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

Abstract

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 km 2 . 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 W...

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

“…The data for this particular study were collected between 2011 and 2019 during the dry season (20 November – 10 March), in sites (usually 4.5 ha; a triangle with three sides of 300 × 50 m) situated between 7°N and 22°N and between 17°W and 42°E (see Figure 3A and 3B in Zwarts et al 2023a). As described in that paper, we used a stratified random sampling regime in order to generate data that are representative for the region.…”

“…The 1901 sites are situated in 178 1° latitude × 1° longitude grid cells (111 × c. 104 km). Since data from grid cells with less than 10 sites were combined with adjacent grid cells, bird densities are given for 150 grid cells (see Figure 3B in Zwarts et al 2023a).…”

“…In a strict sense, the Sahel is the climate zone where the annual rainfall varies between 100 and 600 mm (Figure 5 in Zwarts et al 2023a). We use the term in a wider sense as the transition zone between the arid Sahara in the north (>30 mm rainfall/year) and the humid forests in the south (<1200 mm rainfall/year).…”

“…Some species, usually found in lower woody vegetation, may also forage on the ground or in herbs. Three species of which more than half of the recorded birds foraged on the ground or in the herbaceous layer (Cricket Warbler Spiloptila clamans , Black Scrub Robin Cercotrichas podobe and Rufous-tailed Scrub Robin C. galactotes ; Figure 1) are excluded (but have been included in Zwarts et al 2023a), as well as Woodchat Shrike Lanius senator and Great Grey Shrike Lanius excubitor , which use trees as perches and take food from the ground. The arboreal bird species are categorised as Afro-Palearctic (termed migrants) and Afro-tropical (residents), and as insectivorous, frugivorous or nectarivorous, based on Morel (1968), Morel & Morel (1978), Urban et al (1997), Fry & Keith (2000, 2004) and own observations.…”

“…The aim of the present paper is to show the distribution of arboreal bird species south of the Sahara and to estimate their total number between the Atlantic Ocean and the Red Sea (a distance of 6000 km) in a 1600 km wide band. The paper is complementary to the one describing the distribution of ground-foraging birds (Zwarts et al 2023a). Together they form the descriptive basis for attempts to explain the observed bird distributions and why so many migratory – and African – bird species are in decline.…”