For 30 Afro-Palearctic bird species, the size of the breeding population in Europe is compared to the numbers wintering in the northern dry belt of Africa south of the Sahara, the Sahel. As the distribution of most of these species is wider than just Europe and the Sahel, the estimates are adjusted based on known breeding and wintering ranges. Eight Palearctic species recorded sparsely in the Sahel appeared to winter mainly beyond our delimited study area and so were excluded from the analyses. Species with a wide breeding distribution invariably had larger breeding than wintering ranges, but the opposite was true for species with limited breeding distributions. This outcome was at least partly due to underestimation of the breeding range of species with a small breeding area and an overestimation of the wintering range in species having a large wintering area. Our systematic survey of the Sahel revealed that bird species wintering in the northern and driest part of the Sahel actually wintered further north than indicated on published distribution maps, whereas species from the southern, humid zone wintered further south. The Sahel surveys indicate that the total population size of species breeding mainly in southern Europe, such as Masked Shrike Lanius nubicus, Western Bonelli's Warbler Phylloscopus bonelli, Subalpine Warbler Curruca iberiae + C. subalpina + C. cantillans and Rüppell's Warbler Curruca ruppeli, have so far been underestimated, but that population sizes of Common Redstart Phoenicurus phoenicurus and Common Whitethroat Curruca communis have probably been overestimated.

This paper quantifies the density and the total number of granivorous and insectivorous ground-foraging birds, whether Afro-Palearctic migrants or Afro-tropical residents, in the transition zone between the arid Sahara and the humid Guinea zone. Situated between 17°W and 42°E and between 7°N and 22°N, this is an area covering 10 million km2. The study took place during the northern winter, between 20 November and 10 March (thus covering much of the long dry season) from 2011 up to and including 2019. Using a stratified random sampling regime, we counted birds at 1901 sites of 4.5 ha in area. We present background information about the study region, with maps showing variation in elevation, rainfall, woody cover, land use and human population density. The bird counts were converted into average densities for 43 bird species in 150 grid cells of 1° latitude × 1° longitude. The distribution of the various bird species was predominantly related to annual rainfall, but because woody cover increases with rainfall, species' preferences for arid or more humid zones were partly influenced by an overall preference for open or more wooded landscapes. Bird species such as larks and Tawny Pipit Anthus campestris, even when rainfall was accounted for, selected comparatively open landscapes, whereas species feeding on the ground near trees or using them as perches (e.g. sparrows, finches, shrikes, Tree Pipit Anthus trivialis) preferred relatively more enclosed environments. To estimate total population size, the 150 grid cells were assembled into eleven rainfall categories (per 100 mm rainfall) and six longitudinal bands. To assess the reliability of these estimations, population sizes were calculated separately on the 1901 study sites split in two halves. The estimated population sizes were precise for migrants, especially for insectivores (7% deviation for the split-half estimates), but less precise for residents (22–28% deviation). Most ground-foraging birds were granivorous (at least in the dry season), their total number being estimated at 4000 million residents and 133 million migrants, residents being 30 times as abundant as migrants. Ground-foraging insectivores were less numerous, the total estimated being 920 million birds, of which 694 million were residents and 221 million migrants, the ratio residents/migrants being an order of magnitude smaller than in granivores. The three most abundant granivorous residents were Red-cheeked Cordon-bleu Uraeginthus bengalus (467 million), Sudan Golden Sparrow Passer luteus (375 million birds) and Red-billed Quelea Quelea quelea (311 million). The Greater Short-toed Lark Calandrella brachydactyla (126 million) was the only common granivorous migrant. The most common insectivorous ground-foraging bird was a resident (Greater Blue-eared Starling Lamprotornis chalybaeus; 100 million), and more commonly encountered than all the ground-foraging insectivorous migrants such as Isabelline Wheatear Oenanthe isabellina (32 million), Northern Wheatear Oenanthe oenanthe (27 million) and Western Yellow Wagtail Motacilla flava (24 million) together.

Every year, hundreds of millions of migratory birds cross the Sahara to spend the northern winter in the Sahel. After their arrival in September the region does not receive any rainfall until June while temperatures increase. Birds inhabiting the Sahel have several strategies to cope with this seasonal advent of drought. Most ground-foraging and arboreal migrants actually remain in the desiccating Sahel, although Northern Wheatear Oenanthe oenanthe remains in the arid zone only in a wet year, but moves from the arid to the semi-arid zone in a dry year. Some arboreal migrants stay for 1–2 months in the Sahel during the early dry season, but move on to the more humid zone further south for the rest of the northern winter. Common Redstart Phoenicurus phoenicurus is the only Sahelian arboreal migrant that moves southward in this period. Counter-intuitively, Curruca species move northward after the early dry season to the arid zone where they concentrate in woody plant species whose attractiveness increases later in the dry season. This is either because those plants then gain berries (Toothbrush Tree Salvadora persica) or because they develop flowers (six desert species). In the semi-arid zone, tree-dwelling bird species disappear from tree species when these lose their leaves. However, in tree species which do not shed their leaves, bird numbers remain either constant (those using Desert Date Balanites aegyptiaca) or increase (those using Winter Thorn Faidherbia albida, a tree that foliates during the dry season). On floodplains bird numbers in acacia trees increase during the dry season. As a consequence, birds become concentrated in fewer tree and shrub species during their stay in the Sahel. After wet rainy seasons, trees have more flowers and leaves and shed them later, giving the birds more foraging space. At the end of their stay in Africa after dry rainy seasons, the number of arboreal birds is only half that after wet rainy seasons, suggesting higher mortality in dry years. Clearly, in such years mortality would be even higher without what can be seen as ‘refuge trees’: the acacias on floodplains, and Faidherbia and to a lesser degree Balanites in the rest of the Sahel.

Many migratory bird species, several of which are in severe decline, and African residents spend the northern winter in the Sahel, by nature a huge savannah, half of which has been converted into farmland. We analyse the impact of such large-scale changes on birds. On average, woody cover is 38% lower on farmland than on savannah. More critically, farmers have drastically changed the vegetation communities of their farmland. In the arid and semi-arid zone, they partly removed bird-rich trees such as Umbrella Thorn Acacia tortilis and Desert Date Balanites aegyptiaca, yet further south they created a richer bird habitat by replacing the original woody species by Winter Thorn Faidherbia albida, a preferred tree species for Afro-Palearctic migrants (but less so for Afro-tropical residents). Still further south, two bird-poor trees, Shea Tree Vitellaria paradoxa and African Locust Bean Tree Parkia biglobosa, dominate farmland, causing birds, mainly Afro-tropical residents, to lose habitat. As a consequence of farming, arboreal migrants are confronted with habitat degradation in the northern arid zone and in the southern humid zone, but face more favourable wintering conditions in the sub-humid central zone. Ground-foraging birds are more abundant on savannah than on farmland; 24 bird species from this group, including three wheatear species and many residents, are more than twice as abundant on savannah. Conversion of savannah into farmland has mixed outcomes for ground-foraging birds, but were generally negative except for five species ( including Western Yellow Wagtail Motacilla flava) which were more than twice as abundant on farmland than on savannah. Thus, the conversion of savannah into farmland represents a loss for many but not all bird species.

Shrub-dwelling birds may resort to ground-foraging in the Sahel when opportunities are favourable. Several arboreal and semi-arboreal passerines, both African and European, were frequently recorded foraging on the ground, but not in heavily grazed areas. Grazed, dry savannah probably has fewer insects on the ground, which is often devoid of vegetation in the dry season. Shrub-dwelling birds foraged more frequently on the ground in the eastern Sahel, where grazing pressure is lower. In the Sahel grazing pressure increased fourfold since the 1960s, presumably reducing opportunities for arboreal bird species to facultatively forage on the ground. Due to increased grazing pressure, Common Whitethroats Curruca communis and other shrub-dwelling passerines may have lost a specific niche within their foraging habitat. This has compounded the greater losses associated with declines of woody vegetation during the drought years since the late 1960s.

Arboreal bird species occurring in the wide transient zone between Sahara and tropical rain forest are unequally distributed across the rainfall zones. As this also holds for the woody plant species which they select for foraging, it is possible that birds are bound to specific rainfall zones because their preferred woody species are common there. But it may also be the other way around, i.e. that the distribution of birds is primarily determined by their selection of a specific rainfall zone, with the choice of particular woody plants being collateral. We made maps of the predicted distribution of birds based on their occurrence in different woody species (such as measured from field study sites) multiplied by the average density at which bird species forage in those woody plant species. We then compared these maps with the observed distribution of 13 bird species (7 Afro-Palearctic migrants and 6 Afro-tropical residents). This comparison shows that the distribution of birds is largely determined by the distribution of their preferred woody species rather than rainfall. However, there are small, but systematic differences between observed and predicted bird densities in the most arid and most humid parts of their distributions. Most migrants are commoner than predicted in the semi-arid and arid zone (100–600 mm rainfall/year) and most residents commoner in the humid zone. This was confirmed in a separate analysis of the densities at which these bird species forage in five common and bird-rich tree species occurring over a wide range of rainfall zones. There are no empirical data to support the idea that migrants and residents are spatially separated to avoid interspecific competition, so the question remains what migrants gain by their preference for trees from the (semi)arid zone. In the (semi)arid zones, preferred trees are as fully leafed in the dry season as the same trees farther south, but insectivorous birds in the arid zone had a higher capture rate in those trees, suggesting a larger supply of insect prey. In addition, the driest zones held far fewer avian predators than any other vegetation zone in the sub-Sahara, indicating a lower predation risk. We suggest that arboreal birds find better living conditions in the dry zones than in the more humid zones. But there is a trade-off: arid regions have a higher overall probability of very low rainfall years when trees lose their leaves or even die, than do the more humid regions. In those years, mortality among birds in the arid zones will be disproportionally high.

The sub-Sahara between the Atlantic Ocean and Red Sea and between 5°N (Guinean vegetation zone) and 20°N (southern Sahara) was visited on 466 days during 15 dry seasons (late September – early March) in 1996–2019. Using a combination of field methods, ranging from road counts to surveys of single sites (non-random and random-stratified), a total of 22,696 raptors of 62 species were identified. These were allocated to 1° latitude-longitude grid cells. Palearctic migrants accounted for 13% of the total. Two Afrotropical raptors were by far the most common, Yellow-billed Kite Milvus aegyptius (46%) and Hooded Vulture Necrosyrtes monachus (25%). Diversity and density were lowest in the arid and semi-arid zones but increased with increasing annual rainfall and vegetation cover. Palearctic migrants almost exclusively occupied the driest zones (100–500 mm rainfall per year), African raptors were commonest in the more humid zones. Migrants were concentrated in the western and eastern sections of the sub-Sahara, in longitudinal agreement with the main crossing points on either side of the Mediterranean for the large majority of Palearctic migrants. Comparatively few migrants were encountered in the central Sahel (Mali-Niger-Chad), suggesting that most Palearctic raptors remained either in West or in East Africa upon entering the continent. Even harriers Circus spp., known to cross the full width of the Mediterranean Sea, showed a distinct East Africa bias in their distribution. Afrotropical raptors were more evenly distributed across the width of the sub-Sahara within the 100–1000-mm rainfall zone.

Many studies have shown that rainfall in the Sahel has a great influence on population trends of European bird species that spend the northern winter there. African bird species living in the Sahel, notably those that forage on the ground, have also shown significant declines, but independent of rainfall. This paper summarises the results of field data gathered in the entire Sahel and evaluates the many factors that play a role in the fortunes of birds. (1) Rainfall determines the extent of open water in the Sahel, and by default the fortunes of waterbirds. In recent decades the surface area of open water has increased because water tables have risen. (2) Rainfall south of the Sahel determines river discharge and therefore the surface of floodplains in the Sahel. Rainfall has a cumulative effect: discharges disproportionally decrease after a number of years with little rain, and vice versa. During the dry season (October–May), floodplains gradually dry out. In wet years, water – and hence food – is available for birds up to their departure, but in dry years birds become concentrated at the few remaining pools and so present an easy target for bird-trappers. Further desiccation leads to starvation. (3) After a year with heavy rainfall, seed is available in abundance, but a dry year results in a shift in the plant community and a low seed supply. Mortality among seedeaters increases under dry conditions. (4) In dry years, trees lose their leaves early on, forcing arboreal birds into a diminishing number of trees that retain leaves. In extremely dry years trees die on a massive scale and it takes many years before tree coverage is restored. When droughts occur in quick succession, as in 1972/73 and again in 1984/85, tree recovery is slow and populations of arboreal birds will continue to decline, or recover slowly or only partly (as for Eurasian Wryneck Jynx torquilla and Common Redstart Phoenicurus phoenicurus, whose numbers remain reduced by tenfold when compared to the 1950s, despite a slight recovery). Rainfall in the Sahel gradually recovered after 1990, as did the woody vegetation albeit with a delay, and many migratory bird species responded accordingly. Subalpine Warblers Curruca subalpina and Western Orphean Warblers Curruca hortensis have increased as much as threefold to fivefold since 1990. Southern European bird species, wintering in the arid parts of southern Sahara and Sahel, were hit the hardest during the Great Drought in 1969–1992, but also recovered the fastest, particularly strongly once rainfall had significantly recovered. Despite clear links between migratory birds and rainfall-related variables in their wintering areas, a migrant's world is more complicated than exclusively being constrained by rainfall. In the past century, the human population in sub-Saharan Africa has increased tenfold, with far-reaching consequences. (1) Cattle numbers boomed and grazing pressure increased greatly. Heavy grazing means lower grass seed production, especially of seeds that birds prefer. The steep decline of granivorous birds over the past several decades is therefore no surprise. (2) Every year, 2% of the savannah is converted into farmland. This is unfavourable for most, but not all, bird species. Farmers selectively favour particular tree species on their land (and remove the rest), by which the resulting tree species composition differs from the typical variety found in savannah. The shift in tree composition particularly disfavours birds wintering in both the more arid and more humid vegetation zones. Arboreal birds wintering in the intermediate zone have benefitted from the expansion of farming, because their preferred tree (White Thorn Faidherbia albida) is favoured by farmers and has become more abundant. (3) The expansion of agriculture is most obvious in the low-lying, slightly more humid and more fertile soils, much to the detriment of flooded forests, which being important refuges during droughts are particularly rich in birds. Most flooded forests have disappeared from the Sahel. (4) Millions of hectares of humid woody savannah have been converted into Cashew Anacardium occidentale plantations since 1980. Cashew plantations are almost completely devoid of birds and so constitute a significant loss of a once highly diverse wooded habitat. (5) The proliferation of Prosopis juliflora, an exotic tree species resembling an acacia, has locally and regionally displaced indigenous acacia species. Prosopis attracts few birds in comparison with acacias. (6) Irrigation and dam construction have decimated the size of floodplains in Senegal, Mali, Nigeria and Chad, with corresponding effects on many bird species, especially waterbirds, that concentrate in these areas. (7) Massive bird catches, such as those in the Inner Niger Delta after about 1990, did not occur in the past when cheap nylon nets (initially used for fishing) were not yet available. Furthermore, storage and transportation of trapped birds were impossible until ice and vehicles became available. (8) Large bird species have virtually disappeared from the Sahel due to human predation, especially in the densely populated western part. As a wintering area for most migratory Eurasian birds, the Sahel has become less attractive. Far fewer migratory birds now use the Sahel than half a century ago, not least because many populations have substantially declined in numbers. Furthermore, several European migrant species increasingly are wintering north of the Sahara, which may relate to impoverished conditions in the Sahel, but as likely to significant habitat changes in Europe and the pace of climate change.

Many migratory bird species cross the Mediterranean during autumn migration, but most do so either at the western or eastern ends where they can avoid, or minimise, sea crossings. The intervening 3500 km has long sea crossings, probably adding to the barrier imposed by the Sahara. If this were the general migration pattern, it would result in high concentrations of Afro-Palearctic migrants in West and East Africa and fewer in the central sub-Saharan zones. Unless migrants reorientate upon reaching the sub-Sahara, densities of migratory birds in the central Sahel should be much lower than at either end of the African savannah range. The available studies of birds equipped with GPS or geolocators show that south of the Sahara at least some species perform lateral movements to some extent. However, many remain either in the Sahel's western or eastern parts or continue moving southwards along the same longitudinal axis. We use density counts of arboreal birds from across the full width of the Sahel to explore the extent to which the central Sahel zone is underused by migratory birds. Eleven out of twelve common migratory arboreal species occurred at lower densities in the central Sahel than could be explained by tree-related variables. Western Bonelli's Warbler Phylloscopus bonelli, Western Orphean Warbler Curruca hortensis and Subalpine Warbler Curruca cantillans were most common in the western and (much) less common in the central Sahel, whereas Eastern Olivaceous Warbler Iduna pallida, Eastern Orphean Warbler Curruca crassirostris, Lesser Whitethroat Curruca curruca and Rüppell's Warbler Curruca ruppeli were most common in eastern, but less so in the central Sahel. Woodchat Shrike Lanius senator and Common Redstart Phoenicurus phoenicurus were more common in the western and eastern parts than in the central Sahel. No longitudinal variation was found for Common Whitethroat Curruca communis, which is consistent with the knowledge that many cross the Mediterranean waters upon encountering them. The conclusion is justified that the central Sahel is underused by migratory birds and by consequence, as far as these birds are concerned, not ‘saturated’. The question arises whether in the past, when the number of migratory birds was much greater than today, there might not have been a Gap of Chad.

Bird counts in the Ferlo, NW Senegal had shown that the density of ground-foraging birds was much lower in grazed than in ungrazed savannah: 84% fewer granivorous birds and 64% fewer insectivorous birds. Between 1960 and 2010, in three areas within the same region, granivores declined by 39–97% and insectivores by 61–91%, losses attributable to the steadily increasing livestock grazing pressure. If these trends hold for all Sahelian rangelands, the extrapolation at the time indicated that 1.5 billion birds would have been lost in just half a century. The aim of this space-for-time substitution study was to investigate whether that extrapolation can be substantiated with data from the eastern Sahel. To permit analysis of the spatial and seasonal variation in grazing pressure and its impact on ground-foraging granivorous and insectivorous birds, we counted birds in 1901 sites across the entire region (Mauritania to Ethiopia) and took landscape photos of these sites to evaluate livestock presence. We also estimated livestock density from our counts of cowpats and of droppings of sheep and goats, and the cover of the soil vegetation. Within the same rainfall zone (200–400 mm/year), the grazing pressure was higher in the Ferlo than elsewhere in the Sahel. Grazing pressure declined in the Ferlo in the course of the dry season, indicating that cattle food supply became depleted. No such seasonal decline was recorded elsewhere in the Sahel. The same pattern was found for ground-foraging birds: a decline during the dry season in the Ferlo, but nowhere else in the same Sahel rainfall zone. Indeed, in the 1960s and 1970s, when the grazing pressure was much lower than today, there were no records of a seasonal decline of ground-foraging birds in the Ferlo. The much lower densities of seed-eating birds in the Ferlo were not exhibited elsewhere in the Sahel, which is consistent with the phenomenon of local overgrazing. The previous extrapolated loss of 1.5 billion birds is therefore too high, because conditions in the Ferlo were found to differ from those experienced elsewhere in Sahel's arid zone. The actual overall loss cannot be specified due to lack of bird counts from the past in the eastern Sahel. The comparison of grazing pressure and bird densities across all rainfall zones of the western and eastern Sahel shows that increasing livestock densities negatively impact bird numbers. Livestock grazing pressure in the west was higher than in the east and most ground-foraging bird species were less common in the west than in the east. Furthermore, the majority of ground-foraging species in West Sahelian savannahs were exclusively confined to the arid and semi-arid zone, but in the east, these species were more widely distributed and also occupied the more humid zone to the south.

Bush fires are widespread in African savannahs. Their impact on birds varies. Many insectivores temporarily profit from the insects escaping fire and smoke, whereas the burnt-through grass and herb layer facilitates feeding for some ground-foraging bird species. Nevertheless, bush fires have a direct, negative impact on many other ground-foraging birds. The average density of seed-eating birds in humid, African savannahs (annual rainfall >800 mm) was 15.9 birds/ha in unburned savannahs, compared to 3.3 birds/ ha (–72%) in recently burned areas. No such difference was found for insectivorous bird species. Eleven of the 13 common ground-foraging migratory bird species were not affected by bush fires in Africa because they spend the northern winter in the arid and semiarid zone, beyond the main bush-fire zone. In the long run, savannah-inhabitant birds profit from bush fires, simply because fires prevent open landscape from becoming overgrown with trees. However, the short-term implications of bush fires might be severe for seed-eating birds that rely on humid savannah, because of the more than 3 million km2 in Africa burned annually, most comprises humid savannah.

During the dry season four billion African and European granivorous birds in the Sahel consume, by grand average, 15 g seeds/ha/day, equivalent to an average annual consumption of 4.5 kg/ha. This represents only 4–15% of the estimated average total soil seed bank of some 30–100 kg/ha in the early dry season. Despite this apparent abundance of food, there are many reasons to presume that the number of seed-eating birds is limited by their food supply. First, the birds have to share the seed supply with rodents and insects that eat more seeds than all the birds combined. Second, granivorous birds are constrained by foraging time available to them. They avoid foraging during the midday heat and feeding time is mostly restricted to the early morning and late afternoon, totalling about 4 h per day. This forces them to achieve high intake rates and thus to select feeding sites where the available seeds can be handled quickly and/or are so abundant that the encounter rate is high. Third, only a proportion of the seeds lies on the surface where they are easy to find. Most grass seeds are tiny and even small birds need to eat thousands per day. Because they have so little time to look for food, they cannot afford to search for seeds hidden in the sand. Doves rapidly swallow seeds whole, but all smaller seedeaters have to separate the husk from the seed, a process that takes time too. Fourth, seed-eating birds in the Sahel discriminate between seeds. They ignore 'empty seeds' (husks) and also avoid feeding on common graminoids whose seeds have long awns (Aristida) or spines (Cenchrus) and which are time-consuming to process. Occasionally, granivorous birds may select seeds from forbs, but these, being low in digestibility, are not the preferred choice. Granivorous birds prefer the seeds of Panicum grass and other grass species with highly soluble carbohydrate fractions. Birds switch to marginal seed types at the end of the dry season, when the seed bank of the preferred species is depleted. Fifth, soil seed bank of preferred grass species is much reduced in dry years. Panicum and other preferred annual grasses are found mostly on riverine floodplains and in depressions that are prone to ephemeral flooding during the rainy season. Such sites attract many seed-eating birds, but the total surface area of floodplains is relatively small compared to the extensive drylands, on top of being very much smaller in dry years, circumstances that account for high mortality among seed-eating birds in drought years. The final argument for food-limitation is that the mounting grazing pressure of livestock over the last decades has severely reduced the annual soil seed bank and changed the plant community (preferred grass species replaced by non-preferred grasses and forbs). The combination of these factors caused a very large decline of seed-eating bird populations in the Sahel between the 1970s and 2010, including a handful of Eurasian species. The Sahel is still home to some four billion granivorous birds during the dry season, but just half a century ago the numbers must have been much higher.

The Sahel is thinly covered by trees, but nevertheless forms an important habitat for millions of tree-dwelling birds. We describe tree availability and tree selection of 14 insectivorous Afro-Palearctic migrants and 18 Afro-tropical residents (10 insectivores, 3 frugivores and 5 nectarivores) inhabiting the Sahel from the Atlantic to the Red Sea. Of the 304 woody species identified across the region during systematic fieldwork in stratified plots, we noted height and canopy surface of 760,000 individual woody plants. Birds present in trees and shrubs were recorded separately per individual woody plant. 99.5% of the birds were concentrated in only 41 woody species. For 20 out of 32 bird species, Winter Thorn Faidherbia albida was the tree species most often used. Two other important tree species were Umbrella Thorn Acacia tortilis and Desert Date Balanites aegyptiaca. Representing only 11% of the total woody canopy cover, these three species attracted 89% of Western Bonelli's Warblers Phylloscopus bonelli and 77% of Subalpine Warblers Curruca iberiae + subalpina + cantillans. High selectivity for particular woody species was typical for migrants and residents, irrespective of their diet. Bird species feeding in shrubs used a larger variety of woody species than bird species feeding in tall trees. The highest bird densities (80–160 birds/ha canopy) were found in three shrubs with a limited distribution in the southern Sahara and northern Sahel: the berry-bearing Toothbrush Tree Salvadora persica, the small thorny shrub Sodad Capparis decidua and the small tree Maerua crassifolia. Other bird-rich woody species were without exception thorny (Balanites aegyptiaca, various species of acacia and ziziphus). In contrast, the five woody species most commonly distributed across the region (Cashew Anacardium occidentale, African Birch Anogeissus leiocarpus, Combretum glutinosum, Guiera senegalensis and Shea Tree Vitellaria paradoxa), representing 27% of the woody cover in the study sites, were rarely visited by foraging birds. In this sub-Saharan region, it is not total woody cover per se that matters to birds, but the presence of specific woody species. This finding has important implications: remote sensing studies showing global increase or decline of woody vegetation without identifying individual species have little value in explaining trends in arboreal bird populations. Local people have a large impact on the species composition of the woody vegetation in the Sahel, with positive and negative consequences for migrants wintering in this region. Faidherbia albida, the most important tree for birds in the sub-Saharan dry belt, is highly valued by local people and has the distinction of leafing in winter and being attractive to arthropods. On the other hand, migratory and African bird species have been negatively affected by the rapidly expanding cashew plantations since the early 1980s.

Arboreal birds in the Sahel are highly selective in their tree choice. Most migrant, but also resident, birds are found in a few tree species and within those species often only in trees with abundant leaves and flowers. For this reason alone, preferred trees were expected to teem with birds. This was not the case. Most bird species were present in trees as singletons, even half of the Senegal Eremomela Eremomela pusilla – the most social species of all – were recorded as solitary birds. The probability that two different bird species were in the same tree was also very small, 2.8% on average. Mixed-group foraging flocks of arboreal birds, as so often reported for tropical forests, did not occur in the Sahel. Perhaps birds forage singly because they have no need to fear the raptors that are common in the forests further south. Some species, such as European Pied Flycatcher Ficedula hypoleuca and Common Redstart Phoenicurus phoenicurus, defend winter territories, but for other species individual home ranges show overlap. In the humid forests further south, and among resident species in the Sahel, few agonistic interactions were seen, but migratory birds were often agonistic with congeners and even more frequently with birds of other species. Larger bird species usually won agonistic interactions, but Western Olivaceous Warblers Iduna opaca chased off birds twice their own body size. Subalpine Warbler Curruca iberiae + subalpina + cantillans and other Curruca species, with the exception of Lesser Whitethroat Curruca curruca, were also intolerant. Western Bonelli's Warblers Phylloscopus bonelli, and during migration also Willow Warblers Phylloscopus trochilus, were most often on the receiving end of agonistic interactions. Far fewer agonistic interactions were recorded in the more humid regions to the south of the Sahel. This disparity may hinge on the higher intra- and interspecific encounter rate in the Sahel, where a greater fraction of trees are occupied by birds, than in the humid forests.

SummaryArboreal birds tend to remain in woody vegetation and avoid crossing open areas. Therefore, few tree-dwelling birds are to be expected in scattered trees. We tested this expectation with field data collected in the deserts, savannas and open agricultural parklands of West Africa where woody cover in 1,327 stratified random study sites varied between 0.2 and 29%. We found no evidence that scattered trees were avoided. Instead, bird density in trees was independent of trees occurring clumped or singly. The presence of birds in an individual tree was related to tree species and tree-related variables, but not to woody cover or species composition of the surrounding woody vegetation. We hypothesise that scattered trees are not avoided because (1) travel time between trees is too short to have a negative impact on foraging time, (2) predation risk of arboreal passerines is very low (bird-hunting raptors are scarce in the deserts and savannas of West Africa and mostly prey on ground-feeding, not arboreal, birds), and (3) the probability of being chased away by other arboreal birds is less when trees are more scattered. Scattered trees are ecologically important since hundreds of millions of migratory woodland birds, of which several species are in decline, spend the northern winter in Africa in open, often human-modified, landscapes where trees are well spaced.

The large and widespread decline of European migratory birds spending the northern winter in the Sahel suggests - on top of adverse changes in the breeding quarters - pivotal changes in African overwintering areas. This paper attempts to answer three questions related to the sub-Saharan region: (1) can a change in the woody vegetation explain the decline of migratory birds feeding in trees, (2) ditto for the ground vegetation and bird species searching for food on the ground, and (3) are African bird species also in decline? The analysis is confined to the western Sahel (annual rainfall 100-400 mm, 15.5-18 degrees N and 14.7-16 degrees W), a region intensively used as rangeland but too dry for agriculture. The woody cover, largely stable before 1970, declined in the driest zone by 90% between the 1960s and 2000s, and by 40% in the more humid zone where the woody cover was already low in the 1960s. The woody vegetation changed in many places from an Acacia savanna into an open savanna with scattered bushes and few trees, concomitant with a shift in species composition. These changes took place during a prolonged drought (1969-1992), and were aggravated by increased grazing pressure after the construction of boreholes as evident from the loss of woody cover close to boreholes. A comparison of bird composition and densities in grazed and ungrazed areas and in three study sites intermittently surveyed between 1960 and 1994, with our surveys in the same sites in 2014-2015, revealed about 80% losses for birds feeding on the ground. The increased grazing pressure of livestock must have caused a large reduction of the soil seed bank and most likely also of insects. Between 15 and 16 degrees W an estimated 8 million arboreal birds, mainly migrants, and 30 million ground-feeding birds, mainly granivorous residents, lost their habitat. Assuming that this zone is representative for the Sahelian rangelands as a whole, 1.5-2.0 billion birds have lost their habitat in half a century.

In West Africa, tree preferences of wintering migratory birds (and African residents) were quantified in order to assess the importance of wintering conditions on distribution, abundance and trends of insectivorous woodland birds. This study encompassed 2000 plots between 10–18°N and 0–17°W, visited in October-March 2007–2015, and covered 183 woody species and 59 bird species. Canopy surface (measured in a horizontal plane) and birds present were determined in 308,000 trees and shrubs. Absolute bird density amounted to 13 birds/ha canopy, on average, varying for the different woody species between 0 and 130 birds/ha canopy. Birds were highly selective in their tree choice, with no insectivorous birds at all in 65% of the woody species. Bird density was four times higher in acacias and other thorny species than in non-thorny trees, and seven times higher in trees with leaves having a low crude fibre content than in trees with high crude fibre foliage. Salvadora persica shrubs, but only when carrying berries, were even more attractive. Overall, densities of migratory woodland birds were highest in the (thorny) trees of the Sahelian vegetation zone. This counterintuitive finding, with highest numbers of wintering birds in the driest and most desiccated parts of West Africa (short of the Sahara), also known as Moreau's Paradox, can be explained by the foliage palatability hypothesis. The Sahelian vegetation zone has always been subject to heavy grazing from large herbivores, and as a consequence woody species have evolved mechanical defences (thorns) to withstand grazing of large herbivores, at the expense of chemical defence against arthropods. South of the Sahel, with a much lower grazing pressure, thorny trees (rich in arthropods) are replaced by (usually non-thorny) trees with less palatable foliage and a higher crude fibre content, and hence with less arthropod food for insectivorous birds.

Mangrove forests attract many insectivorous birds. Bird density in West African mangroves in January–March 2014 is higher in Avicennia (21 birds/ha canopy) than in Rhizophora (11 birds/ha). The Palearctic species are dominant in the most northern mangroves (14–16°N), but further south resident birds become as numerous as migrants (11–12°N). The European Reed Warbler Acrocephalus scirpaceus is the most common winter visitor in West African mangroves between 12 and 16°N, with an estimated total of 4–6 million birds, which may account for 30–50% of the European population. The mortality of European Reed Warblers while crossing the Sahara desert in spring is higher when their Sudan-Guinean wintering areas have been drought-stricken in the preceding winter. European Reed Warblers wintering in mangroves suffer the same fate, because mangroves in the Sahel region massively die off in drought years.