Palearctic migrants arrive in the northern tropics south of the Sahara between August and October, as the local rainy season ends. Some species remain in the Sahel and Sudan zones throughout the overwintering period. Others seem unable to tolerate the increasing aridity as the dry season progresses. These remain in the Sahel and Sudan zones for 1–2 months and then fatten before performing a second migration further south, either to dry Guinea savannas in West Africa, or to rainy conditions in equatorial East Africa or southern Africa, where rains begin in November. Between March and May, as southern Africa enters its dry season and the rains begin in the northern tropics, these movements are reversed, but the passage is much more rapid than in autumn. Birds that overwinter too far south to reach the Palearctic in a single journey, put on enough fat to reach the southern edge of the Sahara, where they refatten finally for the Saharan or Arabian desert crossing. Palearctic migrants, like other birds, moult when it is most favorable to do so. Some species wintering in the dry northern tropics moult on the breeding grounds before migration, while others moult in winter quarters in September-December when these areas are still wet. Species wintering at equatorial latitudes tend to moult on their northern stopover sites or in their final winter quarters during the same period. Birds wintering in southern Africa moult in their final destination between November and April.