Marcin Przymencki, Klaudia Litwiniak, Magdalena Zagalska-Neubauer, Grzegorz Neubauer
Ornis Polonica 2021, 62: 169–188
https://doi.org/10.12657/ornis.2021.3.1
Abstract: The Caspian Gull Larus cachinnans is a large gull species, which expands its breeding area in Europe and increases in numbers. The first Polish breeding records of the species come from the early 1980s, and its current population is estimated at 3000–3500 breeding pairs. Despite its widespread occurrence on the continent, the species reproductive ecology has not been well described, and many aspects of its biology still remain unknown. In 2019–2020 we studied the breeding ecology of the Caspian Gull at Mietków Reservoir (Lower Silesia). We recorded a minimum of 260 breeding pairs in 2019 and at least 190 pairs in 2020. Birds started nesting in the first decade of April. Dimensions of nests (medians) were as follows: outer diameter – 47 cm, inner diameter – 23 cm, height – 7 cm, depth – 7 cm. Mean volume of the first eggs, called A-eggs was 84.4 cm3, B-eggs – 82.3 cm3 and C-eggs – 76.6 cm3. Length, width and volume of eggs tended to decrease with the increasing laying date. The full clutch volume was 241.2 cm3 and 245.2 cm3 in 2019 and 2020, respectively. Relative volumes of C-egg in clutches constituted 0.91 and 0.93 of the mean volume of A- and B-eggs in the two study years, respectively. In both study years 15.5–15.9% of eggs from 25.6–32.1% of clutches were lost, and 2.43–2.48 chicks per a breeding pair hatched. In 2019 the daily chick survival rate was estimated at ~0.95, while the probability of the nestling survival until the age of 22 days after hatching was ~0.36 (before the flooding of the colony due to heavy rainfall). The second (after 17 May) flooding of the colony resulted in the death of more than 90% of nestlings. In 2020 the survival rate of the young was lower and more variable than the year before, and the probability of survival until the 17th day of life was only ~0.23. We compare our data with the results obtained in other colonies in Poland and Ukraine.
Keywords: abundance, breeding ecology, Caspian Gull, Larus cachinnans, Mietków Reservoir
