Of birds very few are indigenous; the jackdaw, blue solitary thrush, spectacled warbler, the robin, kestrel and the herring-gull.
They are rich in sea-fowl, the most common being the eider duck, puffin, Manx shearwater, black guillemot, kittiwake and herring gull.
The largest species known is the Stercorarius catarrhactes of ornithologists - the "Skooi" or "Bonxie" of the Shetlanders, a bird in size equalling a herring-gull, Larus argentatus.
It was Herring Gull size but pale and very blotchy, with pale panels mid-wing.
The best-known case is herring gull versus lesser black-backed gull.