Two undescribed species of bird from West Africa

Abstract

Taxonomically undifferentiated western and eastern populations of Dusky Long-tailed Cuckoo Cercococcyx mechowi and Yellow-spotted Barbet Buccanodon duchaillui are known to have very different voices. The cuckoo has two song types, a melodious three-note whistle and a plaintive whinnying in West Africa west of the Bakossi Mountains in Cameroon, and a much less melodious, higher-pitched three-note whistle and a much faster whinnying in Central Africa east of the Bakossi Mountains. The barbet has an accelerating song of some 6-11 hoots west of the Dahomey Gap and a rapid rolling purr to the east. Even though in plumage and morphometrics there is no unambiguous diagnostic distinction between these two vocal groups, analysis of their vocalisations demonstrates a high degree of differentiation. We consider the western groups as species, naming the cuckoo for Françoise Dowsett-Lemaire and the barbet for Robert Dowsett. Dusky Long-tailed Cuckoo Cercococcyx mechowi and Yellow-spotted Barbet Buccanodon duchaillui share much the same geographical range, extending through the Upper and Lower Guinea forest belt of West Africa, from Sierra Leone east discontinuously at the Dahomey Gap to Nigeria and thence Uganda (and for the barbet western Kenya), and south to northern Angola (del Hoyo & Collar 2014). They also share a curious characteristic, that their populations either side of a relatively narrow line (not the same line, however) have, quite recently, been found to possess markedly different songs. The revelation concerning the cuckoo appears first to have been made by Dowsett-Lemaire (1997), and in the case of the barbet by Borrow & Demey (2001). We cite the latter's information on both species in quotation marks in the following paragraph. Each population of Dusky Long-tailed Cuckoo has two song-types. From Sierra Leone to western Cameroon, birds have (1a) a song of 'three rising notes, hu hee wheeu', and (1b) 'a less frequently uttered whinnying series of rather plaintive notes, first accelerating and then slowing and descending tiutiutiutiutittiui-tiu-tiu-tiu…, reminiscent of [a] Halcyon kingfisher'. From western Cameroon eastwards, birds have (2a) a faster song, 'with three similar, less melodious notes wheet-wheet-wheet' and (2b) 'a fast, descending wheewheewheewheewhee…, almost twice as fast as [the] equivalent in Upper Guinea'. For Yellow-spotted Barbet, the main call in the west of the range is 'a series of 7-10 accelerating oop notes similar to a song of Hairy-breasted Barbet' Tricholaema hirsuta, while in the east birds give 'a characteristic purring rrurrrrrr… (lasting 1-2 seconds), unique among [African] barbets, uttered by adult and juvenile'. These remarkable circumstances have apparently gone unstudied. Both species are treated as monotypic in all recent world lists (

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“…Our results, based on the mitochondrial cytb gene, highlight a genetic break congruent with the vocal differences noted by Boesman & Collar (2019), consistent with their taxonomic suggestion to treat these populations as two species. However, our results also suggest a more complex evolutionary history for the Yellow-spotted Barbet than simply a Dahomey Gap split and a more complex pattern of molecular divergence than indicated by vocal variation alone, despite largely congruent sampling of vocal and genetic data.…”

“…Both the vocal analysis provided by Boesman & Collar (2019) and our results suggest greater diversity within this species than previously thought. Discordance between the geographic patterns presented by vocal variation and that of genetic variation are not unexpected (e.g., Nwankwo et al 2018).…”

“…Given this complexity, it is difficult to align our results directly with the simple Dahomey Gap split in vocal variation. We find it noteworthy that Boesman & Collar (2019) found the same vocal dialect in all sampled populations east of the Dahomey Gap, populations among which we found up to 10% average pair-wise molecular divergence. This clearly suggests that vocal and genetic variation in this species are decoupled.…”

“…Discordance between the geographic patterns presented by vocal variation and that of genetic variation are not unexpected (e.g., Nwankwo et al 2018). The complexities of this system presented jointly by the vocal (Boesman & Collar 2019) and molecular variation suggest that this taxon merits further research. How the vocal and genetic variation in a broader sense fit with the described, but not recognised subspecies, is beyond the scope of this note.…”

“…Differences in the vocalisations of the western and eastern populations were first noted by Borrow & Demey (2001). Boesman & Collar (2019) investigated this variation using the number of notes, length of longest note, pace of notes, and acceleration. Following criteria published by , they concluded that western and eastern populations should be recognised as separate species: Western Yellow-spotted Barbet B. dowsetti, occurring west of the Dahomey Gap, and Eastern Yellow-spotted Barbet B. duchaillui, to the east of it.…”