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The Gulf of Guinea thrush Turdus olivaceofuscus is endemic to the islands of São Tomé (nominate olivaceofuscus) and Príncipe (subspecies xanthorhynchus). Relationships between the two island taxa, originally described as two different species, are uncertain. This problem has been difficult to resolve due to the scarcity of information from Príncipe birds. A focused effort to find birds from Príncipe resulted in new observations, the first records of its song, and in the capture of four individuals, which provided new data for analyses. We obtained additional data from museum specimens. Our analyses indicate that the two populations differ substantially in size, bill shape and bill, eye and leg coloration as well as in several plumage characteristics. In addition, xanthorhynchus utters a low call of a type not previously recorded in the genus Turdus. Genetic evidence corroborates the phenotypic evidence: both taxa constitute clearly independent evolutionary lineages (2368 bp from the mitochondrial markers ND2, ND3 and cytochrome b (cyt- b) from four individuals of each population). Genetic divergence between the taxa (cyt- b: uncorrected: 6.4%; corrected: 8.8%) suggests that they may have been isolated for over 4 Myr. These results support the split of T. olivaceofuscus into two species: São Tomé thrush T. olivaceofuscus and Príncipe thrush Turdus xanthorhynchus. The latter is a very rare species, restricted to the most inaccessible parts of Príncipe Island. Phylogenetic inference favoured the African thrush Turdus pelios as the closest living relative to the Gulf of Guinea species. [ABSTRACT FROM AUTHOR]