BREEDING ECOLOGY OF GREAT CORMORANT Phalacrocorax carbo sinensis IN THE SLOWINSKI NATIONAL PARK (NORTHWESTERN POLAND)

Zofia Pajkert & Wojciech Górski

The breeding ecology of the Great Cormorant Phalacrocorax carbo sinensis was studied in 1983-1992 in a ground nesting colony situated on a small treeless islet (Kamienna Island; 54° 40'N, 17° 05'E; 0.6 ha in size) on Lake Gardno in the Slowinski National Park. Ground nesting is exceptional in Great Cormorants in Poland (see Przybysz et al. 1985), so the breeding colony existing at the Kamienna Island provides a good opportunity for detailed studies on the breeding biology of this species here. Observations were carried out from the end of March or the beginning of April up until the beginning of August every year, on average once in every ten-day period.

Spring arrival of Great Cormorants at the breeding grounds occurs mainly in the second half of April, but in some years as early as early April (1992) or even late March (1987; Table 1). Arrival date depends on the ice conditions on the lake and on the water level, since in spring the islet was often flooded with water. Construction of the nests starts from the second half of April onwards or at the end of April (but in some years as late as in mid-May; 1984) and lasts until mid-June (Table 1).

Egg laying starts in May and lasts until the end of June (Table 1). Nestlings hatch from the end of May or the beginning of June until the end of June or even up until the beginning of July (1989; Table 1). Young Great Cormorants enter the water from mid-June until the beginning of August. Departure from the colony occurs from the end of June (1985 - an unsuccessful year) up until mid-August (1989; Table 1).

Great Cormorants settled on Kamienna Island in the early 1980s (Pajkert & Pajkert 1991). This settlement was probably a consequence of the increase in size of the greatest Polish colony at Katy Rybackie where in 1980 there were 608 and in 1985 as many as 1542 Great Cormorant nests (Przybysz et al. 1985, 1988). Between 1983 and 1992 a rapid numerical increase was also observed on Kamienna Island (Table 2), but this was unrelated to local reproductive success and due mainly to immigration of birds from outside the colony.

Clutch size ranged between one and five eggs (mean = 2.57, SD = 1.18, n = 247). Clutches of three and four made up 30.2 and 33.4% respectively of the total number of eggs. The colony was presumably inhabited by relatively young breeding birds (immature individuals predominated among the Great Cormorants present at Kamienna Island in spring), as the clutches of one, two and three eggs constituted almost 75% of the total number of clutches (see Table 3). On the other hand, clutches of one, two and three eggs may also be due to partial nest predation.

Overall breeding success during the ten-year period of investigation was relatively low: only 20.6% of the eggs produced fledged young (Table 4) and the mean number of young reared per nest with eggs was 0.53. The breeding losses were due both to heavy egg predation by Herring Gull Larus argentatus and damaging of eggs or chicks by fishermen or to flooding of the islet with water in some years.

Acknowledgements

The authors thank Professor Zdzislaw Bogucki for valuable comments on the first draft of this paper.

References

Pajkert Z. & Pajkert W. 1991. [Ground nesting of the Cormorant in the S_owi_ski National Park]. In: Górski W. (ed.) Breeding sites of wetland birds and their protection in the centre of the Pomeranian Region. S_upsk 1991: 151-158.

Przybysz A., Przybysz J. & Przybysz K. 1985. [The cormorant Phalacrocorax carbo L. in Poland in 1980 and 1981]. Prz. zool. 29: 199-213.

Przybysz J., Engel J., Mellin M., Mrugasiewicz A. & Przybysz K. 1988. [A quantitative increase of cormorant (Phalacrocorax carbo sinensis Shaw et Nodder) populations in Poland]. Prz. zool. 32: 71-81.

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