The estrildid finches are small passerine birds of the Old World tropics and Australasia. Hybridization occurs in several genera, but only a few cases have been studied in detail.
Sympatric red and black colour morphs of the Gouldian Finch (E. gouldiae) show postzygotic incompatibilities. An experiment comparing pure and mixed breeds found large inviability effects. Mortality was most severe for hybrid females, consistent with Haldane’s Rule (Pryke & Griffith, 2009). These morphs may be on their way to becoming separate species. Assortative mating is probably mitigated by genes controlling colour expression and genes causing hybrid dysfunction on the Z-chromosome (Pryke, 2010).
A genomic analysis of this genus (sampling 11 of the 13 species) uncovered substantial autosomal introgression between sympatric species. There were, however, some divergent genomic regions containing colour genes, which could explain the phenotypic diversity in this group of birds (Stryjewksi & Sorenson, 2017). This study found one hybrid between Great-billed Mannikin (L. grandis) and Chestnut-breasted Mannikin (L. castaneothorax).
A hybrid between Chestnut-breasted Mannikin (L. castaneothorax) and Yellow-rumped Mannikin (Lonchura flaviprymna) was reported in northern Australia (Immelmann, 1962).
Hybrids between Chestnut-breasted Mannikin (L. castaneothorax) and Scaly-breasted Munia (L. punctulata) are sterile, probably due to genetic incompatibilities during spermatogenesis (Swan & Christidis, 1987).
The Long-tailed Finch is endemic to Northern Australia. Based on bill color, you can distinguish between two subspecies: the red-billed hecki and the yellow-billed acuticauda. Interestlingly, hybrids between both subspecies have orange bills. A survey of bill color across their range indicated that there is selection against hybrids (Griffith & Hooper, 2017). Differentiation between these subspecies is mainly concentrated on the Z-chromosome and is associated with two or more putative chromosomal inversions. One genomic region affecting bill colour is on the Z, but the main candidates are on chromosome 8 (Hooper et al., 2019).
Experimental work showed that hybrids probably have lower fertilization success because less sperm cells reach the perivitelline layer of the egg compared to pure pairs (Hurley et al., 2018).
Two subspecies of the Zebra Finch (T. guttata guttata and T. g. castanotis) mate assortatively based morphological and vocal cues (Clayton, 1990a). Several experiments were conducted to assess the effects of cross-fostering on mate choice (Clayton, 1989; Clayton, 1990b).
Mate choice experiments with three species of Blue Waxbill revealed that males and females use different criteria. Males focus on female size, while females rely on male ornamentation. This difference in mate choice criteria could lead to hybridization (Luddem et al., 2004).
Clayton, N. S. (1989). The Effects of Cross-Fostering on Selective Song Learning in Estrildid Finches. Behaviour 109, 163-175.
Clayton, N. S. (1990a). Assortative Mating in Zebra Finch Subspecies, Taeniopygia-Guttata-Guttata and T-G-Castanotis. Philosophical Transactions of the Royal Society of London Series B-Biological Sciences 330, 351-370.
Clayton, N. S. (1990b). The Effects of Cross-Fostering on Assortative Mating between Zebra Finch Subspecies. Animal Behaviour 40, 1102-1110.
Griffith, S.C. & Hooper, D.M. (2017) Geographical variation in bill colour in the Long-tailed Finch: evidence for a narrow zone of admixture between sub-species. Emu – Austral Ornithology, 117: 141-150.
Hooper, D. M., Griffith, S. C., & Price, T. D. (2019). Sex chromosome inversions enforce reproductive isolation across an avian hybrid zone. Molecular Ecology, 28(6), 1246-1262.
Hurley, L.L., Rowe, M. & Griffith, S.C. (2018) Differential sperm-egg interactions in experimental pairings between two subspecies and their hybrids in a passerine bird. Ecology and Evolution.
Immelmann, K. (1962) Besiedlungsgeschichte und Bastardierung von Lonchura castaneothorax und Lonchura flaviprymna in Nordaustralien. Journal of Ornithology 130(4), 344-357.
Luddem, S. T., Collins, S. A., Brooks, M. A. & Winter, M. (2004). Some males are choosier than others: Species recognition in blue waxbills. Behaviour 141, 1021-1039.
Pryke, S. R. (2010). Sex Chromosome Linkage of Mate Preference and Color Signal Maintains Assortative Mating between Interbreeding Finch Morphs. Evolution 64, 1301-1310.
Pryke, S. R. & Griffith, S. C. (2009). Postzygotic Genetic Incompatibility between Sympatric Color Morphs. Evolution 63, 793-798.
Swan, M. A. & Christidis, L. (1987). Impaired Spermatogenesis in the Finch Hybrid L-Castaneothorax by L-Punctulata – Transmission Electron-Microscopy and Genetic-Analysis. Gamete Research 17, 157-171.
Stryjewski, K.F. & Sorenson, M.D. (2017) Mosaic genome evolution in a recent and rapid avian radiation. Nature Ecology & Evolution 1, 1912-1922.