Long-term changes in winter distribution of Danish-ringed Great Cormorants

TY - JOUR

T1 - Long-term changes in winter distribution of Danish-ringed Great Cormorants

AU - Bregnballe, Thomas

AU - Herrmann, Christof

AU - Pedersen, Kjeld Tommy

AU - Wendt, Juliane

AU - Kralj, Jelena

AU - Frederiksen, Morten

PY - 2021/12

Y1 - 2021/12

N2 - We describe long-term changes in the distribution of 2123 freshly dead winter recoveries of 86,427 cormorant chicks ringed between 1940 and 2015 in Denmark. The entire wintering range was divided into four major compartments to look for changes in a) migratory distances within three compartments, and b) the distribution of recoveries among all four compartments. Distances to winter recovery sites declined by more than 500 km (around one third) over ten years in the south-eastern wintering compartment. A gradual decline of similar magnitude occurred in the southern wintering compartment during 1976/77 to 2015/16. There were no changes in migration distance among cormorants wintering in the south-western compartment over the time period. From 1991 onwards, recoveries were recorded in increasing proportions in the south-western compartment (from 17% in 1986/87-1990/91 to 71% in 2011/12-2015/16). The proportion recovered in the southern compartment varied between 40 and 45% up to the beginning of the 1990s, and then fell to 4% during the most recent period. The proportions recovered in the south-eastern compartment ranged from 9 to 18% until 1990/91 but fell subsequently to 0.6-4%. Long-term changes in the geographical origin of cormorants recovered in Croatia confirmed the suspicion that declines in numbers of recoveries of Danish-ringed cormorants in the south-eastern wintering area reflected a true westward shift in winter distribution. The composition of recoveries in Croatia revealed that the south-eastern wintering areas were increasingly ‘taken over’ by cormorants from breeding colonies in the eastern Baltic region. We conclude that Danish cormorants shifted their winter distribution north- and westward from the 1990s onwards, and argue that westward shifts represent a response to increased ‘competition’ with birds from breeding colonies located further east in the Baltic Sea where populations increased markedly from the 1990s onwards.

AB - We describe long-term changes in the distribution of 2123 freshly dead winter recoveries of 86,427 cormorant chicks ringed between 1940 and 2015 in Denmark. The entire wintering range was divided into four major compartments to look for changes in a) migratory distances within three compartments, and b) the distribution of recoveries among all four compartments. Distances to winter recovery sites declined by more than 500 km (around one third) over ten years in the south-eastern wintering compartment. A gradual decline of similar magnitude occurred in the southern wintering compartment during 1976/77 to 2015/16. There were no changes in migration distance among cormorants wintering in the south-western compartment over the time period. From 1991 onwards, recoveries were recorded in increasing proportions in the south-western compartment (from 17% in 1986/87-1990/91 to 71% in 2011/12-2015/16). The proportion recovered in the southern compartment varied between 40 and 45% up to the beginning of the 1990s, and then fell to 4% during the most recent period. The proportions recovered in the south-eastern compartment ranged from 9 to 18% until 1990/91 but fell subsequently to 0.6-4%. Long-term changes in the geographical origin of cormorants recovered in Croatia confirmed the suspicion that declines in numbers of recoveries of Danish-ringed cormorants in the south-eastern wintering area reflected a true westward shift in winter distribution. The composition of recoveries in Croatia revealed that the south-eastern wintering areas were increasingly ‘taken over’ by cormorants from breeding colonies in the eastern Baltic region. We conclude that Danish cormorants shifted their winter distribution north- and westward from the 1990s onwards, and argue that westward shifts represent a response to increased ‘competition’ with birds from breeding colonies located further east in the Baltic Sea where populations increased markedly from the 1990s onwards.

KW - skarv

KW - træk

KW - udbredelse

KW - skarv

KW - træk

U2 - 10.5253/arde.v109i2.a6

DO - 10.5253/arde.v109i2.a6

M3 - Journal article

SN - 0373-2266

VL - 109

SP - 327

EP - 340

JO - Ardea

JF - Ardea

IS - 3

ER -