© Copyright: Images: Jari Peltomäki, Mervi Wahlroos, Jouko Lehmuskallio, Jari Hiltunen, M., W. & F. von Wright: Svenska fåglar (Kansalliskirjasto, The National Library of Finland), Pohjolan linnut värikuvin. Recording: Jan-Erik Bruun. All rights reserved.
- Name also: Great Black Cormorant, Black Shag
- Family: Cormorants – Phalacrocoracidae
- Appearance: Dark, long-necked and long-tailed seabirds belonging to the Suliformes order. Young birds have white colouring on their underparts to varying extents.
- Size: Length 77–94 cm, wingspan 121–149 cm, weight 2.2–3.6 kg.
- Nest: Birds of the nominate species breed in Finland and elsewhere in colonies on craggy rocky islands. Birds of the more southerly subspecies sinensis nest in colonies in trees. Nests are mainly made of twigs.
- Breeding: 3–4 eggs laid in April–May, incubated by both parents for 27–31 days. Fledglings learn to fly within about 50 days.
- Distribution: First bred in Finland only as recently as 1996, but numbers have risen steeply since then. The species’ Finnish breeding population was estimated at almost 13,000 pairs in 2008, 17,700 in 2011 and 24,000 in 2015. The three largest colonies included more than one third of all Finnish Great Cormorant nests
- Migration: May be seen in Finland at any time of year, but most birds migrate from August onwards to spend the winter in Western Europe, returning from early March. Birds breeding in arctic regions may be seen on migration in Finland, especially around the Gulf of Bothnia.
- Diet: Various kinds of fish, especially bottom-feeding fish, also crustaceans and other benthic animals. Cormorants can dive to depths of up to 10 metres. Along the research, made near Hanko (in south-western sea-side) in 2011, ca. 80% of cormorants’ diet consisted of viviparous blennies, roaches, perches and three-spined sticklebacks.
- Calls: A harsh rattling “arrr”.
Adult Great Cormorants gain their breeding plumage during a partial moult in February–March. Their plumage is mainly bronze-sheened black, with a white featherless throat and cheek patch, and a white thigh patch. Adults then moult fully between July and November, taking on their less metallic winter plumage, with brown cap and cheeks, black and white colouring on the white parts of their heads, and no white thigh patches. Birds of the southern subspecies sinensis have much more white on their heads and necks.
Great Cormorants have black legs, yellowish brown bills, and bluish green irises. Juveniles are more brownish with extensive white colouring on their underparts. Their plumage only gains its metallic sheen when they reach the age of 2.5–3 years. Juveniles have no white thigh patches.
Great Cormorants fly in a similar way to geese, and often in flocks with similar formations. But they occasionally interrupt their wing-beats to rise like a helicopter and to glide briefly. In the water, however, they may more easily be mistaken for divers.
The similar and closely related Shag (Phalacrocorax aristotelis), which very occasionally visits Finland, is smaller and more slightly built than the Great Cormorant, and has a thinner neck and a more rounded head. In their breeding plumage Shags have a distinct crest, a greenish sheen to their dark plumage, and no white colouring around the base of the beak. Their wing-beats are more rapid than those of Great Cormorants, and they do not glide in flight. Shags have only been recorded in Finland very rarely (5 confirmed sightings by 2006).