GREAT CORMORANTS Phalacrocorax carbo IN SLOVENIA

Milan Vogrin

In the 1980s numbers of Great Cormorants Phalacrocorax carbo have been increasing in Slovenia, particularly in inland areas (Sovinc 1994, own observations). In the same time large reservoirs were built in some rivers (particularly along the rivers Drava and Sava) and a lot of new fish ponds were constructed as well. Before 1980 the Great Cormorant was considered a rare vagrant on freshwater wetlands in Slovenia.

During the first few years in which Great Cormorants had become regular visitors of inland Slovenia (they do not breed in the country), the birds stayed on only in the winter months. They arrived in October or late September and stayed on until February or March. The last three or four years, however, birds have been seen throughout the year, though numbers in summer are still low and most records refer to immature birds. The peak month is November, when nowadays an estimated 1500 individuals visit our freshwater wetlands (first on rivers, reservoirs and fish ponds). The autumn migration peak occurs mid-November. According to the Atlas of Wintering Birds in Slovenia (Sovinc 1994), there are five main wintering areas for Great Cormorants in the country. Four of these are situated along rivers (Drava, Sava, Krka and Kolpa) and one is coastal. The greatest part of Slovenia's wintering Great Cormorant population is found in the northeast, an area with numerous fish ponds and reservoirs along the Drava river. Along the Slovenian part of this river (between Austria and Croatia) the wintering population is estimated at about 600 individuals. In this area the birds roost on reservoirs in the Drava river, especially in Ptuj and Ormoz. The birds here are feeding in reservoirs, in the river Drava and on neighbouring fish ponds, particularly at Race and Pozeg. When the fish ponds are frozen (usually from December through February) foraging birds are confined to the river and the reservoirs. At this time marked daily migrations occur up and down the river Drava, seemingly up to distances of about 60 km. In spring, the migration peak is in March, but far less pronounced than in autumn. The total inland wintering population will fluctuate between 1200 and 1800 birds in recent years. The coastal area holds a further 300 wintering Great Cormorants (Skornik 1987), although here too an increase in numbers has been recorded in recent years (Skornik 1995).

Field observations tentatively suggest that some of the Slovenian Great Cormorants might belong to the Atlantic race P. c. carbo (own observations). Nonetheless, definite ring recoveries have only revealed birds ringed in Poland (Sere in Sovinc 1994). Unidentified colour-ringed birds (white and black colour rings) have been seen on some occasions.

References

Sovinc A. 1994. Zimski ornitoloski Atlas Slovenije, DOPPS. Tehniska zalozba Slovenije, Ljubljana.

Skornik I. 1987. Pomembno ornitolosko obmocje Evrope - Secoveljske soline. Falco 2: 3-14.

Skornik I. 1995. Wintering of the Cormorant Phalacrocorax carbo on the Slovenian coast. Falco 9: 13-16. Koper. Slovene with English summary.

Milan Vogrin, Hotinjska cesta 108, 62312 Orehova vas, Slovenia