European shag

The European shag, also called the common shag (Gulosus aristotelis), is a type of cormorant and the sole species in its genus, Gulosus. It nests along rocky shorelines in western and southern Europe, southwestern Asia, and northern Africa. Most birds stay within this breeding area during winter, except those from the most northerly regions. In Britain, it is simply known as the shag. The genus name comes from the Latin word for glutton, while the species name honors the Greek philosopher Aristotle.

This bird was once placed in the genus Phalacrocorax, but a 2014 study showed it is more distantly related to the group containing Phalacrocorax and Urile, and sits at the base of the clade with Nannopterum and Leucocarbo. As a result, it was moved to its own genus, Gulosus, a change adopted by the IOC in 2021. The split between Gulosus and the Nannopterum-Leucocarbo lineage is estimated to have occurred between 9.0 and 11.2 million years ago.

Three subspecies exist: G. a. aristotelis, found along Atlantic coasts of northwestern Europe; G. a. desmarestii, in southern Europe and southwestern Asia along the Mediterranean and Black Sea coasts; and G. a. riggenbachi, on the northwest African coast. They differ slightly in bill size and in the breast and leg color of young birds. Recent evidence suggests birds from the Atlantic coast of southwestern Europe may represent a separate, undescribed subspecies. The name "shag" is also used in the Southern Hemisphere for several other cormorant species.

The European shag is a medium-large black bird, 68 to 78 cm long with a wingspan of 95 to 110 cm. It has a longish tail and a yellow throat patch. Breeding adults develop a small crest. It differs from the great cormorant by its smaller size, lighter build, thinner bill, and, in breeding adults, a crest and metallic green sheen on the feathers. Juveniles have darker underparts. The shag’s tail has 12 feathers, while the great cormorant’s has 14. The green sheen sometimes gives it the alternative name green cormorant.

It feeds at sea and, unlike the great cormorant, is rarely seen inland. In winter, it stays along coasts with plenty of fish. It is one of the deepest divers among cormorants, recorded diving up to 61 meters deep using depth gauges. It mainly feeds on the sea bottom, preying on a variety of fish, most commonly sand eels. Shags may travel many kilometers from their roosts to feed. In UK coastal waters, typical dives last 20 to 45 seconds, with about 15 seconds of recovery between dives, consistent with aerobic diving. They leap out of the water before diving to gain extra momentum.

It breeds on coasts, nesting on rocky ledges, in crevices, or small caves. Nests are messy piles of rotting seaweed or twigs, held together by guano. The nesting season is long, starting in late February, though some nests begin as late as May or later. Three eggs are laid. Chicks hatch without down and depend entirely on their parents for warmth, often for two months before they can fly. Fledging occurs from early June to late August, occasionally into mid-October.

The shag is a pursuit-diving seabird that mostly feeds in benthic habitats. Because diet samples (regurgitated food or pellets) are easy to collect, and due to perceived competition with fisheries, its diet has been heavily studied. Evidence from the Isle of May, Scotland, between 1985 and 2014, shows that chick diet has diversified in response to ocean warming. Shags also eat fewer sand eels on windy days, likely because wind affects their flight. The year-round diet of adults at this colony has shifted from sand eel specialists to a more varied prey base over the past three decades.

During the breeding season (late April to mid-July), the European shag can be seen at locations including the Saltee Islands (Ireland), Farne Islands and Isles of Scilly (England), Isle of May, Deerness, and Fowlsheugh (Scotland), Runde (Norway), Iceland, Denmark, Faroe Islands, Galicia (northern Spain), and Dalmatia and Istria (Croatia). In April 2017, eight new shags were born in Monaco. The British population was 18,000 pairs in 2015. A large colony of 2,500 pairs exists in the Cíes Islands, Spain.