Arctic terns are a little smaller than common terns - another difference is that their bills are entirely red (no black tip as the common terns have) and their tails streamers are longer - The rump is white, the underparts are darker and the wing lacks the dark wedge on the outer edge

The males and the females have similar plumage and both have a black mask. The Arctic terns feet are webbed. The legs are red and very short and the birds have a white head with a red pointed beak.

The bottom of their wings are pale grey or ivory. The young arctic terns have pale red coloured legs - they have full plumage in their third year

In Scotland the arctic terns breed along the coast or indeed on the open moorlands and heaths.They are vulnerable to predators such as rats and mink so this means that they frequently breed on offshore islands areas where there are no mammal predators. In winter the arctic tern stays out at sea, resting on floating objects and ice

The arctic terns eat mainly fish and small marine invertebrates. Terns eat by swooping down into the ocean to catch prey eating small fish or crustaceans, although they will catch insects in flight for a small portion of their diet.

"tr-tee-ar" - A harsh 'kee-airr' and a scolding 'kit-it-it-kaar

These terns nest once every one to three years - being dependant upon their mating cycle. The female lays up to three cream coloured - brown speckled eggs and can have two clutches a year. The nests are usually in a grassy area with a few stones between bare rocks.

When they finish nesting they fly for another long southern migration. These birds mate for life and return to the same colony each year. Here in the Western Isles - just this last couple of years there has been quite a colony at Gress near Tosta - Isle of Lewis.

• Courtship starts with the female chasing the male and then descending slowly. The male then offers fish to the female and lots of strutting with tails raised and then the birds fly and circle each other
  
  The arctic terns can be really aggressive in defending the nest and their young - will even attach humans. The chicks are quite quick to be moving around the nest site within a few days - although not straying far. THe adults brood the chicks for ten days after hatching.
  
  The chicks eat the prey brought by the adult terns for about a month before fledging and learning the art of plunge diving
  
  

Arctic terns are known for having the longest migration of any bird. Some arctic terns travel from the Arctic to the Antarctic and back again in one year. Some migrate over 20,000 miles a year

Arctic Terns live quite a long time up to thirty years of age.

A group of terns are collectively known as a "ternery" or a "U" of terns.