A major difference between migrants and residents is in frequency of movements. Because they are not tied to nesting areas, the Palaearctic migrants are free to move around, exploiting temporary local abundances of food in a way that most residents cannot. The same is true for intra-African migrants in their non-breeding season. Many migratory species thus pursue an itinerant lifestyle, periodically moving in relation to seasonal changes in food supplies or even in relation to local flushes of food, such as those produced by patchy rainstorms (Sinclair 1978, Liversidge 1989, Leisler 1992, Herremans 1998). In consequence, such migrants may occupy only part of their potential non-breeding range at any one time, avoiding certain areas altogether in some years. In contrast, many resident African species remain on territories all year and, south of the equator, most African species, both residents and intra-African migrants, are still breeding at the time the Palaearctic migrants are present (Moreau 1972). This limits the effectiveness of the African birds in exploiting transient food supplies, and hence their ability to compete for them with the Palaearctic migrants. In affect, the Palaearctic migrants tend to use temporary habitats which cannot be fully exploited year-round by local species: seasonally flooded grasslands, drying pools, recently burnt savannahs, nearly bare fields, and plantations and secondary growth in the forest zone (Thiollay 1989).
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