of the observations have so far shown that the numbers of individuals and their location in the observed territory are considerably influenced by changes in the height of the water level.
The most numerous and the most frequently occurring species in the observed territory during the spring and summer periods were the following ones: Phalacrocarax carbo, Cygnus olor, Anas platyrhynchos, Aythia fuligula, Fulica atra, Larus ridibundus and Riparia riparia.
Results of the comparison of minimum and maximum numbers of selected indicative species of birds found in the water reservoir and the observed section of the old river-bed of the Danube in individual months are shown in Table 3.
Table 3: Numbers of selected species of birds found in the old river-bed of the Danube and in the Hrusov resevoir in 1994
| SPECIES | Maximal and minimal numbers | ||||||
|---|---|---|---|---|---|---|---|
| March | April | May | June | July | August | ||
| Podiceps cristatus | D | 0 | 0 | 0 | 0 | 0 | 0 |
| Z | 14-27 | 26-110 | 8-67 | 47-61 | 76 | 44 | |
| Phalacrocorax carbo | D | 2-188 | 64-99 | 4-173 | 3-11 | 117 | 6 |
| Z | 63-131 | 45-472 | 28-389 | 143-198 | 1086 | 810 | |
| Cygnus olor | D | 58-107 | 0-64 | 0-71 | 18-19 | 12 | 4 |
| Z | 4-21 | 12-57 | 80-168 | 113-275 | 384 | 689 | |
| Anas platyrhynchos | D | 122-150 | 6-208 | 22-31 | 23-233 | 22 | 250 |
| Z | 104-127 | 87-201 | 14-155 | 141-300 | 174 | 4679 | |
| Aythia fuligula | D | 0 | 0 | 0 | 0 | 1 | 0 |
| Z | 435-656 | 58-1605 | 15-18 | 10-183 | 555 | 0 | |
| Fulica atra | D | 0 | 0 | 0 | 0 | 0 | 0 |
| Z | 235-477 | 72-354 | 32-85 | 52-92 | 660 | 3290 | |
| Larus ridibundus | D | 10-33 | 215-370 | 348-520 | 210-295 | 167 | 196 |
| Z | 129-499 | 1037-2017 | 610-2218 | 734-1588 | 4405 | 536 | |
The occurrence of several rarer species was observed: Gavia stellata, Gavia arctica, Podiceps griseigena, Ardea purpurea, Netta rufina, Aythia marila, clangula hyemalis, Mergus serrator, Haliaetus albicilla, Pandion haliaetus, Falco subbuteo, Numenius arquatus and Recurvirostra avosetta.
A more complete picture of changes in avifauna composition and its adaptation to new conditions will be presented on the completion of a year-long cycle of observations and on obtaining further data from several years of research. In spite of that even the results obtained so far provide a lot of valuable information on the impact of the Hydroelectric power structure Gabcikovo on the qualitative and quantitative composition and spatial preferences of avifauna in the observed territory.
The decrease of the number of species and individuals of birds in the observed section of the old river-bed of the Danube was caused not only by higher attractiveness of the water reservoir, but to a considerable degree also by constructional - technical activities in the immediate surroundings of the Danube river that are in the above section most extensive. With a view to the continued building activities and changes in the Danube environment it is very difficult to make a prognosis of the development and adaptations of the water-fowl in this section of the observed territory.
Besides the higher occurrence and nesting of rare species e.g. Larus argentatus [8] and Recurvirostra avosetta [1], in further periods gradual adaptation to new conditions is expected also with other species, e.g. Aythia fuligula and Aythia ferina, which before the construction of the reservoir occurred in the given area only as winter migrants.