The Passerellidae (New World sparrows or American sparrows) are a large family of seed-eating passerine birds with distinctively finch-like bills. The American Ornithological Society split the family from Emberizidae (Old World buntings) in 2017. 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; Walsh et al., 2019). This genetic exchange might result in adaptive introgression of genes that confer an advantage in the saltmarshes (Walsh et al., 2018). The hybrid zone is maintained by local environmental features and fits a mosaic hybrid zone model (Walsh et al., 2016a). Reproductive isolation between both species might be partly explained by differences in sperm morphology (Cramer et al. 2021). RFLP markers have been developed 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).
A study on adaptation to salt marshes found evidence for gene flow between two other subspecies of Nelson’s sparrow: nelsoni and subvirgatus (Walsh et al., 2019).
Apart from hybridization between Sharp-tailed Sparrows, 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 White-browed Brush-finch (A. torquatus) complex. Several of these species might be hybridizing (Cadena & Cuervo, 2010).
In the foothills of the Andes, Carantón‑Ayala et al. (2018) found a strange specimen of Brush finch (genus Atlapetes). Genetic data suggested that it is a hybrid between White-naped Brush finch (Atlapetes albinucha) and Dusky-headed Brush finch (Atlapetes fuscoolivaceus). The mitochondrial DNA of the hybrid, which is only transmitted through the female lineage, clustered with Dusky-headed Brush finch, indicating that this was the female parent. The nuclear DNA, on the other hand, pointed to White-naped Brush finch as the father. The morphological characteristics of the hybrid were in line with this conclusion.
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).
A study on adaptation to salt marshes found evidence for gene flow between two subspecies of the Song Sparrow (Melospiza melodia: gouldii and pusillula) and two subspecies of the Swamp Sparrow (Melospiza georgiana: georgiana and nigrescens)(Walsh et al., 2019).
In California, a putative hybrid zone between two lineages of the Savannah Sparrow (Passerculus sandwichensis) is suggested by morphological and genetic data (Benham & Cheviron, 2019).
A study on adaptation to salt marshes found evidence for gene flow between two subspecies of the Savannah Sparrow: nevadensis and beldingi (Walsh et al., 2019).
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, 1958). Genomic analyses showed locus-specific patterns of introgression (Kingston et al., 2012; Kingston et al., 2014; Kingston et al., 2017).
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). The latter hybrid has been confirmed with molecular data (Jones et al., 2019).
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).
Benham P.M. & Cheviron Z.A. (2019) Divergent mitochondrial lineages arose within a large, panmictic population of the Savannah sparrow (Passerculus sandwichensis). Molecular Ecology.
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.
Carantón‑Ayala D., Avendaño J.E., Cadena, C.D. (2018) Hybridization in brushfinches (Atlapetes , Emberizidae) from the southeast Andes of Colombia: a consequence of habitat disturbance? Journal of Ornithology
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.
Cramer, E. R., Grønstøl, G., Maxwell, L., Kovach, A. I., & Lifjeld, J. T. (2021). Sperm length divergence as a potential prezygotic barrier in a passerine hybrid zone. Ecology and Evolution, 11: 9489-9497.
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.
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.
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.
Jones, A.W., Boone, A.T., Brennan, C.L., Rodewald, P.G., & Kistler, E. (2019). Documentation of a hybrid Spizella sparrow using spectographic and genetic analysis. Bulletin of the British Ornithologists’ Club, 139(2), 105-110.
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.
Kingston, S. E., T. L. Parchman, Z. Gompert, C. A. Buerkle and M. J. Braun (2017). Heterogeneity and concordance in locus-specific differentiation and introgression between species of towhees. Journal of Evolutionary Biology 30(3): 474-485.
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-&.
Paludan, K. (1940). Contributions to the ornithology of Iran. Ejnar Munksgaard.
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.
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. (1958). Hybridization in the Red-Eyed Towhees of Mexico: The Eastern Plateau Populations. The Condor 60, 85-104.
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.
Walsh, J., Kovach, A.I., Olsen, B.J., Shriver, W.G. & Lovette, I.J. (2018) Bidirectional adaptive introgression between two ecologically divergent sparrow species. Evolution.
Walsh, J., Benham, P. M., Deane‐Coe, P. E., Arcese, P., Butcher, B. G., Chan, Y. L., … & Shriver, W. G. (2019). Genomics of rapid ecological divergence and parallel adaptation in four tidal marsh sparrows. Evolution Letters.
Walsh, J., Clucas, G. V., MacManes, M. D., Thomas, W. K., & Kovach, A. I. (2019). Divergent selection and drift shape the genomes of two avian sister species spanning a saline–freshwater ecotone. Ecology and Evolution, 9(23):13477-13494.
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.