AFRICAN MIGRANTS IN NIGERIA

Owing to the regular alternation of wet and dry seasons and to the relatively simple arrangement of vegetation zones, migration is a well-developed phenomenon in the northern tropics of Africa. Nigeria is well placed for its study. While a vast amount remains to be learned, a systematic treatment of the 672 Nigerian bird species shows that at least 126 of them are migrant. Species newly shown to be migratory include the sunbird Nectarinia pulchella, the bunting Fringillaria (Emberiza) tahapisi, and the sparrow Gymnoris (Petronia) dentata, and corroborative evidence is adduced for many more. New interpretations are placed on the movements of the bustard Neotis denhami, the Painted Snipe Rostratula benghalensis, and other species. Most families that are migrant in the temperate zone (e.g. storks, cuckoos, nightjars, hirundines) have several migrants among their tropical representatives, except flycatchers, thrushes, warblers and shrikes. Important tropical migrant families include the kingfishers and sunbirds. The following ecological correlates emerge: migration appears to be more important at lower than higher trophic levels in the ecosystem; only one primary lowland rain forest and one montane forest species migrate; 95% of African migrants in Nigeria inhabit the five savanna zones, where the 120 migrants comprise 28% of the avifauna; most savanna migrants cross one, two or three vegetation zone boundaries; few are restricted within a single zone or cross four boundaries; twice as many savanna species are eurytopic (habitat-tolerant) as stenotopic (habitat-tied), and twice the proportion of eurytopic as stenotopic species are migrant; the majority of migrants move so as to avoid the winter drought in northern Nigeria (insectivores being little affected) and also the heaviest summer rains in southern savannas; migration is initiated by climatic factors such as rains, dry desert winds, etc. Three to six Nigerian species cross the Equator, but the stork Anastomus lamelligerus is shown to breed in Nigeria and the Chad basin and may not be a trans-equatorial migrant. A quail, a crake, a coucal and five cuckoos have space-time distribution patterns (motograms) in Nigeria that suggest equatorial migration. Motograms figured for 60 species show great variation. Nearly all land-bird migrants move with the progression of the sun and the inter-tropical front, north in spring and south in autumn. The basic pattern is thus a summer wet-season range at higher latitude than the winter dry-season range, and specific patterns vary according to the absolute and relative latitudinal limits af each season, and to speed and dates of migration. Comparison of congeneric migrants having different breeding seasons or motogram patterns suggests that a specific migration pattern is fixed by ecological necessity, and reproduction occurs at that time in the year when the species is stationary longest. Breeding seasons may span migration periods, leading to two-stage early-rains migration in e.g. the kingfisher Halcyon leucocephala. Water-bird migrations are complicated by opportunist breeding and responses to floods and changing water levels in rivers. A few species move with the typical land-bird pattern; others concentrate at perennial marshes in arid northern savannas, and may disperse in all directions from the few west African breeding stations.