Under favorable conditions, a species' population grows rapidly until it encounters an ecological limitation. This could be a lack of habitat or food, disease, pollution or a sudden environmental change caused by human activity, writes bird ecologist Marko Mägi in the Linnuvaatleja blog.

A population increase can also be curbed by a predator for whom the new species — such as a seabird nesting in a large colony — becomes an easy target. For instance, the pelagic cormorant (Phalacrocorax pelagicus), which inhabits the Pacific coast, has started nesting in caves and under bridges to avoid predation by the bald eagle (Haliaeetus leucocephalus).

Birds nesting on islands, along with their eggs and chicks, are vulnerable to predation by cats, foxes, raccoon dogs or rats, often forcing them to seek new breeding grounds. In extreme cases, predation can be so severe that survivors abandon the colony entirely. A nest built in a tree is protected from four-legged predators, but not from winged ones.

On Norway's coastal cliffs, northern gannets (Morus bassanus) and black-legged kittiwakes (Rissa tridactyla) nest in locations where the threat of predation by the white-tailed eagle (Haliaeetus albicilla) is lower; in some cases, past colonies have been abandoned due to eagle attacks. In Finland and Denmark, white-tailed eagles target nesting common eiders (Somateria mollissima), and studies show that in Finland, eider populations have significantly declined in these areas. In Sweden and North America, white-tailed eagles and bald eagles have completely disrupted the breeding of Caspian terns (Hydroprogne caspia), as young birds are either eaten directly or their nests are raided by opportunistic gulls following eagle disturbances.

A recent review of the relationship between white-tailed eagles and cormorants in the Baltic Sea suggests that their feeding grounds overlap: both species hunt fish, but the white-tailed eagle also preys on waterbirds and frequently visits cormorant colonies. As a result, white-tailed eagles influence the lives of cormorants as well.

By the early 20th century, the white-tailed eagle had nearly disappeared from several Baltic Sea countries due to persecution, primarily poisoning. The species began recovering in the 1980s, with 670 breeding pairs recorded around the Baltic Sea in the 1990s and as many as 3,000 pairs by 2012. The continued increase in numbers is regarded as a conservation success story.

Eagle raids on colonies lead to displacement — abandonment and relocation. However, the relationship between eagles and cormorants is not always dictated by the food chain. In some places, the two species appear to coexist peacefully; in certain cases, cormorants have even nested in the same tree as eagles without being disturbed.

White-tailed eagle raids on cormorant colonies began in Germany and Sweden at the start of this century. Today, Swedish eagles target approximately 56 percent of cormorant colonies and some colonies have been observed being attacked simultaneously by four to six eagles hunting both adult birds and chicks.

In Finland's Turku archipelago, reports describe incidents where 11 to 18 eagles have attacked a single cormorant colony at the same time, with such raids becoming increasingly frequent. In Denmark, white-tailed eagles regularly visit cormorant colonies. Estonian data show that the more eagle nests exist within a 10-kilometer radius, the fewer cormorant colonies are found in that area.

The impact of white-tailed eagles on cormorants varies. There are instances where eagles visit colonies simply to rest without bothering the cormorants. Conversely, there are cases where eagles systematically raid one nest after another. While egg predation is relatively rare, eagles frequently target cormorant chicks — particularly those in the middle stage of development or close to fledging. Attacks on adult cormorants are uncommon.

White-tailed eagles are much more likely to raid ground nests. In one recorded case in Sweden, 31 eagles descended upon a single colony in a day, completely destroying all ground nests while leaving tree nests untouched. The following day, 10 to 15 eagles returned and attacked the tree nests, which contained approximately 350 chicks aged three to five weeks. While landing on a tree nest is difficult for an eagle, their flights between the trees panicked the chicks, causing them to jump out, where they were caught on the ground. This pattern repeated the next year. In another colony, adult cormorants abandoned their nests after a young white-tailed eagle visited the site for three consecutive days. However, some instances of nest and chick defense have also been observed.

When adult cormorants flee, gulls and crows take advantage of the situation. In Denmark, following an eagle disturbance, herring gulls (Larus argentatus) looted over 1,400 out of 1,500 nests in a colony. In Finland, cormorants in eagle-visited colonies have shown a significant reduction in clutch size. In Germany's Wallnau colony, where white-tailed eagles were frequently observed in 2012, cormorants produced an average of only 0.3 chicks per nest, compared to the usual two to four. The following year, the number rose slightly to 0.5 chicks per nest.

Over the years, more cormorants have shifted to nesting in trees, leading to the formation of exceptionally large colonies. Since 2010, the number of colonies with more than 500 pairs has increased sharply — coinciding with the rise in white-tailed eagle activity in these areas. In Sweden's Stockholm region, several colonies were abandoned within a short period due to frequent eagle visits. This suggests that cormorants may be clustering to avoid predation, as past colonies were smaller when eagle numbers were lower.

White-tailed eagles have also begun stealing fish from cormorants — a behavior that appears to be relatively new. When an adult cormorant returns to its colony after a fishing trip, an eagle will attack, prompting the cormorant to regurgitate its catch in fear, which the eagle then seizes.

The study's authors suggest that as white-tailed eagle populations continue to grow, cormorants will become an increasingly important food source. Even if direct predation is limited, the disturbance caused by eagle raids may lead to colony abandonment. Current data indicate that further increases in eagle numbers will restrict cormorant breeding opportunities, and in some parts of the Baltic Sea, they are already controlling cormorant populations.

The study was published in the journal Ardea.

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