Geographical variation in the Bar-tailed Trogon Apaloderma vittatum

Distributed in four areas of Africa (‘Eastern’—east of the Gregory Rift Valley from central Kenya south to Malawi; ‘Central’—west of the Gregory Rift from western Kenya and western Uganda south to Burundi and eastern Democratic Republic of Congo; ‘Western’—Nigeria and Cameroon plus Bioko; and Angola), Bar-tailed Trogon Apaloderma vittatum is at present widely judged to be monotypic. However, photographic evidence reveals three different facial patterns in these populations, with a bare ear spot and whitish-green bill in Eastern and Angolan birds, a bare ear spot, golden-yellow bill and bare gape line in Central birds, and these same features plus a distinct bare crescent above the eye in Western birds. Central and Western birds also have broader, more distinct barring on the wing panel. Moreover, there is a stepped cline in sizes in these three distinct populations, from Eastern plus Angola (largest) through Central to Western (smallest), with Bioko birds having notably short tails; and songs also decelerate from east to west, but with too much overlap to be diagnosable. Nevertheless, under the Tobias criteria the morphological differences of Central and Western birds vs. Eastern and Angolan populations accord the former species rank, for which the name camerunense is available. This latter species itself breaks into three subspecies, with the nominate in Nigeria and Cameroon, francisci on Bioko, and a new name for the hitherto taxonomically unrecognised Central populations.