The four species comprising the genera Artisornis and Oreolais are insectivorous warblers restricted to the vine-tangles and dense understorey vegetation of montane forests across east and central Africa (Figure 2, Nguembock et al., 2008; Bowie et al., 2018). The African Tailorbird A. metopias is distributed from northern Tanzania to the mountains on the eastern side of Lake Malawi in southern Tanzania (Matengo Highlands) and northern Mozambique (Serra Jeci, near the east bank of Lake Malawi). Its sister-species, the Long-billed Tailorbird A. moreaui has a strange, disjunct distribution with one population on Serra Jeci and another 1,000 km away, in the lower to middle montane forests of the East Usambara Mountains near the coast of northern Tanzania (Figure 2). This distribution pattern has puzzled many biogeographers (e.g., Stuart, 1981).

Bowie et al. (2018) provided evidence in support of the contemporaneous Pleistocene vicariance of the once continuous range of the African Tailorbird into four distinct clades each reciprocally monophyletic: (1) Usambara Mountains, (2) Central Eastern Arc Mountains, (3) Uluguru Mountains, and (4) the Matengo and Serra Jeci Highlands; Figure 2). The simultaneous separation of the two populations of A. moreaui with that of the divergence of the four populations of A. metopias, suggests that both species were affected at the same time during the Pleistocene, when Africa became more arid (deMenocal, 2004; Trauth et al., 2005; Lyons et al., 2015) and lowland and highland forests were fragmented. To explain the present disjunct distribution of the two extant populations of Long-billed Tailorbird, Bowie et al. (2018) inferred a direct dispersal via lowland forest between the East Usambara Mountains (one of the few places in Africa where montane and lowland forest about) and the forest habitats of northern Mozambique. Molecular data from other montane bird species (e.g., Bowie et al., 2006; Fjeldså et al., 2006; Fuchs et al., 2011) as well as montane frogs (Lawson, 2013; Portik et al., 2019) suggests the existence of such a lowland corridor along coastal Tanzania and possibly across the hilly landscape of southern Tanzania and northern Mozambique. Lowland forests are assumed to have been far more extensive in the past than they are today (Burgess and Clarke, 2000), and therefore A. moreaui could have been more widely distributed through the eastern coastal lowland, but through climate change and anthropogenic modification of the landscapes, the species went extinct in most of the area, leaving behind the disjunct distribution we observe today.

Within the largest genus of songbirds, the cisticola warblers (genus Cisticola), a small group of species with unstreaked dorsal plumage and with duetting songs, have long been recognized as a distinct montane clade with three species (Lynes, 1930; Davies, 2014): Chubb’s Cisticola C. chubbi in the Cameroon Mountains and along the Albertine Rift to western Kenya; Hunter’s Cisticola C. hunteri in the highlands of Kenya and northern Tanzania; and Black-lored Cisticola C. nigriloris in the Rubeho and Udzungwa highlands of Tanzania and the highlands flanking the northern end of Lake Malawi and adjacent Zambia (at 1,100–2,550 m). These Cisticolas inhabit montane forest, scrub, bracken and tall grassy vegetation associated with swamps and seeps (Urban et al., 1997; Ryan, 2006). A fourth member of this clade, the Kilombero Cisticola C. bakerorum, was recently described from the lowland reed-marshes of the Kilombero floodplain in southern Tanzania, a distinct ecological zone at only 240–305 m a.s.l from that occupied by the other three species in the clade (Fjeldså et al., 2021). The Kilombero Cisticola is sister to the geographically neighboring C. nigriloris, and molecular dating methods suggested that these two taxa diverged at the Pliocene-Pleistocene transition (2–3 Mya). Given the montane ancestral state reconstruction for the African duetting Cisticolas, a plausible evolutionary scenario is a down-slope dispersal of the ancestral Kilombero Cisticola that eventually led to the establishment and isolation on an ever-humid floodplain represented today by the lowland expanse of the Kilombero.

Within the speciose genus Batis, the sexually dimorphic Batis capensis superspecies (Hall and Moreau, 1970) is primarily associated with montane forests and thickets that extend from northern Tanzania, through Malawi, the highlands of northern Mozambique and eastern Zimbabwe to reach the Cape Province in South Africa, where montane forest reaches sea level as latitude compensates for altitude. Traditionally considered a single species with: mixta occupying the costal forests of southeastern Kenya and the mountains of Tanzania and extreme northern Malawi; sola the mountain blocks east of Lake Malawi; dimorpha the mountains of southern Malawi and northern Mozambique; and several closely related taxa occupying the montane (erythrophthalma) and lowland hill habitats (kennedyi) of Zimbabwe and South Africa (capensis and hollidayi). An outlying lowland taxon, reichenowi, inhabits the lowland forests on the Rondo Plateau and other sites in the southeastern corner of Tanzania near the Mozambique border. Most of the populations from northern Malawi through Tanzania inhabit montane forest, although there is evidence of seasonal visits to wooded foothills (down to 540 m) in the dry season (Fjeldså et al., 2006, 2010), and some populations of mixta in the coastal zone of Kenya and northern Tanzania are resident to lowland forest like reichenowi.

A phylogeographic study by Fjeldså et al. (2006) demonstrated that the East Africa taxa (mixta, dimorpha, and sola) do not form a mophophyletic clade, because the Rwenzori Batis B. diops of the Albertine Rift Mountains is more closely related to mixta populations in coastal Kenya and the northern Tanzanian Mountains (extending to the Nguru Mts), whereas the populations inhabiting the highlands of south-western Tanzania are genetically distinct. Fjeldså et al. (2006) recognized the latter populations as a new species, the Dark Batis, B. crypta, which extends from north and east of Lake Malawi through the southern Eastern Arc Mountains to the Ukaguru Mountains in central Tanzania. Species rank for B. crypta was awarded because of reciprocal monophyly, diagnostic morphological characters, and the lack of evidence for gene flow with the Forest Batis B. mixta, although the two species are separated by only 25 km of hills (potential habitat in the non-breeding season) between the Ukaguru and Nguru Mountains. Intriguingly the phenotypically distinct reichenowi, which is isolated in the coastal forests of south-eastern Tanzania, was nested within B. mixta in the molecular phylogeny of Fjeldså et al. (2006) – a result corroborated by our expanded analyses in this paper (Figure 3), with the caveat that due to the lack of tissue samples, we are unable to determine the phylogenetic position of Woodward’s Batis B. fratrum, which somewhat resembles mixta and reichenowi and inhabits the lowlands of Mozambique and southern Malawi.

In our expanded phylogenetic analysis that now includes populations of Batis distributed across the Malawi Rift and southern Africa (Figure 3), montane populations of the northern sola and southern dimorpha were recovered as monophyletic clades sister to the Cape Batis B. capensis of southern Africa. Similar to the parapatric distribution of B. mixta and B. crypta in the Eastern Arc Mountains, the ranges of B. crypta and B. (capensis) sola are separated by only 10 km of mid-altitude rangeland and palm savanna. The Albertine Rift B. diops is recovered as closely related to the B. mixta/crypta/reichenowi species complex; an interesting result given that B. diops is phenotypically monomorphic compared to all other populations in this study, which are dimorphic.

The geographically isolated population of B. mixta reichenowi (Fjeldså et al., 2006, treated as an independent species by del Hoyo and Collar, 2016) would represent a phylogenetic species (Cracraft, 1983) based on the distinctive appearance of the female plumage (Figure 3). The nested position of reichenowi in the phylogeny suggests that this small satellite population of Batis diverged by budding from the more widely distributed northern B. mixta, and is likely a relict population left after a humid period when coastal forest extended along the east coast of Africa, where genetic drift and rapid fixation of alleles (possibly the MC1R gene affecting expression of melanin; see Harris et al., 2020) resulted in the distinct plumage of reichenowi. The contraction of coastal forest as Africa became more arid through the Pleistocene and associated anthropogenic change likely left reichenowi as a relictual taxon now restricted to some lowland forests in south-eastern Tanzania.

The African robins comprise a monophyletic clade of c. 45 insectivorous species (Fjeldså et al., 2020), with the majority of these species distributed in forest understorey and thickets across sub-Saharan Africa, especially in the lower montane zone (Voelker et al., 2010). The family as a whole, like many other families of “higher songbirds” (Passerida, see Fjeldså et al., 2020), shows a high thermal flexibility and tendency to radiate in montane regions. However, some species of robin-chats (Cossypha, Dessonornis, and Caffrornis) can be found in thickets at all elevations from the lowlands to the treeline, with a few species even penetrating arid savanna. To illustrate the elevational flexibility of these lineages we provide new analyses of two lineages [Sheppardia species (akalats), forest-chats in the genus Chamaetylas], and place these results in context with those from a published study of a third lineage, the White-stared Robin Pogonocichla stellata (Bowie et al., 2006).

The genus Sheppardia includes at least 10 species that form a monophyletic clade within the larger African forest robin assemblage (Voelker et al., 2010; Fjeldså et al., 2020). Akalat species are typically restricted to montane or lowland forest, with a few species (e.g., East Coast Akalat S. gunningi) occupying the entire elevational gradient. The greatest diversity of Sheppardia species occurs within the Eastern Arc Mountains of Tanzania (five species). These five species comprise three clades (Figure 4, see also Voelker et al., 2010). (1) The Usambara Akalat species complex S. lowei-aurantiithorax-montana primarily occupies the interior of well matured forest with dense shrubbery and liana tangles on upland plateaus at 1,400–2,400 m, with species replacing each other across the different montane highlands of the Eastern Arc Mountains (Beresford et al., 2004). (2) Sharpe’s Akalat S. sharpei inhabits forests in the mid-montane zone (1,030–2,160 m) extending through most of the Eastern Arc Mountains, with subspecies usambarae occupying the northern Eastern Arc to the Nguru Mountains, and sharpei the remainder of the Eastern Arc, the volcanic highlands of southern Tanzania and the northern and central highlands of the Malawi Rift. Our present analysis (Figure 4) and that of Voelker et al. (2010) recover Sharpe’s Akalat as sister to a clade comprising the lowland Gray-Winged Akalat S. polioptera and the elevationally variable Bocage’s Akalat S. bocagei, although with relatively weak bootstrap support. (3) S. gunning is mainly found in lowland forest (to 300 m) along Africa’s east coast, with the pale plumaged nominate subspecies gunningi occupying the coastal forests of north-central Mozambique. The subspecies sokokensis occupies the coastal forests of south-eastern Kenya and northern Tanzania, extending to Zanzibar Island. In contrast, the remaining two subspecies, which are more richly pigmented (Figure 4), are found in the lower montane zone; bensoni in northern-central Malawi and on Mount Mabu in northern Mozambique (500–1,750 m), and alticola on the steep Nguu Mountains (alias Nguru North) in Tanzania (850–1,750 m, Fjeldså et al., 2010). S. gunningi and S. sharpei do not overlap geographically: this results in the northern alticola and southern bensoni having disjunct ranges >600 km apart, and hence, it is surprising that they form a monophyletic clade (Figure 4) to the exclusion of more geographically proximate taxa (sokokensis and gunningi, respectively). This result lends further support to a past long-distance connection between northern Tanzania and northern Mozambique, consistent with interpretation of phylogeographic structure in the Artisornis warblers as described above (see also Bowie et al., 2018).

The White-Starred Robin Pogonocichla stellata is one of the most widespread Afromontane birds, occupying highlands around the montane circle of Africa, with its range extending through the Malawi Rift to the Western Cape in South Africa. Periods of aridity during the early- to mid-Pleistocene resulted in regional structuring of populations with breaks in gene flow separating populations in: (1) the Albertine Rift (ssp. ruwenzorii); (2) Kenyan Highlands (keniensis); (3) the northern Eastern Arc (helleri); and (4) central Eastern Arc and Malawi Rift mountains (Nguru to northern Mozambique; ssp. orientalis) (Bowie et al., 2006). Some sharing of mitochondrial DNA haplotypes occurs between ruwenzorii and orientalis, and between helleri and orientalis, with coalescent modeling suggesting that the sharing of haplotypes by ruwenzorii and orientalis is due to ancestral polymorphism rather than recurrent gene flow (Bowie et al., 2006). In contrast, gene flow seems to be taking place between the montane populations of northern Tanzania (helleri) and southern Malawi (orientalis) to the exclusion of the interior Eastern Arc Mountains and the central and northern Malawi Rift. It is likely that the White-starred Robin undergoes seasonal elevational migration with the coastal forests of Tanzania and northern Mozambique providing a “corridor” of connectivity, in a similar manner as has been inferred for the Long-billed Tailorbird and East Coast Akalat mentioned in the preceding case studies.

The chunky forest chats in the genus Chamaetylas inhabit the dark forest understorey, often foraging near swarms of driver ants (Dorylus), but in the dry season they are also found in riparian forests in the adjacent foothills (Fjeldså et al., 2010). Surprisingly, small/localized breeding populations, which have been classified as C. fuelleborni xuthura (Clancey and Lawson, 1969) have been found much further south, in lowland sand forests between Beira and the Zambezi River in Mozambique. Records of Chamaetylas forest chats from a coastal forest in southeastern Tanzania could be seasonal migrants from the Eastern Arc Mountains, but they could also represent a resident population and an outlier from the Mozambique lowland population (Jensen et al., 2005). Due to the presence of landmines, many parts of northern Mozambique remain difficult to explore, hence, the forest patches that occupy the hills and inselbergs of northern Mozambique are largely unknown.

The distribution of the White-chested Alethe Chamaetylas fuelleborni, with nominate fuelleborni occurring in the montane highlands of the Eastern Arc Mountains and the mountains flanking the northern Malawi Rift, and the form xuthura of the lowland forests in Mozambique, point to a past connection through the once more extensive coastal forests of Tanzania and Mozambique, probably reaching the Lebombo Mountains south of Maputo. We presently lack molecular data for xuthura, but should this taxon be sister to the montane Thyolo Alethe C. choloensis that occupies the mountains of southern Malawi, instead of C. fuelleborni, this would still illustrate extraordinary elevational flexibility in these forest chats.

The genus Illadopsis comprises eight insectivorous species mainly inhabiting understorey habitats in the Guineo-Congolian rainforests. However, Pale-breasted Illadospis Illadopsis rufipennis has a sister species, Mountain Illadopsis I. pyrrhoptera, in the montane forest of the Albertine Rift and an isolated population in northern Malawi. Small local populations of Illadopsis in Tanzania have traditionally been referred to as subspecies (distans or puguensis) of I. rufipennis), Molecular phylogenetic analysis places the Tanzanian populations as a sister-group to the montane pyrrhoptera, and the West African I. rufipennis populations as sister to this clade. Hence, the traditional I. rufipennis was not monophyletic (see also Nguembock et al., 2009) and the Tanzanian populations are now recognized as a separate species I. distans (del Hoyo and Collar, 2016). Further, a high degree of phylogeographic structure is recovered among the Tanzanian populations sampled (Figure 5), with deep genetic divergence (5–7% uncorrected) suggesting that these populations started to diverge over the past 2 Mya. These populations occupy both lowland and montane forest, and are subtle morphologically different.

One Tanzanian lineage (nominate distans) is found in montane forest in the Usambara Mountains and further inland in foothills of the Nguru Mountains and adjacent Mount Kanga. Another lineage (still unnamed) is distributed locally in the Rubeho and Udzungwa Mountains, mainly occurring in shady places along forest streams up to 2,000 m, but locally (or seasonally) also in adjacent foothill forests (Fjeldså et al., 2010). A third lineage (spp. puguensis) is found in some lowland forests near Dar es Salaam and further south near the Rufiji Delta, and genetically similar birds are found in groundwater forests in the foothills of the Uluguru Mountains 160 km inland, and (surprisingly) in montane forest in the Kiboriani Mountains. A small lowland population on Zanzibar Island has not yet been included in molecular phylogenetic analysis. These populations are highly fragmented and are of critical conservation concern.

This group, with at least 18 insectivorous species, is mainly distributed in lowland gallery forests with dense vine-tangles and epiphytes. Adaptation to higher elevation occurs among members of the Yellow-streaked Greenbul Phyllastrephus flavostriatus complex, in Cabanis’s Greenbul P. cabanisi and Placid Greenbul P. placidus, and in two local populations of the Tiny Greenbul (see below). Further, the Gray-olive Greenbul P. cerviniventris of central and eastern Africa is mostly associated with riparian and groundwater forest in montane foothills, but it is also found locally in small riparian thickets or forest swamps up to 1,900 m in Malawi and Tanzania (e.g., Dowsett-Lemaire and Dowsett, 2006; Fjeldså et al., 2010; John and Kiwango, 2021).

Phyllastrephus debilis, which has an isolated position in the phylogeny of the genus (Johansson et al., 2007; Fjeldså et al., 2020), is found across the mosaic of coastal forests in eastern Africa, with subspecies debilis occurring in central Mozambique, and subspecies rabai through the coastal zone of Tanzania and southern Kenya. Distinctly larger birds with more saturated plumage colors inhabit montane forests between 600 and 2,150 m in the West Usambara and Nguru Mountains of Tanzania (ssp. albigula). A multilocus study of 124 specimens from all parts of the range placed debilis and rabai close together but revealed that the albigula populations had been genetically isolated in their montane forest habitats since 2.4–3.1 Mya (Fuchs et al., 2011). The montane-lowland populations are now in secondary contact in Tanzania, as rabai has expanded inland and reaches the lower montane forest on the slopes of some Eastern Arc Mountains. This has resulted in limited recurrent gene flow from rabai to albigula, but the populations appear to have retained their integrity and are now recognized as separate species (Fuchs et al., 2011; Gill et al., 2021). The molecular data could not support or reject the possibility of ongoing gene flow between the montane populations of albigula on the Nguru and Usambara Mountains, which are presently isolated by 125 km of dry lowland plains.

Phyllastrephus flavostriatus is phenotypically and genetically complex. A phylogeographic study based on 248 specimens representing nearly every known allopatric population and using two mitochondrial markers (Lokugalappatti, 2011), placed populations in montane forests distributed across the Albertine Rift (subspecies graueri, olivaceogriseus, itombwensis, and kungwensis) as the sister clade to populations inhabiting variable elevations in southern and eastern Africa; we recover similar results in our analyses (Figure 6). A distinct morphological form (ssp, alfredi, with a brown instead of gray head) inhabits highlands along the Rukwa and Malawi Rifts and may be the first diverging lineage within the Yellow-streaked Greenbul complex, or may form a clade sister to a complex of populations in the Udzungwa Mountains (uzungwensis) and mountain ranges and foothills extending from southern Malawi to the KwaZulu-Natal Province of South Africa (ssp. vincenti and flavostriatus; Lokugalappatti, 2011). Nested among these southern populations are populations (spp. tenuirostris) that inhabit coastal forests from northern Mozambique to southern Kenya, with individuals ascending to montane forest in the northern Eastern Arc Mountains. These data suggests an origin in montane habitats and development of a broad ecological niche during a Pleistocene expansion toward southern Africa, with a final dispersal through the coastal forest zone of East Africa to the adjacent montane forests of the northern Eastern Arc Mountains.

This family of fruit-dependent birds is widespread in African lowland forest and woodlands. The Green Barbet Stactolaema olivacea, with three widely disjunct subspecies primarily restricted to lowland forest (olivacea, woodwardi, and hylophona; Figure 7) and three subspecies restricted to montane forest (belcheri, rungweensis, and howelli), provides an ideal exemplar taxon with which to explore speciation patterns among lowland and montane forests, as well as the age and extent of connectivity between these habitats. Particularly intriguing is Woodward’s Barbet (C. olivacea woodwardi), that occurs only in the Ongoye (Ngoye) Forest in KwaZulu-Natal, South Africa, over 2,000 km from its nearest neighboring population in southern Malawi (belcheri). Due to the sharing of bright yellow-green ear coverts, the taxa woodwardi and hylophona from coastal forests in south-eastern Tanzania (Jensen et al., 2005), are traditionally placed as sister taxa (Clancey, 1989) and are often lumped together as the same species (Sinclair and Ryan, 2010).

Our molecular analyses reveal remarkably shallow sequence divergence among all six subspecies of Green Barbet (max. 2.36%) despite the very large distances separating disjunct populations (Figure 7). This shallow sequence divergence makes the relationship among subspecies difficult to resolve. Three clades are recovered in our molecular phylogenetic analyses. (1) A clade of individuals sampled from the lowland Sokoke Forest along the coast in southeastern Kenya and individuals sampled from the montane forests of the East and West Usambara Mountains (subspecies olivacea). (2) A clade of individuals occupying the montane highlands of the remainder of the Eastern Arc Mountains and the Misuku Hills in extreme northern Malawi (subspecies howelli and rungweensis). (3) A clade comprising three widely disjunct taxa, with woodwardi (lowland) and belcheri (montane) recovered as sister-taxa, and hylophona (lowland) putatively sister to these two taxa. These data point to at least two instances of montane to lowland transitions, suggesting a recent history of dispersal between lowland and montane habitats for this canopy feeding bird.

In order to underscore that flexibility in elevational distribution is not just a rare anomaly, we mention below some further cases, which have not yet been adequately evaluated by phylogeographic methods. The many cases of montane species that are nested within larger clades of lowland bird species, provide evidence of past flexibility in elevational distribution.

Colorful species in the genus Apalis are mainly distributed in canopies of montane forest areas in Central Africa (A. personata, binotata and jacksoni) and the outlier highlands of Cameroon and Angola (A. binotata and jacksoni), with one distinctive species, the White-winged Apalis A. chariessa, with a relictual distribution in East Africa. One population of White-winged Apalis is found in mid-elevation rainforest in southern Malawi (Dowsett-Lemaire and Dowsett, 2006), another within the humid montane forests of the Udzungwa and Uluguru Mountains, and the species also once occurred (at least until 1961) 570 km further north, in the lowland forests of the lower Tana River delta near the Kenya coast. Another interesting case is the Black-headed Apalis Apalis melanocephala, which is patchily distributed from northern Mozambique and from Malawi to Kenya, mainly in highlands but locally also in foothills and coastal forests. Examining material in several museum collections, JF found significant variation in plumage melanization and tail length, both within and between local populations. Age-related variation and polymorphism could play a role as it does in Cisticolas, but there are also signs of introgression between populations in the south (subspecies tenebricosa) and between populations in Tanzanian coastal forests or mountains in the northern part of the Eastern Arc Mountains (ssp. moschi).

Grass warblers of the genus Bradypterus are patchily distributed across sub-Saharan Africa, mainly occurring within the humid undergrowth of montane forests, or in swamp habitats (Kahindo et al., 2017). The Little Rush-warbler, B. baboecala, is genetically structured with local populations both in highlands and lowlands in central and eastern Africa (unpubl. data).

White-eyes, genus Zosterops, represent one of the most remarkable avian cases of rapid radiation in the Pleistocene. Although most of the diversity is found in the Indo-Pacific archipelago, one lineage colonized Africa and diversified throughout the sub-Saharan continent (Cai et al., 2019; Gwee et al., 2020). Recent molecular work suggest that the many populations, which are characterized by broad, white eye-rings and replace each other in different Afromontane areas, do not represent a monophyletic clade but are interspersed in the phylogeny among generally more widespread lowland forms (Martins et al., 2020). This suggests that numerous local populations need to be recognized as independent species (Pearson and Turner, 2017). The results could also be interpreted to suggest several independent cases of speciation along local elevational gradients (Cox et al., 2014), but, given uncertainties about the phylogenetic resolution and remaining gaps in the geographical sampling, it is difficult to exclude the alternative interpretation, that this “island speciator” diversified rapidly across Africa’s “sky archipelago,” to subsequently disperse and settle in the surrounding lowlands.

The sunbirds of Africa have mainly radiated in upland savannas and mountain regions (Bowie, 2003). Recent phylogenetic work has revealed a monophyletic group, Euchloridia (Bowie and Fjeldså, 2020), which is highly heterogeneous in plumage characters, bill shape and diet, resulting in its constituent species having previously been placed in different genera, which we refer to below to avoid confusion. This group appear to represent relict populations from the time when the Miocene evergreen forests extended across tropical Africa from west to east. Most constituent species are dull olive-green birds of lowland rainforest (Deleornis and Anthreptes), but the Banded Green Sunbird Anthreptes rubritorques is a rare inhabitant of mid-elevation forest on some Eastern Arc Mountains. The more colorful East African species Plain-backed Sunbird Anthreptes reichenowi occupies coastal forests, the phenotypically aberrant Rufous-winged Sunbird Cinnyris rufipennis is restricted to wet highland forest in the Udzungwa Mountains and its sister-species, the Amani Sunbird Hedydipna pallidigaster, has a peculiar, patchy distribution, with a small population in the Udzungwa highland forest, another in the East Usambara Mountains, and yet another in coastal forests in Kenya, mainly in Arabuko Sokoke Forest (Fjeldså et al., 2010). These three populations appear to be genetically closely related (unpubl. data) but inhabit very different climates. Thus, even within this relatively old clade with deep divergence between constituent species, we see mixed occupancy of lowland and montane habitats, and elevational flexibility among populations within one species.

The Olive Sunbird Cyanomitra olivacea (now Haagneria; Bowie and Fjeldså, 2020) one of Africa’s most widespread songbirds occupies both montane and lowland forest throughout its range. Analyses by Bowie et al. (2004) of mitochondrial DNA sequence variation across the species range revealed that that birds sampled from the northern Eastern Arc (Taita Hills, Pare, Usambara, and Nguru Mountains) share alleles from the same haplotype clusters as birds from southern Malawi (Mt. Zomba) and northern Mozambique (Mt. Namuli) to the exclusion of the interior Eastern Arc Mountains (Uluguru, Rubeho, and Udzungwa). This suggests that lowland coastal forests along Africa’s east coast have served as a corridor linking coastal forests in the north with those in extreme southeastern Tanzania, and Mozambique, in a similar manner as has been inferred in several of the case studies mentioned above.

Other potential cases of widespread montane forest birds with local populations in coastal forests comprise Eastern Bronze-naped Pigeon Columba delegorguei, Lemon Dove Columba larvata, Silvery-cheeked Hornbill Bycanistes brevis and Black-fronted Bush-shrike Chlorophoneus nigrifrons. The green turacos, the Tauraco persa group, present a very complex case with diverse distributions, occupying both montane and lowland habitats, and would present a very interesting case for detailed phylogeographic study.

Our data analyses reinforce earlier views (e.g., Moreau, 1966) that the distribution of forest birds across East Africa is complex. However, through the synthesis of the above case studies we are able to identify some repeated patterns. First, there are some bird species with populations in the Eastern Arc Mountains and in adjacent lowland forests or in the northern coastal forests (Cisticola, Illadopsis, Phyllastrephus albigula/debilis, Zosterops, and Hedydipna pallidigaster). Second, several species distributed in the branch of the Eastern Afromontane Biodiversity Hotspot located in Malawi, or in the lowlands of Mozambique, also have resident populations in the northern coastal forests or in adjacent mountains in the northern Eastern Arc to the exclusion of the central Eastern Arc Mountains (Artisornis moreaui, Batis spp., Sheppardia gunningi, Phyllastrephus flavostriatus, Stactolaema olivacea, and Apalis spp.), and there are indications of gene-flow between these areas (Pogonocichla stellata, Cyanomitra olivacea). Third, some of the populations appear to be relictual, with several montane species nested within clades of mainly lowland species (see Fjeldså et al., 2020 for comprehensive species phylogenies) and vice versa (e.g., Batis spp.). If we take into account also the many bird species with mixed elevational distributions, which have not yet been included in detailed phylogeographic studies, we judge that approximately 15% of all forest-dependent and forest-associated species of East Africa have mixed elevational distributions presently.

Flexible elevational distributions are mainly seen among birds of the forest understorey or mid-canopy, and most of the species are insectivorous. This is for instance the case with the Phyllastrephus greenbuls, while other greenbul genera, which have mixed (insect/fruit) diets, are mainly associated with lowland forest, or specialized to live in montane forest (Arizelocichla). White-eyes and sunbirds have mixed diets (insects and nectar) and Stactolaema is fruit-dependent.