Changes in the area of optimal tree cover of a declining Afro‐Palaearctic migrant across the species’ wintering range

Abstract

Land use change in sub‐Saharan Africa continues apace, but its role in driving the declines of Afro‐Palaearctic migrant birds is unknown. This is due partly to a lack of knowledge of migrants’ requirements on the wintering grounds, and of spatially explicit assessments of land cover change. We compared tree cover data derived from satellite remote sensing (available for the period 2000–2014) with distributional data from surveys in four West African countries for the Wood Warbler Phylloscopus sibilatrix, one such declining migrant, to determine the extent of, and change in, optimal tree cover. Wood Warblers were most likely to occur where tree cover per hectare was between 40 and 61% (optimal tree cover). Extrapolation to the whole of the wintering range indicated there was a 46.7% net increase in extent between 2000 and 2014. This was due to an alarming 27 683 km2 of previously closed forest being degraded from > 61% cover to between 40 and 61%, an area greater than that of the optimal tree cover that was lost. Increases in optimal tree cover were greatest in countries with greatest forest cover, such as Sierra Leone, Liberia, Côte d'Ivoire and Democratic Republic of Congo. The results suggest that loss of optimal tree cover in the wintering range might not be a key driver of population decline for Wood Warblers, but the degradation will probably impact species that rely upon dense tree cover.