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Shags are close relatives of cormorants, found on the sea coasts of many regions around the world. In fact, cormorants and shags are in the same biological genus, Phalacrocorax, and the differences between cormorants and shags are not hard and fast. Some species are referred to by either name, depending on where they are being discussed, and the birds are obviously closely related genetically, given that they are in the same genus.
Because the difference between shags and cormorants is somewhat nebulous and both names are sometimes applied to one bird, identifying shags can be frustrating. One rule of thumb is that shags produce distinctive shaggy crests on their heads during breeding season, while cormorants do not. The breeding season for shags is also longer than that of cormorants. However, some birds which grow crests are called cormorants anyway, and some “shags” don't produce crests; this confusion is probably due to variable reports from early explorers who would have seen the birds at differing life stages and given them common names which became too well-used to be discarded.
Shags are marine birds, and unlike many birds which are considered cormorants, they live exclusively by the seashore, rarely traveling inland. Many shags nest on exposed sea stacks and rocky outcroppings, building large and messy nests of seaweed and scrap materials which are bound together with guano. Visiting a community of shags can be quite an assault on the nose, as the birds produce notoriously stinky guano thanks to their fish-heavy diets.
As a general rule, birds which are considered to be shags are smaller than cormorants, with narrower beaks and leaner bodies. They are also better divers than cormorants, and they may be able to dive deeper than any other bird, according to studies conducted by ornithologists. Shags are what are known as benthic feeders, meaning that they collect food from the sea floor, so it is important for them to be able to make deep dives. Many shags also have patches of bright color on their necks and wings.
Shags are especially common in Europe, especially in Britain, where one species, the European or Common Shag, as quite abundant. This species is also sometimes referred to as the Green Cormorant, due to the green tinge which its feathers acquire during breeding season. Shags are also found in New Zealand, where several species including the Bronze Island Shag and the King Shag are endangered due to habitat destruction and a limited range.