Icterids are colourful, small to medium-sized birds restricted to the New World. Hybridization occurs in several genera, but only a few cases have been studied in greater detail.
Red-winged Blackbirds (A. phoenicurus) are represented by two colour morphs. However, in California and Mexico “bicoloured” birds show a mosaic distribution. A genetic study indicated gene flow between these bicoloured birds and the typical colour morphs (Dufort & Barker, 2013).
Baltimore Oriole (I. galbula) and Bullock Oriole (I. bullockii) hybridize across the Great Plains in North America (Corbin & Sibley, 1977; Sibley & Short, 1964). Several parts of the hybrid zone have been studied (Anderson, 1971; Corbin, Sibley & Ferguson, 1979; Rising, 1983; Rising, 1996; Sutton, 1968; Sutton, 1938). The hybrid zone was characterized genetically by analysing the samples collected by Sibley and Short (1964). This study suggested selection against hybrids (Carling, Serene & Lovette, 2011), possibly due to different moult and migration schedules (Rohwer & Johnson, 1992). Estimates of gene flow revealed that nuclear loci introgress beyond the hybrid zone, in contrast to mtDNA (Jacobsen & Omland, 2012). A genomic study showed that this hybrid zone has shifted westwards and became narrower, suggesting stronger selection against hybrids (Walsh et al., 2020).
Other Icterus species that are known to hybridize include Yellow-backed Oriole (I. chrysater) and Yellow-tailed Oriole (I. mesomelas) (Olson, 1983), and Epaulet Oriole (I. cayanensis) and Moriche Oriole (I. chrysocephalus) (D’Horta, Da Silva & Ribas, 2008). The latter two have recently been classified as one species. Two subspecies of Orchard Oriole (I. spurius spurius and I. s. fuertesi) are not reciprocally monophyletic, but there is no evidence for gene flow (Baker et al., 2003).
The hybrid zone between two subspecies of the Common Grackle (Q. quiscula quiscula and Q. q versicolor) has been studied based on morphology (Huntington, 1952; Yang & Selander, 1968) and genetic markers (Zink, Rootes & Dittmann, 1991). The latter study suggested high levels of gene flow.
Great-tailed Grackle (Q. mexicanus) and Boat-tailed Grackle (Q. major) live sympatrically, but do not interbreed, possibly due to behavioural isolation mechanisms (Selander & Giller, 1961).
The Eastern Meadowlark (S. magna) and Western Meadowlark (S. neglecta) interbreed in North America (Szijj, 1966).
Anderson, B. W. (1971). Man’s influence on hybridization in two avian species in South Dakota. Condor, 342-347.
Baker, J. M., Lopez-Medrano, E., Navarro-Siguenza, A. G., Rojas-Soto, O. R. & Omland, K. E. (2003). Recent speciation in the orchard oriole group: Divergence of Icterus spurius spurius and Icterus spurius fuertesi. Auk 120, 848-859.
Carling, M. D., Serene, L. G. & Lovette, I. J. (2011). Using Historical DNA to Characterize Hybridization between Baltimore Orioles (Icterus Galbula) and Bullock’s Orioles (I. Bullockii). Auk 128, 61-68.
Corbin, K. W. & Sibley, C. G. (1977). Rapid evolution in orioles of the genus Icterus. Condor, 335-342.
Corbin, K. W., Sibley, C. G. & Ferguson, A. (1979). Genic Changes Associated with the Establishment of Sympatry in Orioles of the Genus Icterus. Evolution 33, 624-633.
D’Horta, F. M., Da Silva, J. M. C. & Ribas, C. C. (2008). Species limits and hybridization zones in Icterus cayanensis-chrysocephalus group (Aves: Icteridae). Biological Journal of the Linnean Society 95, 583-597.
Dufort, M. J. & Barker, F. K. (2013). Range dynamics, rather than convergent selection, explain the mosaic distribution of red-winged blackbird phenotypes. Ecology and Evolution 3, 4910-4924.
Huntington, C. E. (1952). Hybridization in the Purple Grackle, Quiscalus-Quiscula. Systematic Zoology 1, 149-170.
Jacobsen, F. & Omland, K. E. (2012). Extensive introgressive hybridization within the northern oriole group (Genus Icterus) revealed by three-species isolation with migration analysis. Ecology and Evolution 2, 2413-2429.
Olson, S. L. (1983). A Hybrid between the Orioles Icterus-Chrysater and Icterus-Mesomelas. Auk 100, 733-735.
Rising, J. (1983). The progress of Oriole hybridization in Kansas. The Auk, 885-897.
Rising, J. D. (1996). The stability of the Oriole hybrid zone in western Kansas. Condor, 658-663.
Rohwer, S. & Johnson, M. S. (1992). Scheduling Differences of Molt and Migration for Baltimore and Bullocks Orioles Persist in a Common Environment. Condor 94, 992-994.
Selander, R. K. & Giller, D. R. (1961). Analysis of sympatry of great-tailed and boat-tailed grackles. Condor, 29-86.
Sibley, C. G. & Short, L. L. (1964). Hybridization in the orioles of the Great Plains. Condor, 130-150.
Sutton, G. (1968). Oriole hybridization in Oklahoma. Bull Ok Ornit Soc 1, 1-7.
Sutton, G. M. (1938). Oddly plumaged orioles from western Oklahoma. The Auk, 1-6.
Szijj, L. J. (1966). Hybridization and the nature of the isolating mechanism in sympatric populations of meadowlarks (Sturnella) in Ontario. Zeitschrift für Tierpsychologie 23, 677-690.
Walsh, J., Billerman, S. M., Rohwer, V. G., Butcher, B. G., & Lovette, I. J. (2020). Genomic and plumage variation across the controversial Baltimore and Bullock’s oriole hybrid zone. The Auk, 137(4), ukaa044.
Yang, S. Y. & Selander, R. K. (1968). Hybridization in Grackle Quiscalus Quiscula in Louisiana. Systematic Zoology 17, 107-&.
Zink, R. M., Rootes, W. L. & Dittmann, D. L. (1991). Mitochondrial-DNA Variation, Population-Structure, and Evolution of the Common Grackle (Quiscalus-Quiscula). Condor 93, 318-329.
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