A PRELIMINARY ASSESSMENT OF SOME FACTORS INFLUENCING THE DENSITY AND DISTRIBUTION OF PALEARCTIC PASSERINE MIGRANTS WINTERING IN THE SAHEL ZONE OF WEST-AFRICA

Population densities of Palearctic migrant and African birds were esti mated from 444 point counts, that estimated minimum density, at 10 sit es in the Sahel zone of northern Nigeria during December-January 1993/ 94. In total, 15 Palearctic species were recorded during point counts averaging 5 species per site, and 76 African species averaging 24 spec ies per site. The densities of Palearctic migrants varied from 1 bird ha(-1) in semi-desert and highly degraded woodland, tip to 8 birds ha- l in dense Sahelian woodland. Common Whitethroats Sylvia communis were most abundant (up to 0.7 birds ha(-1)) in sites rich in Piliostigma r eticulata trees; Subalpine Warblers S. cantillans were most abundant ( tip to 5.9 birds ha(-1)) in sites rich in Acacia spp. and Cassia siebe riana; Lesser Whitethroats S. curruca and Redstarts Phoenicurus phoeni curus were associated with high overall tree densities (both up to 0.6 birds ha(-1)); Northern Wheatears Oenanthe oenanthe occurred at densi ties of about 1 bird ha(-1) at very low tree density falling to about 0.25 birds ha(-1) over a wide range of intermediate tree densities. Fa rmland supported densities of Palearctic migrants up to 6 birds ha(-1) where large trees or hedgerows remained. The densities of Palearctic Sylviidae were positively correlated with the combined densities of al l ecologically similar African species of small insectivorous and/or f rugivorous passerines: Palearctic warblers were half as common as thei r African counterparts.