INTERACTIONS BETWEEN GREAT CORMORANT Phalacrocorax carbo sinensis AND HERRING GULL Larus argentatus argentatus IN THEIR COMMON BREEDING SITES

Wojciech Górski & Zofia Pajkert

Two types of biological interactions were observed between Great Cormorant Phalacrocorax carbo sinensis and Herring Gull Larus argentatus argentatus at their common breeding colonies in northwestern Poland (Fig. 1). Competition for places to nest was ascertained on the small, treeless Kamienna Island (54° 40'N, 17° 05'E) on the coastal Lake Gardno in the S_owi_ski National Park. Herring Gulls occupied this site at the beginning of the 1970s as a breeding colony of 24 pairs was recorded in 1972, but already 162 nests were found in 1978 (Bednorz 1983). The gullery further increased to 320 nests of Herring Gulls in 1985 and then a significant decrease in numbers was observed (Pajkert 1992). The Great Cormorant settled on the island in the beginning of the 1980s (Pajkert & Pajkert 1991) and its numbers increased from year to year (Fig. 1). The Great Cormorant was more successful in the competition for places to nest and gulls were pushed to the edges of the breeding colony (Górski et al. 1990). The increase in numbers of cormorants and their nests was followed by a significant decrease in numbers of Herring Gulls' nests (Figs. 2, 3). As a possible consequence of this competition the foundation of a new breeding colony of Herring Gulls was recorded in the neighbouring Ustka harbour, where since the mid-1980s Herring Gulls started to breed on roofs of buildings and 162 such nests were found in 1989 (Zió_kowski 1992).

Fig. 1. Locations of the common breeding colonies of Great Cormorant and Herring Gull in the Pomeranian region (NW Poland).

Fig. 2. Relationship between the number of Great Cormorant nests and the number of Herring Gull nests on Kamienna Island (1983-1992).	Fig 3. Relationship between the number of Great Cormorants present in May and the number of Herring Gull nests on Kamienna Island (1983-1992).

On the other hand, on the afforested islands on the inland lakes Lake Somi_skie (54° 02'N, 17° 48'E) and Lake Wielimie (53° N, 17° 45'E) in the Pomeranian Region Herring Gulls nest in breeding colonies of Great Cormorants. The gulls build their nests under trees with cormorants' nests and partly feed on the remnants of their prey.

Lake Somi_skie became inhabited by the Great Cormorant in 1988 (33 nests located in trees on a small islet) and in the same year three breeding pairs of Herring Gull joined the colony and built their nests under the trees with the cormorants' nests. In 1989 there were 16 nests of Hering Gulls on the islet (Górski et al. 1990).

On Lake Wielimie Great Cormorants started to breed in 1983, when 30 pairs settled on the small islet (Karwacki 1991). The breeding colony increased to 250 nests in 1991-1992. Herring Gulls have been accompanying the cormorants since 1989 when the first nest was found. In 1992 there were eight breeding pairs while in 1995 13 nests of Herring Gulls were recorded.

It was concluded that both interactions (competition for places to nest on a treeless islet and commensalism on the afforested islands) depend on the spatial structure of the environment at the common breeding localities of the species under study.

Acknowledgements The authors thank Professor Zdis_aw Bogucki for useful comments on the first draft of this paper.

References

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Wojciech Górski & Zofia Pajkert, Department of Zoology and Genetics, Pedagogical University, 76 - 200 S_upsk, Poland