Savannah Trees Attract More Migratory Bird Species Than Residents, But Why?

Abstract

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 Gap of Chad, a Dearth of Migratory Birds in the Central Sahel

“…The lines are dashed when connected to a longitudinal zone where birds were expected but not observed (thus ratio = 0). Observed bird densities are given in Supplementary Material of Zwarts et al (2023b) and observed and predicted densities in Figure 3 and Figures S5–S14 in Zwarts et al (2023f). χ 2 -tests show that longitudinal variation was in all cases significant ( P < 0.001).…”

“…This allowed us to convert bird counts in the study sites into bird densities per ha, but also to calculate bird density per ha of canopy, separately per tree and shrub species. We used the distribution of different woody species (Zwarts et al 2023d) to formulate a prediction of bird distribution on the assumption that their density per ha of canopy in different woody species would be the same across their entire distribution areas and used the deviation of the observed from the predicted distribution to test whether the density of bird species varies per longitude; see Zwarts et al (2023f) for details.…”

“…proportion of the birds spend some weeks in the Acacia belt early October in the northern Sahel, 400–500 km north of their wintering quarters (Zwarts et al 2023f). – Northern Wheatear Oenanthe oenanthe in the western Sahel remained near the southern edge of the Sahara until late December, then moved c. 200 km southward (Arlt et al 2015; see also Schmaljohann et al 2012), although the tendency to move southward is almost absent in wet years (Zwarts et al 2023f). – Tawny Pipits Anthus campestris moved c. 400 km SW (five birds; Briedis et al 2016), at least in a relatively dry year and possibly not in wet years (Zwarts et al 2023f).…”

“…– Northern Wheatear Oenanthe oenanthe in the western Sahel remained near the southern edge of the Sahara until late December, then moved c. 200 km southward (Arlt et al 2015; see also Schmaljohann et al 2012), although the tendency to move southward is almost absent in wet years (Zwarts et al 2023f). – Tawny Pipits Anthus campestris moved c. 400 km SW (five birds; Briedis et al 2016), at least in a relatively dry year and possibly not in wet years (Zwarts et al 2023f). – Ortolan Buntings Emberiza hortulana from NW Europe remained the entire wintering period in West Africa at about 10°W and congeners from Ukraine in Ethiopia occurred at 40°E where the individual birds used different wintering sites, but all sites were situated along the same longitude (Jiguet et al 2019).…”

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

“…The data summarised in this paper will be used to address various related questions, notably (1) can the observed distribution be explained by the distribution of their preferred woody plants, and (2) if not, what other factors may be involved (Zwarts et al 2023c,e,f)?…”