Sunday August 14, 2022

White-tailed Eagles affect Great Cormorants in Baltic Sea

Published : 04 Aug 2022, 02:30

  DF Report

Photo: Heikki Kotiranta/SYKE.

The growing White-tailed Eagle population affects both the location of the Great Cormorant’s nesting colonies and their breeding performance, according to a new study.

The White-tailed Eagle plays an increasingly important role in limiting the Great Cormorant population. Finland’s Great Cormorant population has not grown since 2015, said the Finnish Environment Institute (SYKE) in a press release on Wednesday referring to the study.

The abundance of the White-tailed Eagle, both in Finland and elsewhere in the Baltic Sea, is one of the great success stories of nature conservation.

In addition to the White-tailed Eagle, the Great Cormorant population also increased significantly in Finland from the turn of the millennium.

In recent years, the interaction between eagles and cormorants has become more common, and observations of eagles in the vicinity of colonies of Great Cormorants have increased in different parts of the Baltic Sea, especially since the 2010s.

The White-tailed Eagle can eat both Great Cormorant eggs and offspring, and also steal fish caught by cormorants. In addition, the eagles scare adult cormorants from their nests, exposing eggs and offspring to predatory gulls and crows.

“The effects of the White-tailed Eagle are already visible in the incubation phase of Great Cormorants. The cormorant colonies of the Gulf of Finland and the Archipelago Sea, where the White-tailed Eagles were seen, had fewer eggs than the colonies where the eagles were not seen,” said Senior Curator Aleksi Lehikoinen from the Finnish Museum of Natural History of the University of Helsinki.

In order to avoid predation of eggs and offspring, Great Cormorants often return to their nests if the White-tailed Eagle stays calm in the colony.

However, one colony can be visited by several eagles at the same time, as especially the non-breeding young eagles gather in flocks. Situations have been observed in both Finland and Germany where there are up to 30 White-tailed Eagles in a Great Cormorant colony at the same time.

“The predation and harassment caused by the White-tailed Eagle can sometimes be so severe that the colony does not produce any offspring in the summer. In just a few years, continuous predation has led to the end of a breeding colony of two thousand nests,” said Researcher Pekka Rusanen from the Finnish Environment Institute.

In a number of European countries, including Finland, Great Cormorant colonies have moved their nesting sites because of the excessive predation of eagles.

In Estonia, for example, the number of colonies is highest in the regions with the least White-tailed Eagle nests. In Finland, the Great Cormorant colonies in the outer archipelago have decreased dramatically and the nesting is increasingly concentrated in the inner archipelago.

Since the 2010s, the majority of the Great Cormorant’s breeding population in Finland has been concentrated in large colonies of more than 500 pairs.

In a large colony, some birds can succeed in their nesting despite the predation of eagles. Although the predation of the White-tailed Eagle has continued to increase in recent years, the eagles do not cause losses to colonies in all nesting sites.

The reduced offspring rate of the Great Cormorant affects the size of the breeding population by a delay of two to three years, as fewer Great Cormorants return to their native regions. The Finnish Great Cormorant population has not increased since 2015, and in recent years the population has shown a slight decline.