Aarhus University Seal / Aarhus Universitets segl

No. 187: Denmark's breeding population of cormorants in 2020

Sterup, J. & Bregnballe, T. 2020. Danmarks ynglebestand af skarver i 2020. Aarhus Universitet, DCE – Nationalt Center for Miljø og Energi, 40 s. - Teknisk rapport nr. 187. http://dce2.au.dk/pub/TR187.pdf

Summary

This report presents the results of the annual count of all apparently occupied Great Cormorant nests in Denmark in 2020. Furthermore, it describes the extent of management measures in the colonies.

In 2020, a total of 31,964 nests were registered in Denmark. This is an increase of 4.8 % compared to 2019. Nevertheless, the population has been rather stable since 2014 with between 30,500 and 33,200 breeding pairs and annual fluctuations below 5 %. The current population size is around 20 % lower than during 1996-2005, when the population peaked at around 40,000 pairs.

The largest regional population change was registered in southwestern Kattegat, where the number of nests increased by 1,000 or 20 %. In the other regions, nest numbers were relatively stable with changes in nest number of up to 350.

In total, 90 breeding colonies were found in Denmark in 2020. This is 15 more than in 2019 and the largest number of colonies ever registered in Denmark. The largest colony in 2020 held 2,540 nests and was located in Stavns Fjord on the island of Samsø. Another eight colonies had more than 1,000 nests.

The large increase in the number of colonies may be explained by deteriorating breeding conditions in existing colonies due to management measures, predation/disturbance by White-tailed Eagles or other natural causes. The growing population of White-tailed Eagles in Denmark seems to have an increasing impact on Cormorant colonies. In some colonies the presence of eagles cause a decline in breeding success and a delay in the timing of onset of breeding.

The Danish Nature Agency, the Ministry of the Environment, implemented management measures to reduce breeding success in eleven colonies in 2020 and gave permission to private landowners to undertake management in another nine colonies. In 2020, a total of 3,695 nests were exposed to management, mainly by preventing the eggs from hatching by spraying them with vegetable oil or by removing the nests. Furthermore, a total of 291 Cormorants were shot around five colonies. The proportion of nests exposed to management measures was approximately as in 2016-2019, but higher than in 2010-2015.