Skip to content

Bird Diversity Fell After Cormorant Colonies Became Established on Baltic Islands

Great cormorant standing on nesting material in Estonia
Research · Ecology

Long-term monitoring of small islands in the northeastern Baltic Sea found that breeding-bird diversity declined after colonies of great cormorants (Phalacrocorax carbo sinensis) became established. The study analysed annual breeding-bird data from 261 islands along the Estonian coast from 1960 to 2022, allowing the same island communities to be followed before, during and, in some cases, after cormorant occupation.

The researchers used Shannon’s diversity index, which combines species richness with how evenly individuals are distributed among species. A lower value therefore means that an island has fewer breeding species, a community more strongly dominated by a few species, or both.

Diversity declined after cormorant colonies became established

Breeding-bird diversity was negatively associated with the establishment of cormorant colonies. The strongest negative species responses included European herring gull (Larus argentatus), common gull (Larus canus) and velvet scoter (Melanitta fusca), while mute swan (Cygnus olor) showed the clearest positive response.

The authors interpret the community-level pattern as evidence of strong competition for limited breeding habitat on small marine islands. Dense cormorant colonies can also transform terrestrial vegetation through nesting, trampling and nutrient inputs, changing the structure of the habitat available to other birds. The study therefore points to both direct competition for space and colony-driven habitat change as mechanisms that can reshape island bird communities.

Diversity began to recover after colonies were abandoned

The change was not permanent on every island. On islands where cormorants later abandoned breeding colonies, Shannon diversity began to recover towards its earlier level. This before–during–after pattern strengthens the link between colony presence and the bird-community change, although individual species did not all respond in the same direction.

The authors conclude that highly abundant colonial birds can become major ecological modifiers on small islands. The unusually long monitoring series — 261 islands over more than six decades — allowed the study to detect community changes that would be difficult to distinguish from ordinary island-to-island variation in a short survey.

Sources and image credit

  1. Verliin, A., Meitern, R., Rasmann, E., Vetemaa, M., Eschbaum, R., Albert, A., Taal, I., Linnas, T.-T., & Saks, L. (2026). Loss of avian biodiversity on Baltic islands after establishment of cormorant colonies. Proceedings of the Royal Society B: Biological Sciences, 293(2071), 20253138. https://doi.org/10.1098/rspb.2025.3138Primary open-access peer-reviewed study published online 27 May 2026. Annual breeding-bird monitoring from 261 islands along the Estonian Baltic coast covering 1960–2022 showed a significant negative association between great cormorant colony establishment and Shannon diversity. Diversity began to recover after cormorants abandoned colonies.

Image: Kristian Pikner / Wikimedia Commons — Great Cormorant photographed on nesting material in Estonia, 8 July 2017; representative Estonian species image, not a bird from the monitored island dataset · CC BY-SA 4.0 · Source

About this content: This story was produced with AI assistance within an editorial workflow developed by Wildlife Vagabond. Editorial responsibility remains with Wildlife Vagabond.How AI is used

Ecology

SPECIES IN THIS STORY

Species in this story

Great Cormorant Phalacrocorax carbo Explore species European Herring Gull Larus argentatus Explore species Common Gull Larus canus Explore species Mute Swan Cygnus olor Explore species

Independent research and conservation news archive

Did you find this information useful?

Wildlife Vagabond is independently built and maintained. Voluntary support helps cover source verification, hosting and continued work on research and conservation news.

Support Wildlife Vagabond

The news archive will remain freely available.