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). This explanation was confirmed by genomic analyses (Taylor et al., 2021).
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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