The New World Warblers or Wood-Warblers are a group of passerine birds restricted to the New World. Hybridization is common in this family (Willis, Symula & Lovette, 2014) and speciation seems to be context-specific (Irwin et al., 2018). Several intergeneric hybrids have been documented:
- Northern Waterthrush (Seiurus novaeboracensis) x Blackpoll Warbler (Setophaga striata) (Short & Robbins, 1967). However, Parkes (1995) argues that the Setophaga species in this cross was a Cape May Warbler (S. tigrina)
- Black-and-White Warbler (Mniotilia varia) x Cerulean Warbler (Setophaga cerulea) (Parkes, 1978)
- Black-and-White Warbler x Blackburnian Warbler (S. fusca) (Parkes, 1983)
- Mourning Warbler (Oporornis philadelphia) x Common Yellowthroat (Geothlypis trichas) (Bledsoe, 1988)
- Blue-winged Warbler (Vermivora pinus) x Kentucky Warbler (Oporornis formosus) (Graves, 1988)
- Black-and-White Warbler (Mniotilia varia) x Yellow-rumped Warbler (Setophaga coronata) (Vallender et al., 2009a)
- Sutton’s Warbler (S. americana) x Yellow-throated Warbler (S. dominica) (Anich et al., 2012)
Toews et al. (2018) described a three-way hybrid: a female hybrid between a golden-winged warbler (Vermivora chrysoptera) and a blue-winged warbler (V. cyanoptera) mated with a chestnut-sided warbler (Setophaga pensylvanica).
Pictures of a possible Blackpoll Warbler x Bay-breasted Warbler (S. castanea) can be found here.
Hybrids between MacGilivray’s Warbler (O. tolmiei) and Mourning Warbler (O. philadelphia) have been documented (Cox, 1973; Hall, 1979; Patti & Myers, 1976). Genetic analyses uncovered an extensive hybrid zone in British Columbia (Irwin et al., 2009). Songs of these species are differentiated in allopatry, but they converge in the hybrid zone (Kenyon et al., 2011).
Hybrids between Nashville Warbler (O. ruficapilla) and Tennessee Warbler (O. peregrine) have been documented (Parkes, 1996).
Ralston et al. (2015) describe a hybrid between Orange-Crowned Warbler (O. celata) and Nashville Warbler (O. ruficapilla) based on genetic and morphometric data.
Two subspecies of the Orange-Crowned Warbler (O. c. celata and O. c. lutescens) might be interbreeding in Alaska. This suggestion is based on genetic (Bull et al., 2010) and morphological analyses (Gilbert & West, 2015).
There is a hybrid zone between Colima Warbler (Oreothlypis crissalis) x Virginia’s Warbler (O. virginiae) in the Davis Mountains of Texas (Bryan & Lockwood, 2018).
Phylogenetic analysis of the Phaeothlypis wood-warbler complex in South America revealed a phenotypic hybrid zone between bright and dark plumage forms. Furthermore, a contact zone between highly divergent mitochondrial haplotypes was discovered about 1000 km north of the phenotypic hybrid zone (Lovette, 2004).
The 27 species of Setophaga (previously Dendroica) wood-warblers represent a spectacular adaptive radiation that possibly started in the Late Miocene or Early Pliocene (Lovette & Bermingham, 1999; Lovette et al., 1999). Several species engage in hybridization:
- Townsend’s Warbler (S. townsendi) x Black-throated Gray Warbler (S. nigrescens) (Rohwer, 1994)
- Townsend’s Warbler (S. townsendi) x Black-throated Green Warbler (S. virens) (Rohwer, 1994; Toews, Brelsford & Irwin, 2011)
- Magnolia Warbler (S. magnolia) x Yellow-rumped Warbler (S. coronata) (Latta, Parkes & Wunderle, 1998)
- Hermit Warbler (S. occidentalis) x Black-throated Gray Warbler (S. nigrescens) (Rohwer, Wood & Bermingham, 2000)
- Kirtland’s Warbler (S. kirtlandii) x Blackburnian Warbler (S. fusca) (Latta & Parkes, 2001)
- Chestnut-sided Warbler (S. pensylvanica) x Magnolia Warbler (S. magnolia) (Burrell et al., 2016)
- Cerulean Warbler (S. cerulea) x Black-throated Blue Warbler (S. caerulescens) (Delancey et al., 2019)
In a hybrid zone between Black‐throated Green Warbler (Setophaga virens) and Townsend’s warbler (S. townsendi) song seems to play a minor role in reproductive isolation. If song is an important reproductive barrier between these species, there should be a strong relationship between song and genotype. This was not the case (Kenyon et al., 2017).
Hybrids between Hermit Warbler (S. occidentalis) and Townsend’s Warbler (S. coronata) were first documented based on morphology and song (Jewett, 1944; Morrison & Hardy, 1983). Rohwer and Wood (1998) described three hybrid zones, two in Washington and one in Oregon. The frequency of hybrid in these hybrid zones was related to the time since contact and dispersal of birds into the hybrid zone (Rohwer & Martin, 2007). Hybrids lay smaller clutches compared to the parental species (Pearson & Rohwer, 1998), but they showed no evidence of inviability (Smith & Rohwer, 2000). Rohwer and Wood (1998) also indicated that the hybrid zone was moving, possibly because of the dominance of Townsend’s Warbler. This hypothesis was supported by subsequent studies, Townsend’s Warblers lay bigger clutches (Pearson & Rohwer, 1998), were more successful in maintaining territories and attracting mates (Pearson, 2000), and were more aggressive (Pearson & Rohwer, 2000), this latter observation was also confirmed by higher androgen levels (Owen-Ashley & Butler, 2004). Patterns of habitat use were also consistent with Townsend’s Warbler dominance (Pearson & Manuwal, 2000). It was thus concluded that the mechanism for hybrid zone movement is competitive exclusion (Krosby & Rohwer, 2010). Genetic analysis revealed that mtDNA spreads across the hybrids zones much wider than phenotypic characters (Rohwer, Bermingham & Wood, 2001). This might be a historical footprint of the moving hybrid zone. Probably, northern populations of Hermit Warbler were diverging in a refugium before being displaced by aggressive Townsend’s Warblers (Krosby & Rohwer, 2009). However, a study comparing samples from 1987-1994 with 2015-2016 found no evidence for hybrid zone movement (Wang et al., 2019).
The Yellow-rumped Warbler complex comprises four subspecies (coronata, auduboni, nigrifrons and goldmani). Phylogenetic analysis of this complex showed that coronata and auduboni cluster together (Mila, Smith & Wayne, 2007). Indeed, a hybrid zone between these subspecies has been described early on (Barrowclough, 1980; Hubbard, 1969; Hubbard, 1970). Despite weak assortative mating, these subspecies remain distinct, suggesting strong postmating selection (Brelsford & Irwin, 2009). The mechanisms of selection against hybrids remain unknown. One study tested the hypothesis that hybrids have higher parasite prevalence, but did not find support for this mechanism (Cozzarolo et al., 2018). There are also no differences between sex and age classes (Toews et al., 2018)
Detailed genetic analyses revealed that mtDNA introgressed from coronata into auduboni, a patterns that coincides with a shift to migratory behaviour (Toews et al., 2014b). Further research indicated a migratory divide between the two subspecies (Toews et al., 2014a). This hybrid zone has also been used to unravel the genetic basis of plumage coloration in these birds (Brelsford et al., 2017).
Apart from the hybrid zone between coronata and auduboni, a cryptic hybrid zone between auduboni and nigrifrons was uncovered (Mila et al., 2011). These complex hybridization patterns also led to the suggestion that auduboni might be a hybrid lineage between coronata and nigrifrons (Brelsford, Mila & Irwin, 2011).
The study of hybridization between Golden-winged Warbler (V. chrysoptera) and Blue-winged Warbler (V. pinus) has a long history (Berger, 1958; Carter, 1944; Meeker, 1906; Parkes, 1951; Sage, 1889; Short, 1963) and some excellent reviews have been published (Confer, 2006; Gill, 2004). I will nonetheless provide an overview of the relevant findings. The hybrid forms were first described as distinct species, Brewster’s Warbler (V. leucobronchialis) and Lawrence’s Warbler (V. lawrencei) (e.g., Bishop, 1889; Carter, 1944; Eames, 1888; Eames, 1889; Palmer & Sage, 1885; Sage, 1893). Millicent Ficken and Robert Ficken (1967; 1968a; 1968b; 1968c; 1968d; 1969; 1970) studied numerous behavioural and morphological aspects of this hybrid zone. Their work was continued by Gill and Murray (1972a; 1972b; 1976).
Blue-winged Warblers increased dramatically between 1880 and 1920, this expansion ultimately resulted in hybridization with Golden-winged Warblers (Gill, 1980). In some areas about 10% of the males were hybrids (Confer & Tupper, 2000). The increase in Blue-winged Warblers was strongly correlated with a decrease in Golden-winged Warblers (Confer & Knapp, 1981). The decline of Golden-winged Warblers might be intensified by hybridization. Hybrids do not seem to be at a disadvantage in reproduction (Neville et al., 2008; Vallender et al., 2007a), parasite susceptibility (Vallender et al., 2012). Experiments do show sexual selection against hybrid phenotypes (Leichty & Grier, 2006). It seems that Golden-winged Warblers are being outcompeted and “out-hybridized” by the invading Blue-winged Warblers. But there could be a safe haven for them in swamp forests (Confer et al., 2010).
An allozyme study showed that both species display little divergence (Gill, 1987) and mtDNA analyses revealed extensive asymmetrical introgression from Blue-winged mtDNA into Golden-winged Warblers in Pennsylvania and New Jersey (Gill, 1997). However, this pattern does not hold across the entire hybrid zone, in other areas introgression is bi-directional (Dabrowski et al., 2005; Shapiro et al., 2004; Vallender et al., 2009b). Inclusion of nuclear genetic markers uncovered even more cryptic introgression (Vallender et al., 2007b). There is thus genetic exchange across the entire range of these species, with the lowest levels of introgression in Manitoba, Canada (Moulton et al., 2017). A study of habitat associations in New York and Pennsylvania revealed that habitat use of hybrids may promote contact with Golden-winged Warblers and likely facilitate genetic introgression (Wood et al., 2016). A hybrid zone has also been used to unravel the genetic basis of plumage coloration in these birds (Toews et al., 2016; Baiz et al., 2020).
Despite their genetic similarity, both species display different migration strategies. Golden-winged Warblers spend their winter in northern Colombia, while Blue-winged Warblers fly to the Yucatan peninsula in Mexico and the western tip of Cuba. Hybrids show an intermediate choice wintering in Cuba and Nicaragua (Bennett et al. 2017). These results confirm the notion that migratory behavior is heritable in passerines with genetic hybrids showing intermediate strategies. The largely undifferentiated genomes of these birds allowed researchers to pinpoint the gene underlying migratory direction: VPS13A (Toews et al., 2019)
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