Loss of avian biodiversity on Baltic islands after establishment of cormorant colonies

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 negatively related to the establishment of cormorant breeding colonies but started to recover on islands after cormorants abandoned the colonies. This indicates considerable competition between great cormorant and other species for breeding habitat on these islands and stresses the impact that highly abundant colonial bird species can have on the biodiversity of small marine islands.