Long-Term Changes in Winter Distribution of Danish-Ringed Great Cormorants

We describe long-term changes in the distribution of 2249 freshly dead winter recoveries of 94,352 Great Cormorant chicks ringed between 1940 and 2018 in Denmark. The entire wintering range was divided into four major compartments to assess changes in (1) migratory distance and (2) the spatial distribution of recoveries. In the south-eastern wintering compartment, the mean distance to winter recovery sites declined from the winters 1946/47–2000/01 to those of 2001/02–2018/19 by 528 km (corresponding to a reduction of 36%). In the southern-central wintering compartment the change was gradual from before the mid-1980s to the winters 2006/07–2018/19 with a reduction of c. 700 km (corresponding to 41%). There were no temporal changes in migration distance for Cormorants wintering in the south-west. From 1991 onwards, recoveries were recorded in increasing proportions in the south-western compartment (from 21% in 1946/47–1990/91 to 60% in 2001/02–2018/19). The proportion recovered in the southern-central compartment varied between 34 and 45% up to the mid-1990s and then fell to 4–6% during the winters 2006/07–2018/19. The proportions recovered in the south-eastern compartment ranged from 9 to 18% until 1990/91 but fell subsequently to 0.6 to 2%. Long-term changes in the geographical origin of Cormorants recovered in Croatia further confirm that declines in numbers of recoveries of Danish-ringed Cormorants in the south-eastern wintering area reflect a true westward shift in winter distribution. The composition of recoveries in Croatia revealed that the south-eastern wintering areas were increasingly dominated by Cormorants from breeding colonies in the central and eastern Baltic region. We conclude that Danish Cormorants shifted their winter distribution westward from the 1990s onwards and shortened their migration by wintering further north. We hypothesise that this westward shift represents 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.