This large family contains birds known as Buntings in the Old World and (American) Sparrows in the New World. Other North American birds in this family are Juncos and Towhees. Hybridization has been documented in several genera. Intergeneric hybrids between Junco, Zonotrichia and Melospiza have been reported (Dickerman, 1961).
Genetic analysis of the Sharp-tailed Sparrow showed two distinct groups (Rising & Avise, 1993), that were recognised as distinct species in 1995, namely Nelson’s Sharp-tailed Sparrow (A. nelsoni) and Saltmarsh Sharp-tailed Sparrow (A. caudacutus). These species overlap and hybridize in southern Maine (Hodgman, Shriver & Vickery, 2002), which results in asymmetrical introgression (Shriver et al., 2005; Walsh et al., 2016b). This hybrid zone is maintained by local environmental features and fits a mosaic hybrid zone model (Walsh et al., 2016a). Another study developed RFLP markers to detect hybrids (Walsh et al., 2011). Hybrids and backcrosses can be identified using genetic markers, in contrast to morphological features (Walsh et al., 2015). Also, detailed analysis of the population structure of A. caudacutus revealed five populations connected by gene flow (Walsh et al., 2012).
Apart from hybridization between Sharp-tailed Sparrows, Murray (Murray, 1968) described a hybrid between Le Conte’s Sparrow (A. leconteii) and Saltmarsh Sharp-tailed Sparrow. And in a captive breeding program, the now extinct Dusky Seaside Sparrow (A. maritimus nigricans) was crossed with a closely related subspecies, Scott’s Seaside Sparrow (A. m. peninsulae) (Zink & Kale, 1995).
An extensive study combining morphological, vocal, ecological and genetic data culminated in the recognition of eight species in the A. torquatus complex. Several of these species might be hybridizing (Cadena & Cuervo, 2010).
Yellowhammer (E. citronella) and Pine Bunting (E. leucocephalos) hybridize in Russia (Panov, Rubtsov & Monzikov, 2003; Panov, Roubtsov & Monzikov, 2003). Genetic analyses showed low divergence in mtDNA compared to nDNA, which could be attributed to recent divergence or introgression (Irwin, Rubtsov & Panov, 2009). The latter hypothesis (introgression) was indicated as more likely by a phylogenetic analysis which showed that these species are not sister species (Rubtsov & Opaev, 2012). Despite large morphological differences, both species produce indistinguishable territorial songs (Tietze, Wassmann & Martens, 2012).
A study on the vocal variation among three subspecies of the Reed Bunting (E. schoeniclus) uncovered a “hybrid zone” between E. s. schoeniclus and two southern subspecies (E. s. intermedia and E. s. witherbyi). This hybrid zone has not been confirmed by morphological or genetic data (Matessi, Pilastro & Marin, 2000).
A phylogenetic analysis of “African brown buntings” lacked reciprocal monophyly, possibly due to incomplete lineage sorting. The analysis revealed one hybrid specimen in the dataset (Olsson, Yosef & Alstrom, 2013). House Bunting (E. sahari) and Striolated Bunting (E. striolata) are considered to be distinct species. A phylogeographic study across the Saharo-Arabian range of these species revealed incongruence between mtDNA and morphology in certain populations. This pattern can be explained by incomplete lineage sorting or introgressive hybridization (Schweizer, Shirihai et al. 2017).
Gholamhosseini et al. (2017) revisited a hybrid zone between Black-headed Bunting (Emberiza melanocephala) and Red-headed Bunting (E. bruniceps) in northern Iran that has been studied by Paludan (1940) and Haffer (1977). The hybrid zone has expanded westward by approximately 170 km. From a climatic point of view, the Black-headed Bunting could occur farther to the east, but it doesn’t. Probably, it is out-competed by the Red-headed Bunting which might be expanding eastward due to land use changes by humans (i.e. deforestation and extension of agriculture).
The Dark-eyed Junco (J. hyemalis) complex exemplifies a case of extremely rapid diversification. Several subspecies have been described and continue to hybridize in certain locations. This complex has been studied using molecular (Mila et al., 2007), morphological (Ferree, 2013) and vocal (Reichard, 2014) approaches, but remains largely problematic.
The Song Sparrow (M. melodia) complex comprises numerous (morphological) subspecies, but lacks genetic population structure (Zink & Dittmann, 1993). This complex can be regarded as a ring species around the Sierra Nevada and Mojave Desert (Patten & Pruett, 2009). The outer subspecies of this ring (heermanni and fallax) differ substantially in plumage, song and habitat use (Patten, Rotenberry & Zuk, 2004).
Mitochondrial haplotypes of the Fox Sparrow (P. iliaca) fall into four distinct groups (Zink, 1994) of which two hybridize (megarhyncha and schistacea), although they are not sister clades (Zink & Weckstein, 2003). Microsatellites lack differentiation, which can be attributed to incomplete lineage sorting and hybridization (Zink, 2008).
The hybrid zone between Spotted Towhee (P. maculatus) and Collared Towhee (P. ocai) in Mexico has been intensively studied from a morphological point of view (Sibley, 1950; Sibley, 1954; Sibley & Sibley, 1964; Sibley & West, 1959). Recent genomic analyses showed locus-specific patterns of introgression (Kingston et al., 2012; Kingston et al., 2014).
A hybrid between Abert’s Towhee (P. aberti) and Canyon Towhee (P. fuscus) has also been reported (Johnson & Hopp, 2010). These species are sometimes included in the genus Melozone.
Only two hybrids have been reported: a possible hybrid between Chipping Sparrow S. passerina [in the paper called arizonae]) and Clay-colored Sparrow (S. pallida) in Mexico (Parkes, 1990) and a hybrid between Field Sparrow (S. pusilla) and Clay-colored Sparrow in Northern Vermont (Hoag, 1999).
Several hybrid have been reported, such as Golden-crowned Sparrow (Z. atricapilla) x White-throated Sparrow (Z. albicollis), Harris’ Sparrow (Z. querula) x White-crowned Sparrow (Z. leucophrys), and Golden-crowned Sparrow x White-crowned Sparrow (Miller, 1940; Payne, 1979). Intergeneric hybrids between Dark-eyed Junco (Junco hyemalis) and White-throated Sparrow have been described (Jung et al., 1994; Short & Simon, 1965; Townsend, 1883).
Phylogenetic relationships, based on mtDNA, are consistent with patterns of hybridization (Zink, Dittmann & Rootes, 1991), for example the hybridizing White-crowned and Golden-crowned Sparrows possess nearly identical haplotypes. This result was questioned by another study which showed that these identical haplotypes may be the result of recent introgression (Weckstein et al., 2001).
A study comparing gene flow of mtDNA and nDNA across an elevational gradient of Rufous-collared Sparrows (Z. capensis) populations found restricted gene flow of mtDNA, which may indicate selection against certain mitochondrial haplotypes (Cheviron & Brumfield, 2009).
Cadena, C. D. & Cuervo, A. M. (2010). Molecules, ecology, morphology, and songs in concert: how many species is Arremon torquatus (Aves: Emberizidae)? Biological Journal of the Linnean Society 99, 152-176.
Cheviron, Z. A. & Brumfield, R. T. (2009). Migration-Selection Balance and Local Adaptation of Mitochondrial Haplotypes in Rufous-Collared Sparrows (Zonotrichia Capensis) Along an Elevational Gradient. Evolution 63, 1593-1605.
Dickerman, R. W. (1961). Hybrids among the fringillid genera Junco-Zonotrichia and Melospiza. The Auk, 627-632.
Dickerman, R. W. (1968). A Hybrid Grasshopper Sparrow× Savannah Sparrow. The Auk, 312-315.
Ferree, E. D. (2013). Geographic Variation in Morphology of Dark-Eyed Juncos and Implications for Population Divergence. Wilson Journal of Ornithology 125, 454-470.
Gholamhosseini, A., Aliabadian, M., Darvish, J., Töpfer, T. & Sætre, G.-P. (2017). An Expanding Hybrid Zone between Black-Headed and Red-Headed Buntings in Northern Iran. Ardea 105, 27-36.
Haffer, J. (1977). Secondary contact zones of birds in northern Iran. Zoologisches Forschungsinstitut und Museum Alexander Koenig.
Hoag, D. J. (1999). Hybridization between Clay-colored sparrow and Field sparrow in northern Vermont. Wilson Bulletin 111, 581-584.
Hodgman, T. P., Shriver, W. G. & Vickery, P. D. (2002). Redefining range overlap between the Sharp-tailed Sparrows of coastal New England. Wilson Bulletin 114, 38-43.
Irwin, D. E., Rubtsov, A. S. & Panov, E. N. (2009). Mitochondrial introgression and replacement between yellowhammers (Emberiza citrinella) and pine buntings (Emberiza leucocephalos) (Aves: Passeriformes). Biological Journal of the Linnean Society 98, 422-438.
Johnson, R. R. & Hopp, S. L. (2010). The First Reported Hybridization of Abert’s and Canyon Towhees (Pipilo spp.). Wilson Journal of Ornithology 122, 399-402.
Jones, A. L., Shriver, W. G., Bulgin, N. L., Lockwood, R. & Vickery, P. D. (2003). A probable grasshopper X savannah sparrow hybrid singing a song sparrow song. Wilson Bulletin 115, 231-236.
Jung, R. E., Morton, E. S. & Fleischer, R. C. (1994). Behavior and Parentage of a White-Throated Sparrow X Dark-Eyed Junco Hybrid. Wilson Bulletin 106, 189-202.
Kingston, S. E., Jernigan, R. W., Fagan, W. F., Braun, D. & Braun, M. J. (2012). Genomic variation in cline shape across a hybrid zone. Ecology and Evolution 2, 2737-2748.
Kingston, S. E., Navarro-Siguenza, A., Garcia-Trejo, E. A., Vazquez-Miranda, H., Fagan, W. F. & Braun, M. J. (2014). Genetic differentiation and habitat connectivity across towhee hybrid zones in Mexico. Evolutionary Ecology 28, 277-297.
Matessi, G., Pilastro, A. & Marin, G. (2000). Variation in quantitative properties of song among European populations of reed bunting (Emberiza schoeniclus) with respect to bill morphology. Canadian Journal of Zoology-Revue Canadienne De Zoologie 78, 428-437.
Mila, B., McCormack, J. E., Castaneda, G., Wayne, R. K. & Smith, T. B. (2007). Recent postglacial range expansion drives the rapid diversification of a songbird lineage in the genus Junco. Proceedings of the Royal Society B-Biological Sciences 274, 2653-2660.
Miller, A. H. (1940). A hybrid between Zonotrichia coronata and Zonotrichia leucophrys. Condor, 45-48.
Murray, B. G. (1968). Relationships of Sparrows in Genera Ammodramus Passerherbulus and Ammospiza with a Description of a Hybrid Le Contes X Sharp-Tailed Sparrow. Auk 85, 586-&.
Olsson, U., Yosef, R. & Alstrom, P. (2013). Assessment of species limits in African ‘brown buntings’ (Emberiza, Passeriformes) based on mitochondrial and nuclear sequence data. Ibis 155, 534-543.
Paludan, K. (1940). Contributions to the ornithology of Iran. Ejnar Munksgaard.
Panov, E., Rubtsov, A. & Monzikov, D. (2003). Relationships between Two Species of Buntings (Emberiza citrinella and E. leucocephalos) Hybridizing in the Zones of Overlap of Their Ranges. Zool. Zh 82, 470-484.
Panov, E. N., Roubtsov, A. S. & Monzikov, D. G. (2003). Hybridization between yellowhammer and pine bunting in Russia. Dutch Birding 25, 17-31.
Parkes, K. C. (1990). Additional Records of Birds from the Distrito Federal, Mexico, Including a Possible Hybrid Spizella. Condor 92, 1080-1081.
Patten, M. A. & Pruett, C. L. (2009). The Song Sparrow, Melospiza melodia, as a ring species: patterns of geographic variation, a revision of subspecies, and implications for speciation. Systematics and Biodiversity 7, 33-62.
Patten, M. A., Rotenberry, J. T. & Zuk, M. (2004). Habitat selection, acoustic adaptation, and the evolution of reproductive isolation. Evolution 58, 2144-2155.
Payne, R. B. (1979). Two apparent hybrid Zonotrichia sparrows. The Auk, 595-599.
Reichard, D. G. (2014). Male Dark-eyed Juncos (Junco hyemalis) Respond Differentially to Playback of Local and Foreign Song. Wilson Journal of Ornithology 126, 605-611.
Rising, J. D. & Avise, J. C. (1993). Application of Genealogical-Concordance Principles to the Taxonomy and Evolutionary History of the Sharp-Tailed Sparrow (Ammodramus-Caudacutus). Auk 110, 844-856.
Rubtsov, A. S. & Opaev, A. S. (2012). Phylogeny reconstruction of the yellowhammer (Emberiza citrinella) and pine bunting (Emberiza leucocephala) based on song and morphological characters. Biology Bulletin 39, 715-728.
Schweizer, M., H. Shirihai, H. Schmaljohann and G. M. Kirwan (2017). Phylogeography of the House Bunting complex: discordance between species limits and genetic markers. Journal of Ornithology: 1-15.
Short, L. L. & Simon, S. W. (1965). Additional hybrids of the Slate-colored Junco and the White-throated Sparrow. Condor, 438-442.
Shriver, W. G., Gibbs, J. P., Vickery, P. D., Gibbs, H. L., Hodgman, T. P., Jones, P. T. & Jacques, C. N. (2005). Concordance between morphological and molecular markers in assessing hybridization between sharp-tailed sparrows in New England. Auk 122, 94-107.
Sibley, C. G. (1950). Species formation in the red-eyed towhees of Mexico. University of California Press.
Sibley, C. G. (1954). Hybridization in the Red-Eyed Towhees of Mexico. Evolution 8, 252-290.
Sibley, C. G. & Sibley, F. C. (1964). Hybridization in the red-eyed towhees of Mexico: the populations of the southeastern plateau region. The Auk, 479-504.
Sibley, C. G. & West, D. A. (1959). Hybridization in the Rufous-sided Towhees of the Great Plains. The Auk, 326-338.
Tietze, D. T., Wassmann, C. & Martens, J. (2012). Territorial song does not isolate Yellowhammers (Emberiza citrinella) from Pine Buntings (E. leucocephalos). Vertebrate Zoology 62, 113-122.
Townsend, C. (1883). Description of a hybrid sparrow (Zonotrichia albicollis x Junco hyemalis). Bull. Nuttall Ornithol. Club 8, 78-80.
Walsh, J., Kovach, A. I., Babbitt, K. J. & O’Brien, K. M. (2012). Fine-Scale Population Structure and Asymmetrical Dispersal in an Obligate Salt-Marsh Passerine, the Saltmarsh Sparrow (Ammodramus Caudacutus). Auk 129, 247-258.
Walsh, J., Kovach, A. I., Lane, O. P., O’Brien, K. M. & Babbitt, K. J. (2011). Genetic Barcode Rflp Analysis of the Nelson’s and Saltmarsh Sparrow Hybrid Zone. Wilson Journal of Ornithology 123, 316-322.
Walsh, J., Rowe, R. J., Olsen, B. J., Shriver, W. G. & Kovach, A. I. (2016a). Genotype-environment associations support a mosaic hybrid zone between two tidal marsh birds. Ecol Evol 6, 279-94.
Walsh, J., Shriver, W. G., Olsen, B. J. & Kovach, A. I. (2016b). Differential introgression and the maintenance of species boundaries in an advanced generation avian hybrid zone. BMC Evol Biol 16, 65.
Walsh, J., Shriver, W. G., Olsen, B. J., O’Brien, K. M. & Kovach, A. I. (2015). Relationship of phenotypic variation and genetic admixture in the Saltmarsh-Nelson’s sparrow hybrid zone. The Auk 132, 704-716
Weckstein, J. D., Zink, R. M., Blackwell-Rago, R. C. & Nelson, D. A. (2001). Anomalous variation in mitochondrial genomes of White-crowned (Zonotrichia leucophrys) and Golden-crowned (Z-atricapilla) sparrows: Pseudogenes, hybridization, or incomplete lineage sorting? Auk 118, 231-236.
Zink, R. M. (1994). The Geography of Mitochondrial-DNA Variation, Population-Structure, Hybridization, and Species Limits in the Fox-Sparrow (Passerella-Iliaca). Evolution 48, 96-111.
Zink, R. M. (2008). Microsatellite and Mitochondrial DNA Differentiation in the Fox Sparrow. Condor 110, 482-492.
Zink, R. M. & Dittmann, D. L. (1993). Gene Flow, Refugia, and Evolution of Geographic-Variation in the Song Sparrow (Melospiza-Melodia). Evolution 47, 717-729.
Zink, R. M., Dittmann, D. L. & Rootes, W. L. (1991). Mitochondrial-DNA Variation and the Phylogeny of Zonotrichia. Auk 108, 578-584.
Zink, R. M. & Kale, H. W. (1995). Conservation Genetics of the Extinct Dusky Seaside Sparrow Ammodramus-Maritimus-Nigrescens. Biological Conservation 74, 69-71.
Zink, R. M. & Weckstein, J. D. (2003). Recent evolutionary history of the fox sparrows (Genus : Passerella). Auk 120, 522-527.