Distribution and Numbers of Arboreal Birds between the Hyper-Arid Sahara and the Hyper-Humid Guinea Forests

This study quantifies the spatial variation in the density of 50 arboreal bird species (17 insectivorous Afro-Palearctic migrants and 33 Afro-tropical residents: 20 insectivores, 7 frugivores and 6 sunbirds) in the transition zone between the arid Sahara and the humid Guinea zone, measured during the dry season in 2011–2019. The distribution of bird species was related to annual rainfall, with Afro-Palearctic migrants found mainly in the relatively dry zone and Afro-tropical residents in the more humid zone. As woody cover increases with rainfall, bird species from the dry zone are by default found in more open habitats than species from the humid zone. This effect of woody cover largely dissolves when corrected for rainfall. The data – pertaining to absolute bird counts in stratified random sites – were used to estimate the total number of birds in this region. To assess the reliability of these estimations, population sizes were calculated separately per species on half-split data. The two estimates deviated about 10% from the averages calculated for the full data set. Among arboreal birds (1322 million), insectivorous residents were most abundant (547 million), followed by insectivorous migrants (326 million), nectarivorous residents (272 million) and frugivorous residents (177 million). The two most numerous arboreal bird species were insectivorous residents: Tawny-flanked Prinia Prinia subflava (128 million) and Green-backed Camaroptera Camaroptera brachyura (103 million). Among the migrants, the three most abundant were Subalpine Warbler Curruca iberiae + subalpina + cantillans (62 million), Lesser Whitethroat Curruca curruca (48 million) and Western Bonelli's Warbler Phylloscopus bonelli (30 million).