Analysis for Effects of Cormoran Abundance to Avian Breeding Communities

Breeding colonies of the Phalacrocorax carbo sinensis (continental subspecies of the Great Cormorant) have rapidly expanded in Europe during the past half-century. To assess the impact of cormorant breeding colonies on bird biodiversity, we used annual monitoring datasets of breeding bird communities from 261 islands in the northeastern Baltic Sea along the Estonian coast, covering the period of 1960-2022. We found that Shannon's diversity index values for birds were significantly negatively related to the establishment of cormorant breeding colonies but started to recover on islands after cormorants abandoned the colonies. This indicates some competition between Great Cormorant and other species for breeding habitat on these islets