Cormorant Numbers and Distribution

The Great Cormorant of the subspecies Phalacrocorax carbo sinensisis is now found throughout Europe outside the breeding season, and it has returned as a breeding bird to almost all European countries. Since at least the early 1900s up to the 1970s the European distribution of this subspecies was restricted to a few breeding colonies. The other subspecies P. c. carbo which mainly bred on rocky coasts in Norway and the United Kingdom was not persecuted to the same extent. This page briefly describes planned and previous monitoring of cormorants in Europe. It also gives a few examples of what we know about cormorant numbers and their distribution in Europe.

Deliverables by the ‘CorMan’ project

The current EU CorMan project will organise new pan-European counts of cormorant colonies in 2012 and of wintering cormorants in January 2013. The counts will to a large extent be conducted by volunteers. The organisation is carried out in collaboration with the IUCN/Wetlands International Cormorant Research Group. Major results from these counts will become available on this Platform and give updated information on the distribution of cormorants in Europe in summer and winter.

The CorMan project will also collate published results from the various national monitoring schemes of breeding and wintering cormorants. Some of this material will become available through this Platform, partly as links.

Monitoring of the breeding population

The development of the European population of cormorants is well known, partly because the colonies can be found without great difficulties and partly because the nests in the breeding colonies are fairly easy to count. Recorders in some countries have counted nests annually in all or almost all their breeding colonies for more than 40 years or since the species returned as a breeder in their country. However, the breeding colonies have not been counted annually in all countries in Europe, so a very precise and complete picture of the development of the breeding population does not exist for the whole of Europe.

Besides the independent national monitoring, a coordinated count of colonies took place in 2006. This Pan-European count was organised by the IUCN/Wetlands International Cormorant Research Group. Some of the results from this count are presented here.

Monitoring outside the breeding season

In a number of countries cormorants are counted regularly at specific sites outside the breeding season. More organised counts covering a large number of sites are carried out in winter in several countries. There are two types of winter counts of cormorants:

The general mid-winter water bird counts. These counts have been conducted for many decades, usually on the second weekend in January, where observers count all the species of waterbirds they can see on relevant water bodies, rivers and lakes. These counts in daytime deliver good information on trends within a certain observed region.
Special cormorant counts at sleeping roosts (night roosts). In their wintering regions the cormorants spend the night in communal sleeping roosts. When counts are conducted simultaneously at all roosts in a given area, it is possible to derive reliable estimates of all the birds present in the region. In some regions it is, however, difficult to ensure that all sleeping roosts are covered. In the well-covered countries, roost counts are delivering very reliable estimates of trends and numbers.

In January 2003, simultaneous counts of sleeping roosts were organised by the IUCN/Wetlands InternationaI Cormorant Research Group in (almost) all countries of Europe. Results from this count can be found here and in van Eerden et al. (2011), Proceedings 7th International Conference on Cormorants, Wetlands International Cormorant Research Group available at this site).

Both counting methods deliver very useful insights into the regional distribution and numbers of wintering birds in the surveyed areas. The winter counts do not give an absolutely complete measure of the total size of the European cormorant population, but they provide a good general estimate of the numbers involved and highly valuable information about how cormorants are distributed throughout Europe in winter.

Overall development in Europe

The continental subspecies sinensis was exposed to persecution for more than a century, and by the early 1960s, total breeding numbers were as low as 3,500-4,300 pairs in the main breeding areas being the Netherlands, Germany, Denmark, Sweden and Poland. Protection increased, to begin with in the Netherlands, and thereafter numbers began to increase. In these five countries, constituting the core breeding area, population growth rates were on average 11% per year during the 1970s and 18% per year during the 1980s. By 1995 breeding numbers had reached 95,000 pairs in these five countries.

Although breeding numbers stabilised in some of the core breeding areas already in the early 1990s, the sinensis population continued to extend its range into central Europe and along the Baltic Sea coast thereby returning to breeding areas from where it had been extinct for a century or more.

A comparison of the results from the 2006 pan-European count of breeding colonies with a compilation made by BirdLife International showed that between 2000-2002 and 2006 breeding numbers of sinensis in Europe (excluding Russia, Ukraine and a few other countries) had increased from c. 169,000 breeding pairs to c. 219,400 pairs. This increase of c. 50,000 pairs over a five-year period was mainly due to an increase by 43,000 pairs in the Baltic Sea region (mainly in Sweden and Poland). Total breeding numbers in Central Europe and the Central and Eastern Mediterranean had not increased over this five-year period.

Breeding numbers of sinensis have also increased markedly in the areas around the Black Sea and Azov Sea in Ukraine and Russia. It has been estimated that about 100,000 pairs of cormorants were breeding here around 2006. Breeding cormorants were apparently moving more and more into inland rivers and wetlands.

The population of the carbo subspecies has gradually increased in some periods in Iceland and Norway, but apparently not along the coast of the United Kingdom. In Norway numbers increased from 27,000 nests in 1995 and 25,000 nests in 2000 to 30,000 nests in 2006. Numbers in Iceland increased from 2,350 nests in 1995 to 4,500 nests in 2007. Breeding numbers of carbo have also increased in Greenland.

Reasons for Population Increase

There are thought to be a number of reasons behind these large population increases. The reasons might be somewhat different among countries. However, there is broad consensus about some key influences. Firstly, since the 1970s the birds have been subject to improved legal conservation measures throughout much or all of their range, mainly during the 1970s and 1980s. This has enhanced the protection afforded to cormorants and cormorant breeding sites and introduced more stringent controls of disturbance and the use of lethal measures.

The previous use of DDT and its metabolites also hampered breeding success in some of the colonies, at least in the Netherlands but probably also in other parts of Europe until the 1970s.

There is broad consensus that the very fast increase in cormorant numbers in part was possibly because the birds had easy access to habitats with high prey availability. Colonies in shallow coastal areas could grow large because of access near to the colonies of high amounts of fish of the right size. It is thought that some huge and very fertile man-made water bodies together with the eutrophication of coastal areas contributed to the initial population rise, and that additionally also feeding opportunities in (partly artificial) inland waters had improved due to man-induced changes.

The large increases in the numbers of cormorants across Europe over the past 30-40 years is mirrored by equally dramatic rises in cormorant populations in other parts of the world - for example, in the Double-crested Cormorant (P. auritus) in North America. These changes in numbers of cormorants have been explained by factors similar to those believed to have affected the sinensis population in Europe.