Bulbuls are a family of medium-sized passerine songbirds, distributed across most of Africa and Asia. Hybridization has been documented in a few genera, namely Criniger, Pycnonotus and Phyllastrephus. Some cases in the latter two genera have been studied in greater detail.
These Bulbuls represent a group of closely related passerines that are continuously distributed in Indo-Malaya. This distribution fits a ring species model driven by geographic isolation. Genetic analyses revealed significant gene flow between parapatric taxa, and reduced ancient gene between two taxa at the extremities of the ring (Fuchs et al., 2015; Pereira & Wake, 2015).
In the Eastern Arc Mountains of Africa, the Tiny Greenbul (P. debilis) is divided into two subspecies, the green-headed albigula and the grey-headed rabai. Coalescent analyses suggest limited gene flow, mostly from rabai into albigula (Fuchs, Fjeldsa & Bowie, 2011).
Several hybrids have been described, such as Red-vented Bulbul (P. cafer) x Himalayan Bulbul (P. leucogenys) in India (Sibley & Short, 1959). Hybridization among three South-African species, Black-eyed Bulbul (P. barbatus), Cape Bulbul (P. capensis) and Red-eyed Bulbul (P. nigricans), was mostly based on isolated reports (Keith, 1992; Markus, 1963; Markus, 1966; Markus, 1967). But recent genetic work demonstrated that the transition zone between the three species is characterized by an ecotone of vegetation types. These results suggest a bounded-hybrid-superiority model (Lloyd et al., 1997). A study of the vocal variation of these species showed that the Red-eyed Bulbul exhibits song characteristics of the other two species (Lloyd, Hulley & Craig, 1996).
In China, two subspecies of the Light-vented Bulbul (P. sinensis sinensis and P. s. hainanus) show little genetic divergence. This could be attributed to recent divergence or gene flow (Wu et al., 2011).
A genomic study found signatures of past gene flow events during the ice ages between island populations of Olive-winged Bulbul (Pycnonotus plumosus), Asian Red-eyed Bulbul (Pycnonotus brunneus), and Cream-vented Bulbul (Pycnonotus simplex) in the Indonesian archipelago (Cros et al., 2020).
Cros, E., Chattopadhyay, B., Garg, K. M., Ng, N. S., Tomassi, S., Benedick, S., Edwards, D. P. & Rheindt, F. E. (2020). Quaternary land bridges have not been universal conduits of gene flow. Molecular Ecology, 29(14): 2692-2706.
Fuchs, J., Ericson, P. G. P., Bonillo, C., Couloux, A. & Pasquet, E. (2015). The complex phylogeography of the Indo-Malayan Alophoixus bulbuls with the description of a putative new ring species complex. Molecular Ecology 24, 5460-5474.
Fuchs, J., Fjeldsa, J. & Bowie, R. C. K. (2011). Diversification across an altitudinal gradient in the Tiny Greenbul (Phyllastrephus debilis) from the Eastern Arc Mountains of Africa. Bmc Evolutionary Biology 11.
Keith, S. (1992). Hybridization among bulbuls of the genus Pycnonotus in southern Africa, vol. 63, pp. 187-188. SOUTH AFRICAN ORNITHOL SOC PO BOX 84394, GREENSIDE 2034, SOUTH AFRICA.
Lloyd, P., Craig, A. J. F. K., Hulley, P. E., Essop, M. F., Bloomer, P. & Crowe, T. M. (1997). Ecology and genetics of hybrid zones in the southern African Pycnonotus bulbul species complex. Ostrich 68, 90-96.
Lloyd, P., Hulley, P. E. & Craig, A. J. (1996). Comparisons of the vocalizations and social behaviour of southern African Pycnonotus bulbuls. Ostrich 67, 118-125.
Markus, M. (1963). Bulbuls from the zone of contact between Pycnonotus barbatus layardi Gurney, 1879, and Pycnonotus nigricans (Vieillot) in the Transvaal. Ostrich 34, 110.
Markus, M. (1966). Systematic notes on Pycnonotus from the southwestern Transvaal. Ostrich 37, 234.
Markus, M. B. (1967). Secondary intergradation amongst bulbuls of the genus Pycnonotus in the Transvaal Province, South Africa. Bull. Brit. Orn. Club 87, 17-23.
Pereira, R. J. & Wake, D. B. (2015). Ring species as demonstrations of the continuum of species formation. Molecular Ecology 24, 5312-5314.
Sibley, C. G. & Short, L. L. (1959). Hybridization in some Indian bulbuls Pycnonotus cafer x P. leucogenys. Ibis 101, 177-182.
Wu, Y. C., Li, H., Zou, F. S. & Hsu, Y. C. (2011). Lineages differentiation and population structure of the light-vented bulbul Pycnonotus sinensis in China based on mitochondrial DNA and microsatellite analysis. Integrative Zoology 6, 387-398.