According to the study's lead author, Aare Verliin, chief fisheries specialist at the University of Tartu, all species on Earth are closely tied to their surrounding environment, relying on it for essential resources. 

"For birds, the most critical factors for survival and reproduction are sufficient food supply and safe nesting sites. Therefore, it is not surprising that competition for these limited resources can be very intense," he explained.

Ecologists have so far focused heavily on food competition and how food webs function. Much less attention has been paid to competition between species for habitat use, writes Lauri Saks, associate professor of ichthyology and fisheries at the University of Tartu.

Verliin notes that it is particularly interesting to examine competition among bird species inhabiting small marine islands.

"The number of cormorants in the Baltic Sea has continued to grow rapidly in recent decades. Since they are fish-eating birds, considerable attention has been given to their impact on fish populations. However, far less is known about the broader environmental effects of expanding cormorant colonies," he said.

Researchers from the University of Tartu's Estonian Marine Institute, the Estonian University of Life Sciences, and Rail Baltic Estonia analyzed breeding bird monitoring data collected on small Estonian islands. Professional conservation officers, scientists, and amateur ornithologists gathered data from 261 islands along the Estonian coast between 1960 and 2022.

Cormorants nested on 71 of these islands during the study period. The long-term dataset made it possible to assess how island bird communities changed after cormorants arrived and to compare colonized islands with those not yet occupied.

Intense competition on the ground and in the treetops

The analysis confirmed that the arrival of cormorants quickly reduced the diversity of local breeding birds. The effect varied in intensity but was observed in ten bird species. On islands inhabited by cormorants, the numbers of ground-nesting birds declined, including mallards, velvet scoters, mergansers, eiders, herring and common gulls, terns, and white wagtails. At the same time, species nesting in reed beds, such as the mute swan and the sedge warbler, increased.

The results show that cormorants reduce biodiversity mainly through competition for space. Because they nest in dense colonies, they occupy large portions of an island, taking over areas suitable for other birds. They do not directly eliminate other birds by predation — rather, neighboring species are forced to find new nesting sites.

Ducks, gulls, and terns are typical small-island specialists that inhabit open, grassy, or stony areas — precisely the habitats used by ground-nesting cormorants.

Although some cormorants nest in trees, most vegetation beneath their nests disappears during the breeding season. As a result, adjacent areas become unsuitable for species that previously nested in grass or shrubs, such as eiders, red-breasted mergansers, and other ducks.

As long as suitable islands remain available

The impact of spatial competition is further supported by the finding that on the few islands where cormorants stopped nesting, bird diversity began to increase again. The "real estate" freed up by cormorants was quickly occupied by other species.

However, the data show that recovery of breeding bird communities takes time — just as it takes years for a forest to recover after logging.

The regional impact of this competition depends on how the cormorant population develops and how much nesting area they occupy. Cormorants themselves are not "bad," but their presence reduces island bird diversity: other birds are forced to leave when they arrive. When cormorants are few, displaced birds can move to neighboring islands. But if their numbers grow across the archipelago, other species may have nowhere left to go.

During the study period, cormorants nested on 16% of the surveyed islands. Until 2022, other birds were still able to find new nesting sites on islands not yet colonized by cormorants. However, if new colonies continue to emerge, such refuges will eventually run out. This could lead to an overall decline in the affected species. Figuratively speaking, the situation resembles a conflict in which some groups are forced to migrate from uninhabitable areas to new locations.

The study confirms that the rapid growth of cormorant populations has environmental impacts extending beyond changes in aquatic food webs. The restructuring of breeding bird communities due to lack of space may also trigger chain reactions, affecting islands to which displaced birds relocate.

As birds move to new islands, competition within and between species for nesting sites intensifies, meaning these cascading effects will require further research in the future.

The article, "Loss of avian biodiversity on Baltic islands after establishment of cormorant colonies," was published in the journal Proceedings of the Royal Society B: Biological Sciences.

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