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).
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).
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