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Across 2022–2023, 61.1% of Monitored Great Cormorant Eggs on Giresun Island Produced Fledged Young

Great cormorant with wings spread on a rock by the sea in Giresun, Türkiye
Research · Reproduction & development

A two-year study on Giresun Island in the eastern Black Sea found that 61.1% of the Great Cormorant (Phalacrocorax carbo) eggs in monitored nests produced young that reached fledging. Researchers counted nests across the island in 2022 and 2023 and followed accessible nests through laying, hatching and chick development.

In 2022, 322 Great Cormorant nests were recorded on the island and 258 were monitored. In 2023, 431 were recorded and 267 monitored. Across the 525 monitored nests, 1,116 eggs were laid; 816 hatched and 682 young reached fledging, normally at about 45–50 days of age. Hatching success was therefore 73.1%. Of the chicks that hatched, 83.6% survived to fledging, leaving an overall egg-to-fledgling survival rate of 61.1%.

Great Cormorant breeding performance was slightly higher in 2023 than in 2022. Egg-to-fledgling survival rose from 59.4% to 62.7% and hatching success from 71.4% to 74.8%, while survival from hatching to fledging was almost unchanged at 83.3% and 83.8%.

The study also followed 75 European Shag (Gulosus aristotelis) nests and 455 Yellow-legged Gull (Larus michahellis) nests. Egg-to-fledgling survival was 37.6% in European Shags and 19.6% in Yellow-legged Gulls, compared with 61.1% in Great Cormorants. Yellow-legged Gulls had particularly low hatching success, at 25.6%, but 76.3% of chicks that did hatch survived to fledging. Their main loss therefore occurred before or around hatching rather than among chicks that had already hatched.

The figures describe the monitored nests during the 2022 and 2023 breeding seasons, not a long-term average for the entire island colony. Within those two seasons, Great Cormorants had the highest egg-to-fledgling survival of the three seabird species monitored on Giresun Island.

Sources and image credit

  1. Arslan, Y. C., & Arpacık, A. (2026). Breeding success of Great Cormorant (Phalacrocorax carbo), European Shag (Gulosus aristotelis) and Yellow-Legged Gull (Larus michahellis) on Giresun Island, Türkiye. Acta Zoologica Bulgarica, 78(2), 195–207. https://doi.org/10.71424/azb78.2.003000Primary peer-reviewed article published online 30 May 2026. In 2022, 322 Great Cormorant nests were recorded on Giresun Island and 258 were monitored; in 2023, 431 were recorded and 267 monitored. Across 525 monitored nests, 1,116 eggs produced 816 hatchlings and 682 fledged young, corresponding to 73.1% hatching success, 83.6% chick-to-fledgling survival and 61.1% egg-to-fledgling survival.

Image: Zeynel Cebeci / Wikimedia Commons — Great Cormorant photographed at Espiye in Giresun Province, Turkey, 10 September 2016; representative regional image, not from the study · CC BY-SA 4.0 · Source

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