They are medium to large in size, with strong feet and bills, rictal bristles, and a single moult each year (most passerines moult twice). Corvids are found worldwide except for the tip of South America and the polar ice caps. The majority of the species are found in tropical South and Central America and in southern Asia, with fewer than 10 species each in Africa and Australasia. The genus "Corvus" has re-entered Australia in relatively recent geological prehistory, with five species and one subspecies there. Several species of raven have reached oceanic islands, and some of these species are now highly threatened with extinction or have already become extinct.