Abstract Breeding colonies of the continental subspecies of the great cormorant (Phalacrocorax carbo sinensis) 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 1960–2022. We found that Shannon’s diversity index values for breeding birds were significantly