Cormorant

"He [Toothless] flew across the beach in his usual, untidy, lopsided fashion, shrieking a few insults along the way at any cormorants that looked smaller than him."

- How to Train Your Dragon

Cormorants are birds mentioned as residing on the Isle of Berk in How to Train Your Dragon.

Description
Cormorants, also sometimes called 'Shags', are medium to large sea birds belonging to the Family Phalacrocoracidae. Typically they have a dark coloration, but a few species are more brightly colored. Cormorants and primarily fish eaters, and dive into the water to catch them. Their beaks a long with a hook at the end to obtain their fishy goal.

Two species living in Scandinavia are the Great cormorant (Phalacrocorax carbo) and the European shag (Phalacrocorax aristotelis).

Function
Cormorants have no mentioned function in the Book series. It is possible historic peoples ate these shore birds and their eggs if the opportunity arose.

Some cultures - mostly from Eastern Asia - historically utilized trained cormorants to fish for them. In Norway the Great cormorant is a game bird, but also a sign of good luck if they appear near villages in the North of the country.

How to Train Your Dragon
Toothless harasses some cormorants on the Long Beach while going out to train to fish.