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The Arabian population of the Abyssinian White-eye Zosterops abyssinicus arabs occurs in the Asir Mountains in southwest Saudi Arabia, the highlands of Yemen, and southwest Oman. In Saudi Arabia, a recently discovered, very small, and enigmatic population of white-eyes Zosterops sp. indet. occurs in stands of mature mangroves on the Kingdom’s southern Red Sea coast 80–100 km west of the nearest locality of Z. a. arabs in the Asir Mountains. We obtained biometric data and blood samples from individuals of both populations, in the Asir Mountains and the Red Sea mangroves, complemented by measurements from museum specimens from throughout the range of Z. a. arabs. The mangrove-dwelling white-eyes were found to differ in morphometrics and several plumage characters from montane Abyssinian White-eyes Z. a. arabs. However, no differences were revealed by one mitochondrial marker between four ‘Mangrove White-eyes’ and five Z. a. arabs from the Asir Mountains. ‘Mangrove White-eyes’ might thus represent the result of a recent colonization followed by niche divergence, and a putative case of incipient speciation in one of the fastest-radiating vertebrate lineages. A review of the scant available literature and our own surveys indicate that the ‘Mangrove White-eye’ is very scarce within a highly restricted and threatened range, and presumably at risk of extinction.

East-African mountain forest species often occur in small and isolated populations, whereas species inhabiting the dry lowland savannahs exist in large and interconnected population networks. Taxa with closely related highland and lowland species, such as the East- African White-eye birds, allow testing for the potential effects of the two contrasting distribution patterns, moun- tain disjunction versus lowland panmixia. In this study, we compare the population genetic and bioacoustic differen- tiation of two representatives of the genus Zosterops: Zo- sterops poliogaster is exclusively found in forests at higher elevations; in comparison, Zosterops abyssinicus, only occurs in the dry and warm lowland savannahs. Both species were analysed across a similar geographical scale. Population genetic differentiation was inferred using the same set of 15 polymorphic microsatellite loci for both species. In addition, we quantitatively analyzed bioacoustic traits. Both data sets indicate a strong population differ- entiation among populations of the highland species, but an absence of differentiation in the lowland species. In addi- tion, the lowland Z. abyssinicus was characterised by a twofold higher genetic diversity than detected for the highland Z. poliogaster. These two contrasting intraspecific population structures may reflect the opposite ecology and distribution of these species: the strong population isolation of Z. poliogaster resulting from long-term restriction to the cool and moist mountain forests at higher elevations has led to strong differentiation among local populations and resulted in a comparatively low level of intraspecific var- iability. In contrast, population panmixia in the lowland Z. abyssinicus provides a high level of gene flow allowing the maintenance of high genetic diversity and avoiding strong population structuring. These findings need to be consid- ered when planning conservation actions.

The Eastern Afromontane biodiversity hotspot consists of isolated mountain massifs embedded within the dry lowland savannas of East Africa and of which the peaks and ridges are covered by cloud forest remnants. These cloud forests are home to the Mountain White-eye (Zosterops poliogaster), while three congeneric species (Abyssinian White-eye, Zosterops abyssinicus; Yellow White-eye, Zosterops senegalensis; Pemba White-eye, Zosterops vaughani) inhabit the adjacent lowland savannas. We sampled individuals of all four species across Kenya to analyse interspecific genetic relationships as well as intraspecific differentiation among mountain populations of Z. poliogaster. While the level of genetic differentiation among the four species was rather low, genetic differentiation within Z. poliogaster was very high, even between geographically neighbouring populations. Overall, levels of genetic variation varied strongly across all four species, with much higher diversity detected within the three lowland ones. The highland species was characterised by numerous private alleles that were geographically restricted at populations from single mountains, some of which showed evidence of recent population bottlenecks. We conclude that Z. poliogaster populations are both of high conservation value and conservation concern, given the high proportion of endemic alleles and the genetic signatures of high genetic drift and low gene flow that are typical for small and isolated populations.

Background Closely related species often occur in geographic isolation, yet sometimes form contact zones with the potential to hybridize. Pre-zygotic barriers may prevent cross breeding in such contact zones. In East Africa, White-eye birds have evolved into various species, inhabiting different habitat types. Zosterops poliogaster is found in cool and moist cloud forests at higher elevations, whereas Z. abyssinicus is distributed across the dry and hot lowland savannahs. In most areas, these two species occur allopatrically, but in the contact zone where the mountain meets the savannah, the distributions of these species sometimes overlap (parapatry), and in a few areas the two taxa occur sympatrically. Acoustic communication is thought to be an important species recognition mechanism in birds and an effective prezygotic barrier for hybridisation. We recorded contact calls of both the lowland and highland species in (i) distinct populations (allopatry), (ii) along contact zones (parapatry), and (iii) in overlapping populations (sympatry) to test for species and population differentiation. Results We found significant differences in call characteristics between the highland and lowland species, in addition to call differentiation within species. The highland Z. poliogaster shows a strong call differentiation among local populations, accompanied by comparatively low variability in their contact calls within populations (i.e. a small acoustic space). In contrast, calls of the lowland Z. abyssinicus are not differentiated among local sites but show relatively high variability in calls within single populations. Call patterns in both species show geographic clines in relation to latitude and longitude. Calls from parapatric populations from both species showed greater similarity to the other taxon in comparison to heterospecific populations found in allopatry. However, where the two species occur sympatrically, contact calls of both species are more distinct from each other than in either allopatric or parapatric populations. Conclusion The contrasting patterns reflect divergent spatial distributions: the highland Z. poliogaster populations are highly disjunct, while Z. abyssinicus lowland populations are interconnected. Higher similarity in contact calls of heterospecific populations might be due to intermixing. In contrast, sympatric populations show reproductive character displacement which leads to strongly divergent call patterns.

The white-eye birds of the genus Zosterops have been recognized for their high speciation rates in the past, but the relationships of the East African populations are not yet fully resolved. We sequenced and annotated mitogenomes of four populations currently assigned to three East African white-eye species, Zosterops senegalensis, Z. abyssinicus and Z. poliogaster. For Z. senegalensis specimens from two distant populations were sequenced; for the other taxa we used samples collected at one site. The mitogenomes ranged between 17,827 and 17,974 bp, in size similar to previously published mitogenomes analyzed for this genus from other geographic regions. The mitogenomes contain the classical set of 13 coding genes, two structural rRNA genes and 22 tRNA genes. We constructed a phylogeny using all complete mitogenomes currently available for the genus. The phylogeny supports an Asian or Oceanic origin of the genus Zosterops. The East African species represent a monophyletic clade, but the two specimens of Zosterops senegalensis from different regions do not group together, supporting previous hypotheses of cryptic species within the genus. The new genetic resources provided here may help to further explore the relationships and evolution of the genus.

Closely related species often occur in geographic isolation, yet sometimes form contact zones with the potential to hybridize. Pre-zygotic barriers may prevent cross breeding in such contact zones. In East Africa, White-eye birds have evolved into various species, inhabiting different habitat types. Zosterops poliogaster is found in cool and moist cloud forests at higher elevations, whereas Z. abyssinicus is distributed across the dry and hot lowland savannahs. In most areas, these two species occur allopatrically, but in the contact zone where the mountain meets the savannah, the distributions of these species sometimes overlap (parapatry), and in a few areas the two taxa occur sympatrically. Acoustic communication is thought to be an important species recognition mechanism in birds and an effective prezygotic barrier for hybridisation. We recorded contact calls of both the lowland and highland species in (i) distinct populations (allopatry), (ii) along contact zones (parapatry), and (iii) in overlapping populations (sympatry) to test for species and population differentiation. We found significant differences in call characteristics between the highland and lowland species, in addition to call differentiation within species. The highland Z. poliogaster shows a strong call differentiation among local populations, accompanied by comparatively low variability in their contact calls within populations (i.e. a small acoustic space). In contrast, calls of the lowland Z. abyssinicus are not differentiated among local sites but show relatively high variability in calls within single populations. Call patterns in both species show geographic clines in relation to latitude and longitude. Calls from parapatric populations from both species showed greater similarity to the other taxon in comparison to heterospecific populations found in allopatry. However, where the two species occur sympatrically, contact calls of both species are more distinct from each other than in either allopatric or parapatric populations. The contrasting patterns reflect divergent spatial distributions: the highland Z. poliogaster populations are highly disjunct, while Z. abyssinicus lowland populations are interconnected. Higher similarity in contact calls of heterospecific populations might be due to intermixing. In contrast, sympatric populations show reproductive character displacement which leads to strongly divergent call patterns.

Species distribution patterns range from highly disjunct to continuous, depending on their ecological demands and the availability of respective habitats. East African savannahs are mostly interconnected and ecologically comparatively homogenous and thus provide a prerequisite for a rather panmictic distribution pattern for species occurring in this habitat. The Abyssinian white-eye Zosterops abyssinicus is a savannah inhabiting bird species, representing such a continuous distribution. This species occurs in high abundances and is very mobile, and past population genetic studies have suggested that gene flow is high and genetic differentiation is low even across relatively large geographic distances. Further, only little morphological differences were found. In order to test for potential divergence in acoustic traits despite its interconnected geographic distribution, we analyzed 2795 contact calls of Z. abyssinicus, which were recorded at 19 sites across Kenya. Our data indicate weak, but significant differentiation in call characteristics across latitudinal gradients. We found strong changes in call characteristics in populations where Z. abyssinicus occurs in sympatry with its highland congener, Zosterops poliogaster. However, the changes in call characteristics in sympatry were in different directions and lead to strong differentiation of the sympatric populations to other conspecific populations potentially representing a case of cascade reinforcement. The detected spatial gradients likely result from ecological differences and balancing effects of natural and sexual selection.

The white-eye birds of the genus Zosterops have been recognized for their high speciation rates in the past, but the relationships of the East African populations are not yet fully resolved. We sequenced and annotated mitogenomes of four populations currently assigned to three East African white-eye species, Zosterops senegalensis, Z. abyssinicus and Z. poliogaster. For Z. senegalensis specimens from two distant populations were sequenced; for the other taxa we used samples collected at one site. The mitogenomes ranged between 17,827 and 17,974 bp, in size similar to previously published mitogenomes analyzed for this genus from other geographic regions. The mitogenomes contain the classical set of 13 coding genes, two structural rRNA genes and 22 tRNA genes. We constructed a phylogeny using all complete mitogenomes currently available for the genus. The phylogeny supports an Asian or Oceanic origin of the genus Zosterops. The East African species represent a monophyletic clade, but the two specime...