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The maximum height of the island is 44m, and there is the remains of a cairn at the highest point.
The island has been used for summer grazing of sheep. A field wall and a sheepfold between Reeli Geo and Loch of Colsay and next to a rectangular wall structure do still exist.
At the southwest tip of the island is 'Black Skerry' and at the north end, the 'Grey Stacks'. North of Reeli Geo, right in the centre of the north western headland there is the Ox Eye (formerly known on OS maps as the Bulls Eye), which is a blowhole linked by a subterranian passage to a geo to the west. The Ox Eye is understood to have been a source of top quality local millstones.
Local folklore tells that the island was once inhabited, by a woman and her son, but there is no known proof of this. Nevertheless, next to the Ox Eye there are two rectangular buildings (c. 12.5m by 3.3m and c. 15.8m by 3.3m respectively), suggested by F T Wainwright to be the remains of Norse long houses, but by J R C Hamilton to be medieval 'seaters'. According to RCAHMS the latter view is supported by nearby 'old' (but undated !) field walls.
Described in 1882:
Colsay, a small island of Dunrossness parish, Shetland, 8 miles NNW of Sumburgh Head. It used to pasture a good many sheep of a large English breed, but it is now devoted to the grazing of cattle.
From A Description of Dunrossness by Mr James Kay Parish Minster thereof. circa 1680: "...toward N.W. is a little Isle of Holme called Colza, not a mile in circumference. This Isle is not inhabited nor manured, but excellent pasture for Sheep & Neat: Ye sheep here are extraordinary fat".
During their research Venables & Venables recorded the following species of birds breeding on Colsay 1949-53.
Regular and common: Storm Petrel; Fulmar; Starling; Shag; Eider; Oyster Catcher; Common Gull; Herring Gull; Common Tern; Lesser Black-backed Gull; Arctic Tern; Razor Bill; Black Guillemot; Puffin; Rock Dove; Hooded Crow; Wren; Wheatear; Pock Pipit and Twite.
Irregular or rare: Mallard; Greater Black-backed Gull; Skylark and Raven.