The greenbuls and bristlebills are a group of birds within the bulbul family Pycnonotidae, found only within Africa. They are all largely drab olive-green above, and paler below, with few distinguishing features.
The "aberrant greenbuls" of the genera Bernieria and Xanthomixis are actually Malagasy warblers (Cibois et al. 2001). To recognize this, they are also called Bernieria and tetrakas rather than greenbuls. Likewise, the Golden Greenbul (Calyptocichla serina) is not a typical greenbul, but apparently the representative of a distinct and ancient lineage of bulbuls, which might include the Black-collared Bulbul, Neolestes torquatus (Moyle & Marks 2006).
Cibois, Alice; Slikas, Beth; Schulenberg, Thomas S. & Pasquet, Eric (2001): An endemic radiation of Malagasy songbirds is revealed by mitochondrial DNA sequence data. Evolution55(6): 1198–1206. DOI:10.1554/0014-3820(2001)055[1198:AEROMS]2.0.CO;2 PDF fulltext
Moyle, Robert G. & Marks, Ben D. (2006): Phylogenetic relationships of the bulbuls (Aves: Pycnonotidae) based on mitochondrial and nuclear DNA sequence data. Molecular Phylogenetics and Evolution40(3): pp. 687–695. doi:10.1016/j.ympev.2006.04.015 (HTML abstract)