Traditionally, numbers of wintering Great Cormorants in the Netherlands are derived from January counts for the International Waterbird Census (IWC) of Wetlands International. However, as the species has become widespread and also inhabits small waterbodies in areas less well covered during the IWC, we organised a national census of all known night-roosts in The Netherlands. These are expected to yield a better coverage of the wintering population as all birds usually gather at communal roosts from late afternoon onwards. In January 2003, coverage was medium, but at least all important sites were checked for Great Cormorants.
In January 2004, 215 out of 224 known roosts were counted, including all major sites (Tab. 1, Fig. 1). The number of birds counted was 25 745 in January 2004. Based on data from sites covered in both years, it was estimated that 16 400 individuals were present in January 2003. The core wintering areas are situated around Lake IJsselmeer (40% of numbers in 2004) and along the large rivers (mainly IJssel, Rhine, and Waal, 26%, see Fig. 1).
The largest roosts were all found around Lake IJsselmeer (maximum 3251 on an island near Den Oever). In the North See and Wadden Sea, which support important concentrations during late summer and autumn, hardly any roosting birds were observed, and birds attending roosts close to these regions were reported to originate from fresh waterbodies.
The number of Great Cormorants counted in January 2004 is the largest midwinter count ever for the Netherlands. Results from (daytime) IWC-counts in 2001-2003 ranged from 18 100 to 23 700 individuals. Both the nightroost counts and daytime observations constitute only a small proportion (about 5%) of the European wintering population. The large differences in numbers between 2003 and 2004 are attributed to weather conditions during the period prior to the census. As a result of cold spells in December 2002 and January 2003, many Great Cormorants were forced to move south, whereas the prevailing mild temperatures in 2003/04 provided excellent wintering opportunities in The Netherlands, with many birds returning early to the breeding colonies.
First breeding in 2003/04 was already reported from late December and there has been a tendency for earlier breeding in many colonies recently.
van Rijn S. & Nienhuis J.
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