Numbers, migration and origin of Cormorants Phalacrocorax carbo at a wintering site on the Island of Helgoland (German Bight, North Sea)

The numbers of Cormorants resting on the island of Helgoland (54 degrees 11'N, 7 degrees 53'E) have increased considerably since the 1970s. Annual totals of the maxima per five-day period rose by 47% per year from 1984 to 1989. Although the observation effort decreased, annual maxima further increased to 661 birds in 1996 (Fig. 1). Up to the late seventies, annual maxima of resting birds were occurred during the migration periods, whereas since about 1980 Helgoland is increasingly used as a wintering site. Resting numbers grew mainly in September/October, reached their maxima from December to March and fell again from the end of March to May (Fig. 2). Today arrival takes place more than 100 days earlier and departure 43 days earlier than in 1979 (Fig. 3). Migrating Cormorants were mainly observed in March and April (median: 12. April) and from the end of July to October (median: 16. September, Fig. 4). They prefer eastern and north-eastern directions in spring, and western and southwestern directions in autumn (Fig. 5). 67% of the individuals were observed to migrate in flocks of 11 to 40 individuals (median 27, Fig. 6), and 69% of migrating individuals were counted before midday (median: 10:05 hours, Fig. 7). Six ringed birds were found or observed at Helgoland. They were ringed in The Netherlands (1), Denmark (4) and Norway (1). According to their body measurements, 19 out of a total of 36 Cormorants found dead at Helgoland are assumed to belong to the subspecies carbo, whereas only 9 are presumably P. c. sinensis. Hence, we believe that most birds wintering at Helgoland belong to P. c. carbo. This subspecies winters less far south than sinensis and prefers marine habitats and rocky shores. Winter maxima of sinensis are only known from wintering sites further south but not from the North Sea and the Wadden Sea. where maxima occur during spring and autumn migration. Breeding numbers of P. c. sinensis have increased considerably in Denmark and south Sweden since 1980/81, whereas those P. c. carbo numbers have increased at a lesser rate. Coinciding with the increase of wintering sinensis in the Skagerrak and Kattegat area (a formerly typical wintering site of carbo), there were at least 9 recoveries of birds ringed in Norway in central and south Europe from 1983-85 and 6 in northeastern Germany from 1976-59. Probably, the establishment and growth of the winter population at Helgoland is a result of increasing competition in the Skagerrak and Kattegat. Similar developments between carbo and sinensis have been observed in northwest France.