Geologically, the Sperrins are formed mostly from Precambrian metamorphic rocks, with some younger Ordovician igneous rocks in the southern flank of the range. The Mountains are very sparsely populated and provide habitat for a diverse range of birds and mammals. Red fox, Sika Deer, Pine Marten and Red Squirrels are commonly found there alongside Peregrine Falcons, Buzzards and Sparrowhawks. Visiting Golden Eagles from the neighbouring reintroduced Donegal population have been sighted across the range from Strabane to Draperstown although no breeding population has yet been established. The Mountains also provide one of the last refuges of Red Grouse in Northern Ireland.