## MONITORING OF THE BIRDS IN THE DANUBE FLOODPLAIN

####
Alzbeta DAROLOVA

Institute of Zoology, Slovak Academy of Sciences, Dubravska cesta 9, 84206 Bratislava, SLOVAKIA

CONCLUSIONS In those parts of the Danube floodplain, where the changes of the hydrological regime have not a visible impact on the vegetation structure, the ornithocoenoses have not changed considerably up to present. A decrease in species diversity was registered in those localities, which were directly influenced by human activities related with the construction and operation of the Gabcikovo barrage system, or in those localities, whose immediate surrounding was influenced by such activities. Deeper changes in the structure of bird communities in the Danube floodplain may be expected as early after a longer period, when the vegetation structure will have been changed more conspicuously. Limitation of breeding and occurrence of birds, especially of larger and shyer species, in the floodplain was caused by a larger degree of disturbance by people in consequence of opening almost whole within-dike area by cutting off the Danube branches from its main stream, construction of dams cross the branches, construction of forest roads for timber hauling etc.

New-arisen water tables, especially the Cunovo reservoir, by-pass canal, tailrace canal, seepage canals and various depressions arisen during the Gabcikovo barrage system construction and filled with water have a significance for waterfowl during passage and wintering, but in no case for their breeding.

#### INTRODUCTION

The biomonitoring of impacts of the Gabcikovo barrage system (further GBS) on birds includes a wide scale of problems, because interference resulting from the GBS construction and operation has been very differentiated in individual parts of the affected area. In this area, some new artificial water habitats have arisen (Hrusovska zdrz reservoir, by-pass canal, seepage canals, depressions temporarily filled with water etc.), but many natural habitats habe been destroyed and those, which have been preserved are influenced by GBS. My earlier investigations concerning the bird communities in the Danube floodplain carried out before the start of GBS operation make possible to evaluate the present state of birds fauna in this area more precisely.

The data concerning the bird communities in individual monitoring plots in the forest habitats were obtained by recording of birds on one hectar square plots. The one hour recording was carried out once a month all the year round. In some months, mostly due to the floods, the recording were not done.

The results used in this contribution were obtained in the period 1991-1994 in the following localities: Dobrohost, Sulany, Kralovska luka, Istragov, Trstena na Ostrove and also from some other places in the Danube within-dike area. For a more detailed evaluation I have used the data obtained in Dobrohost and Sulany, because these localities were of a sufficient extent and their structure was rather close to the structure of the natural soft-wood floodplain forests. So they represented the most suitable object to monitore the response of the rich bird communities on the impact of construction and operation of GBS.

#### LOCALITY SPECIFICATION

#### Locality Dobrohost

The vegetation cover (Salici-Populetum) in the monitored plot near Dobrohost was heterogeneous. The tree etage was formed by willows and poplars, but the shrub layer consisting of the species Cornus sanguinea, Salix spp., Viburnum opulus and Padus racemosa was developed only on a part of this plot. The wooded parts alternated with open places with a rich herb layer and several solitaire willows. A depressi on in the study plot was frequently flooded before the start of GBS operation. Since that time its bottom dried out and more xerophilous plants appeared there.

The immediate surrounding of this study plot was influenced by construction of a canal which supplies the remnants of the branch system by water from the diversion canal. In the connexion with it, a field in the immediate neighbourhood of the study plot was liquidated. During the canal construction the area was considerably disturbed by work of mechanisms and people. A part of the tree stand surrounding the study plot was cut out.

#### Locality Sulany

The study plot near Sulany was represented a relatively very well preserved, almost natural remainder of an old willow-poplar-alder stand growing on sides of a Danube branch. When comparing with the previous locality, the stand was closer and in average older. Shrub and herb layer had asimilar composition as in Dobrohost and both were developed more homogenously over whole area. During the four years of monitoring, the direct human interference were unsignificant.

#### THE OBSERVED CHANGES

Significant changes in ornithocoenoses structure in the Danube floodplain may be expected after deep changes of vegetation structure. The character of vegetation is influenced by changes of hydrological regime after the start of GBS operation, but more expressive changes in its structure can appeare only after a longer period. The speed of these changes is reduced by supply of a part of branch system by water. The tempo of changes in vegetation differs in individual parts of the Danube floodplain in dependence from degree of artificial supplying of the within-dike area by water. Further the direct influence of the changed hydrological regime is increased by impact of various human activities related with the construction of GBS, roads, harvesting the timber and opening the whole within-dike area for public. The Danube branches were gradually cutted off from the main stream and some of them, as a result of decrease of water level in the main stream, do not represent any serious obstacle for people at present, which was earlier to be overcome by boats mostly. The road for lories along the Danube old main river bed as well as on the whole within-dike area were built up and the dams across the branches were gradually elevated. So, at present there does not exist any hardly accessible place here. This results in an enormous disturbance by people, makes impossible breeding of some larger shyer birds (e. g. Haliaeetus albicilla, Ardea cinerea, Milvus migrans, Pernis apivorus, Nycticorax nycticorax, Ciconia nigra and others) and limits considerably their occurrence.

On those localities, which have not been influenced strongly by the human activity (e. g. clear cutting of forest, building of roads) and in which the ground water level has not decreased so drastically as immediately along the former main river bed, where the mainly the willous have gradualy died, I have not observed some significant changes in structure of ornithocoenoses. The monitoring plot near Sulany is one of such places. The number of observed bird species did not changed there significantly during four years of the monitoring and there is no visible trend of changes at present (Table 1). On the contrary, in the study plot near Dobrohost, where the immediate surrounding of the study plot has been changed, a gradual decrease of total number of observed species was registered (from 52 species in 1991 to 43 species in 1994) as well as a decrease of the species occurring in that locality during breeding season (from 38 species in May-July 1991 to 25 species in 1994). Especially in the last year of the monitoring, some migratory species (e. g. Ficedula albicollis, Phylloscopus sibilatrix, Jynx torquilla, Lanius collurio, Acrocephalus palustris, Sylvia nisoria), among which S. nisoria has been included in the Red Book, were not observed in 1994 in contrast to 1993. In 1994, there were not found also some species occurring by us over whole year like Dendrocopus minor, Dryocopus martius, Garrulus glandarius and Corvus corone which did not nest there, but which were observed there sometimes earlier. In 1994, on the contrast to 1994, only two species occurred there, which were not recorded earlier (Phoenicurus ochruros, Anas platyrhynchos). Although the most significant signal of habitat changes is a change of the breeders species occurrence, also absence of some migratory species indicates a change of suitability degree of a habitat as a temporary place of their occurrence. Although the bird communities are relatively dynamic and partial changes in them may be caused by population fluctuations of some species in some periods, a decreasing trend of number of species occurring in the locality Dobrohost is obvious during last four years of observations. The long term observations are necessary on these localities in order to be able to estimate and reliably interpret the future succession trends.

As to the abundance of individual bird species occurring in the studied localities, there has not been observed an unambiguous tendency of changes. Abundance oscillations occuring out of the breeding period are not so much significant, because the community structure in that period may be influenced by the accidental presence of migrating birds and/or, in winter, by wandering polyspecies flocks. In the breeding period itself, no significant changes of abundance has been observed.

During the biomonitoring or my other investigations in the Danube floodplain, I studied the birds communities also in some poplar monocultures. A comparison of the ornithocoenoses from them and form the relatively natural floodplain forests near Sulany and Dobrohost show, that the bird communities in the poplar monocultures are distinctly poorer in number of species as well as of individuals. This fact is illustrated in Table 2. In poplar monocultures, the total number of occurring species, the number of species registered in the breeding period and the average number od species registered during one observation is distinctly lower. This indicates, that the impoverishment of bird communities in the Danube floodplain has been caused also by forest management system. The seminatural floodplain forests were subsituted by poplar monocultures, in which almost no ill or hollow trees occur. Therefore the cavity nesting birds (mostly those of the genera Parus and Ficedula) have a very reduced chance to breed there. In addition, also shrubs having a great importance as a shelter, nesting place or a variable food resource are sometimes artificially removed from the forest stands. If destruction of the existing seminatural habitas by various silvicultural measures is not stopped, there will remain no refugia, which could give the birds a chance to survive in stands with a relatively more suitable structure and more favourable hydrological conditions. The poplar monocultures prevailing in the within-dike zone are not able to play a role of such refugia.

A further complex of problems connected with impact of GBS on birds is represented by necessity to create new artificial water habitas for birds, especially in Cunovo reservoir, by-pass canal, tail-race canal, seepage canals and various depressions arisen during the GBS constructions filled with water.

It is a fact that large numbers of some species of waterfowl (above all the species of family Anatidae and Lariidae) concentrate in the Cunovo reservoir and in diversion canal from autumn to spring. However, they occur there almost only during passage and wintering. A major part of them are only the winter guests comming from the northern areas. In the past, these birds were disperged in the system of the discharging branches, which offered them much more suitable food and cover conditions. For those species, which nest in our geographical conditions, the flowing water does come into consideration a nesting a habitat. Neither the banks of the canals offer them a chance for nesting (the banks of by-pass canal are asphalted, banks of the tailarace are covered by quarry stone, the banks of seepage canals are regularly mown up to the shore line).

Cunovo reservoir has a little specific character. The conditions for waterfowl are variable in dependence on water level. Great number of waterfowl concentrate there in winter, but similarly as the by-pass canal, this water body is used by waterfowl as a concentration place during passage and wintering. Eventually, in the breeding period, it can be visited by the birds nesting on other places and comming here occasionaly when searching for food. In spring, however the reservoir functionates as a trap for those birds, which try to built nests on small islands. Just at the time of sitting on eggs in May and June, the water level increases due to snow thawing in Alps and regularly floods the majority of nests. In this way, the most affected species are Larus ridibundus, Sterna hirundo, Charadrius dubius, Riparia riparia and some pairs of very rare species Recurvirosra avosetta, Himantopus himantopus, Sterna albifrons and Larus cachinans. The only solution, how to make possible a succesfull breeding of the birds nesting of shores of shallow waters, is instalation of small flowing islands covered by gravel. There are very good experiences with them in abroad. Some other waterfowl, which sometimes may be observed on the reservoir in large number (e. g. Phalacrocorax carbo, Ardea cinerea, Egreta alba), need an undisturbed or a little disturbed forest habitat. The reservoir water table represents for them only a place for hunting or resting. In addition, these species do not nest at all in our territory in the whole stretch from the village Sap near Gabcikovo to Bratislava. Also many other birds with a close relation to water habitats nest on the trees and require an undisturbed nest surrounding as possible. Very few places satysfying the ecological requirements of the birds persist in the Danube floodplain and number of the nesting shy birds decreases continuously. From this reason, many species do not nest here at all (Haliaeetus albicilla, Nycticorax nyctycorax, Egretta alba) or number of their breeding pairs decreases (Ciconia nigra, Milvus migrans).

Along the whole Danube, an international winter census of waterfowl was carried out every year in mid-January. Number of observed birds fluctuated considerably in individuals years. In 1991 we counted 26340 individuals, in 1992 37716 individuals, in 1993 740451 ind., in 1994 10966 ind. and in 1995 51629 ind. For the moment, it can not be sed unambiguously, that the construction of GBS would have an influence on number of wintering waterfowl. Irrespectively of it, it is known that after rise of a new water table, the number of birds occuring on it increases in the first three years of its existence, than it decreases and stabilizes on a lower value around which it oscilates. It is to be empasized further, that before the start of the GBS operation, the water flowed through the branch system more intensively and therefore it froze later and in less extent. The waterlfowl could be disperged over a wider area of the branch system. From this reason it was more difficult to count them. At present, less birds remain in the branch system in winter because its biotical quality has decreased considerably - the numerous dams opened the branch system to poachers, anglers and turist who disturbe or endanger the birds, the slower flowing water freze earlier and in larger extent. So the waterfowl is forced to concentrate on the reservoir and diversion canal, where a layman, who has never seen many birds at a time, may belive that there are very many birds.

As for the birds with a close relation to water habitats and nesting in reed stands, these continue to nest in them if the reed is flooded by water. The floods of the extensive reed stands on the Istragov were stoped after start of GBS operation. It resulted in change of representation of bird species nesting here. Acrocephalus arundinaceus does not occur there as a breeding species. Also the number of other species of reed buntings and warblers decreased. No extensive reed stands, which could compensate this empoverishment have been formed in the wihtin-dike area. Some new reed stands have arisen only in the Cunovo reservoir. However, they have an other character (they grow in permanently flowing water) and they are not suitable for breeding of Acrocephalus arundinaceus due to changes of water level. A similarly unfavourable conditions as in Istragov are for the reed stand in Dobrohost.

Table 1. Number of bird species registered on two monitored localities in 1991-1994.

Locality | Years | MONTHS | T | B
--- | --- | --- | --- | ---
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12
užany | 1991 | 13 | 9 | 11 | 21 | 19 | 23 | 20 | 18 | 18 | 15 | 9 | 13 | 47 | 33
| 1992 | 9 | 13 | 8 | 20 | - | 13 | 14 | 17 | 21 | 9 | 5 | 16 | 39 | 20
| 1993 | 6 | 11 | 6 | 20 | 17 | 20 | 14 | 21 | 17 | 18 | 11 | 13 | 43 | 28
| 1994 | - | 10 | 14 | 21 | 20 | 27 | 22 | 20 | 17 | 11 | 16 | 9 | 45 | 35
Dobroho | 1991 | 7 | 6 | 5 | 19 | 22 | 24 | 22 | 22 | 13 | 12 | 6 | 7 | 52 | 38
| 1992 | 7 | 7 | 9 | 15 | 23 | 22 | 13 | 18 | 22 | 14 | 7 | 13 | 48 | 30
| 1993 | 10 | 6 | 5 | 14 | 15 | 19 | 22 | 15 | 17 | 14 | 14 | 14 | 45 | 35
| 1994 | - | 10 | 12 | 18 | 17 | 17 | 12 | 14 | 17 | 12 | 7 | 10 | 43 | 25

Symbols:
T - total of registered species
B - number of species registered only during breeding period from May to July

Table 2 Number of species and individuals of birds in several localities with different vegetation cover.

Locality and its character | A | B | C
--- | --- | --- | ---
užany, soft-wood floodplain forest | 51 | 33 | 47.0
Dobroho, soft-wood floodplain forest with clearings | 49 | 38 | 39.4
Bodíky, predominantly a willow stand | 44 | 32 | 45.0
Istragov, willow stand | 34 | 27 | 32.3
Trstená n. O., young poplar monoculture | 28 | 13 | 11.4
Krážovská lúka, poplar monoculture with shrub layer | 28 | 16 | 17.5
Krážovská lúka, poplar monoculture without shrub layer | 25 | 10 | 6.1

Symbols:
A - total number of species,
B - number of species in breeding period,
C - average number of individuals in one record.