Not everything is black and white. Abundance and head colour of great cormorants Phalacrocorax c. carbo and P. c. sinensis on Heligoland, Germany
The island of Heligoland is the only place in Germany where great cormorants of the carbo and sinensis subspecies are regularly present in significant numbers. The last estimates of their abundance date back to the winters of 1996/1997 and 2000/2001, when subspecies were identified using head colour. We counted great cormorants in breeding plumage over a thirty day period in February and March 2019 and recorded their head colour. While the proportion of black-headed birds had decreased only slightly compared to 1996 (5%), the total number of wintering carbo had decreased by 58 % due to a general decrease of great cormorants on the island. To estimate the error rate of identification using head colour, we photographed 75 great cormorants in breeding plumage, measured their gular pouch angle (GPA) and noted whether they were white- or black-headed. Our data indicate that head colour is not diagnostic for identifying subspecies in single great cormorants, but can be used to infer proportions in large groups. Therefore we believe that the decline of carbo on Heligoland in winter is real. Possible causes are a decline of carbo populations, or an increase of the wintering population of sinensis in Central Europe, especially in Norway.