ZWARTS L, BIJLSMA RG, VAN DER KAMP J & SIKKEMA M (2019) Distribution, density and habitat use of migratory arboreal birds in the Sahel. LIMOSA 92 (3): 125-137.

To be able to quantify the impact of habitat loss in Africa on numbers of wintering European migrants, we need to know where they winter, and in which habitats. Our data were collected in West-Africa between 2007-2018 in 1954 random plots, of which 1462 during the winter months (Fig. 1). Each plot consisted of three transects of 300 × 50 m (Fig. 2). In these plots, all trees and shrubs were identified, counted and their height and width measured. The observed birds were noted separately per individual tree and shrub.
The woody cover in West-Africa, related to rainfall, increases from north to south (Fig. 3). Arboreal birds are less common in the humid, woody south than in the more sparsely wooded savanna in the north. This counterintuitive finding is even more pronounced for European migrants than for African bird species (Fig. 2). When bird density is calculated as number/ha canopy, the northern savanna trees are richest with more than 40 migrants/ha canopy at an annual rainfall of less than 200 mm/year, decreasing to 7 migrants/ha canopy in the zones with more than 800 mm rainfall/year (Fig. 3).
Arboreal birds are highly selective in tree choice. Seventy percent of all European arboreal birds were seen in only three tree species (which together account for 19% of the total woody cover), i.e. 35% in Faidherbia albida, 21% in Acacia tortilis and 14% in Balanites aegyptiaca. Only 1.7% of the migrants were observed in the three most common nonpreferred woody plants (Guiera senegalensis, Combretum glutinosum and Vitellaria paradoxa; together 20% of the total woody cover).
Acacia tortilis is the dominant tree in savannas with an annual rainfall of 100-500 mm, while Faidherbia is characteristic for farmland in the zone with 300-700 mm of rain per year (Fig. 6). The two most numerous arboreal migrants in the Sahel, Bonelli’s Phylloscopus bonellia and Subalpine Warbler Sylvia cantillans, reach their highest densities where these two tree species are most common (Fig. 6). However, the distribution of Palearctic birds is not entirely determined by the occurrence of their preferred tree species. On average, migrants - using the preferred trees - are more common in the western part of the Sahel and where there is less rainfall (Fig. 7).
During the Great Drought (1969-1993), 80% of the trees in the northern Sahel died. The migratory birds wintering there mainly breed in southern Europe (Fig. 3). Those birds must have suffered huge declines, as also apparent from a comparison between our recent bird counts and those carried out in the Sahel half a century ago. This decline went unnoticed in Europe as monitoring breeding birds in southern Europe only started recently.
Arboreal migrants in the Sahel mostly occur in scattered trees in open landscapes (Fig. 3), often in man-made landscapes where the original woody savanna has been transformed into farmland with Faidherbia trees. The future of many arboreal migrants mainly depends on the management of this type of parkland habitat in the Sahel.

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