Numbers of Great Cormorants Phalacrocorax carbo Wintering in the Western Palaearctic in January 2013

Great Cormorants were censused on a pan-European level in January 2013. Cormorants were found in an enormous winter range, spanning from the Baltic Sea and Atlantic Ocean in the north to the Mediterranean and coasts of North Africa. This large-scale exercise in which more than 5000 volunteers took part resulted in a total of 641,650 Cormorants counted (630,000–672,000 estimated). Based on the breeding census data of 2012 we estimated a total number of birds in January 2013 of slightly over 1 million, including the birds from the Ukrainian and Russian parts of Black Sea, Sea of Azov and north-western Caspian. Using the summer counts to produce a corrected estimate for the area that was actually covered during the winter count gave an estimated 695,000 individuals for January 2013. Total coverage was considered good and comparison to the previously conducted winter count of 2003 revealed, corrected for coverage, a 15% increase. As both counts were carried out during a period of cold weather, it is unlikely that birds were missed due to movements to the east and north-east of the range, from or into areas that are difficult to assess. Some 33% of all Cormorants were found to occur in areas with a temperature of –5°C or lower, suggesting that many birds can survive under conditions that may be marginal, i.e. at a high risk and/or cost. Compared to the previous count a relatively lower number of Cormorants were found under low temperature conditions, –5 °C down to –10°C, coinciding with the moment of active ice formation of shallow and stagnant freshwaters. It may well be that the actual ice cover in 2013 was such that birds had to leave these areas. In contrast to the opinion that wintering under low temperatures is marginal, it is possible that these northerly wintering grounds are more rewarding in terms of food profitability. This is because cold-blooded fishes congregate at certain spots and are less mobile at low temperatures and thus relatively easy to catch. Given the current and previous work carried out we recommend a long-term monitoring of these processes, which operate at a huge geographic scale. A repeated pan-European count with intervals of about ten years could detect the major patterns, while also providing a useful method of monitoring changes due to the expected further warming of winter conditions.