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.
Céspedes-Arias et al. (2021) documented a hybrid zone between subspecies of the Golden-fronted Redstart (M. ornatus chrysops) and the Spectacled Redstart (M. melanocephalus ruficoronatus), located in Ecuador and Colombia.
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). Genomic analyses of this hybrid zone indicated that a small genomic regions containing several pigmentation genes might contribute to reproductive isolation between these warblers (Wang et al., 2020).
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)
Anich, N. M., Swelstad, J.A. & Pierce, B. (2012). The First Report of Sutton’s Warbler (Northern Parula × Yellow-throated Warbler) in Wisconsin. The Passenger Pigeon 74, 149-155.
Baiz, M. D., Kramer, G. R., Streby, H. M., Taylor, S. A., Lovette, I. J., & Toews, D. P. (2020). Genomic and plumage variation in Vermivora hybrids. The Auk, 137(3), ukaa027.
Barrowclough, G. F. (1980). Genetic and Phenotypic Differentiation in a Wood Warbler (Genus Dendroica) Hybrid Zone. Auk 97, 655-668.
Bennett, R.E., Barker Swarthout S., Bolsinger, J.S., Rodewald, A., Rosenberg, K.V. & Rohrbaugh, R. (2017). Extreme genetic similarity does not predict non‐breeding distribution of two closely related warblers. Journal of Field Ornithology.
Berger, A. (1958). The Golden-winged-Blue-winged Warbler complex in Michigan and the Great Lakes area. Jack-Pine Warbler 36, 37-73.
Bishop, L. B. (1889). Helminthophila pinus, H. chrysoptera, H. leucobronchialis, and H. lawrencei in Connecticut in the Spring of 1888. The Auk 6, 192-193.
Bledsoe, A. H. (1988). A Hybrid Oporornis-Philadelphia X Geothlypis-Trichas, with Comments on the Taxonomic Interpretation and Evolutionary Significance of Intergeneric Hybridization. Wilson Bulletin 100, 1-&.
Brelsford, A. & Irwin, D. E. (2009). Incipient Speciation Despite Little Assortative Mating: The Yellow-Rumped Warbler Hybrid Zone. Evolution 63, 3050-3060.
Brelsford, A., Mila, B. & Irwin, D. E. (2011). Hybrid origin of Audubon’s warbler. Molecular Ecology 20, 2380-2389.
Brelsford, A., D. P. L. Toews and D. E. Irwin (2017). Admixture mapping in a hybrid zone reveals loci associated with avian feather coloration. Proceedings of the Royal Society B: Biological Sciences 284(1866).
Bryan, K.B. & Lockwood, M.W. (2018) Plumage Characteristics and Song Patterns of Presumed Colima x Virginia’s Warbler Hybrids in the Davis Mountains of Texas. North American Birds, 70(2): 142-154.
Bull, R. D., McCracken, A., Gaston, A. J., Birt, T. P. & Friesen, V. L. (2010). Evidence of Recent Population Differentiation in Orange-Crowned Warblers (Veramivora Celata) in Haida Gwaii. Auk 127, 23-34.
Burrell, K. G. D., Skevington, J. H., Kelso, S., Burrell, M. V. A., Leclair, D. L. & Mackenzie, S. A. (2016). A Previously Undocumented Hybrid New World Warbler (Setophaga pensylvanica x S. magnolia) Captured at Long Point, Ontario. Wilson Journal of Ornithology 128, 624-628.
Carter, T. D. (1944). Six years with a Brewster’s Warbler. The Auk, 48-61.
Céspedes-Arias, L. N., Cuervo, A. M., Bonaccorso, E., Castro-Farias, M., Mendoza-Santacruz, A., Pérez-Emán, J. L., Witt, C. C. & Cadena, C. D. (2021). Extensive hybridization between two Andean warbler species with shallow divergence in mtDNA. Ornithology, 138(1), ukaa065.
Confer, J. L. (2006). Secondary contact and introgression of golden-winged warblers (Vermivora chrysoptera): Documenting the mechanism. Auk 123, 958-961.
Confer, J. L., Barnes, K. W. & Alvey, E. C. (2010). Golden- and Blue-Winged Warblers: Distribution, Nesting Success, and Genetic Differences in Two Habitats. Wilson Journal of Ornithology 122, 273-278.
Confer, J. L. & Knapp, K. (1981). Golden-winged Warblers and Blue-winged Warblers: The relative success of a habitat specialist and a generalist. The Auk, 108-114.
Confer, J. L. & Tupper, S. K. (2000). A reassessment of the status of Golden-winged and Blue-winged Warblers in the Hudson Highlands of southern New York. Wilson Bulletin 112, 544-546.
Cox, G. W. (1973). Hybridization between Mourning and MacGillivray’s warblers. The Auk, 190-191.
Cozzarolo, C.S., Jenkins, T., Toews, D.P.L., Brelsford, A. & Christe, P. (2018) Prevalence and diversity of haemosporidian parasites in the yellow-rumped warbler hybrid zone. Ecology and Evolution
Dabrowski, A., Fraser, R., Confer, J. L. & Lovette, I. J. (2005). Geographic variability in mitochondrial introgression among hybridizing populations of Golden-winged (Vermivora chrysoptera) and Blue-winged (V-pinus) Warblers. Conservation Genetics 6, 843-853.
Delancey, C.D., MacDonald, G.J. & Islam, K. (2019) First confirmed hybrid pairing between a Cerulean Warbler (Setophaga cerulea) and a Black-throated Blue Warbler (Setophaga caerulescens). The Wilson Journal of Ornithology, 131(1), 161-165.
Eames, E. H. (1888). Notes on Helminthophila leucobronchialis. The Auk, 427-428.
Eames, E. H. (1889). Notes on the Blue-Winged Warbler and Its Allies (Helminthophila pinus, H. leucobronchialis, H. lawrencei, and H. chrysoptera) in Connecticut. The Auk, 305-310.
Ficken, M. S. & Ficken, R. W. (1967). Singing behaviour of Blue-winged and Golden-winged warblers and their hybrids. Behaviour, 149-181.
Ficken, M. S. & Ficken, R. W. (1968a). Courtship of blue-winged warblers, golden-winged warblers, and their hybrids. The Wilson Bulletin, 161-172.
Ficken, M. S. & Ficken, R. W. (1968b). Ecology of blue-winged warblers, golden-winged warblers and some other Vermivora. American Midland Naturalist, 311-319.
Ficken, M. S. & Ficken, R. W. (1968c). Reproductive isolating mechanisms in the Blue-winged Warbler-Golden-winged Warbler complex. Evolution, 166-179.
Ficken, M. S. & Ficken, R. W. (1968d). Territorial relationships of Blue-winged Warblers, Golden-winged Warblers, and their hybrids. The Wilson Bulletin, 442-451.
Ficken, M. S. & Ficken, R. W. (1969). Responses of Blue-winged Warblers and Golden-winged Warblers to their own and the other species’ song. The Wilson Bulletin, 69-74.
Ficken, M. S. & Ficken, R. W. (1970). Comments on Introgression and Reproductive Isolating Mechanisms in the Blue-Winged Warbler-Golden-Winged Warbler Complex. Evolution, 254-256.
Gilbert, W. M. & West, G. C. (2015). Morphological Expression in Putative Intergrades between Two Subspecies of Orange-Crowned Warbler (Oreothlypis Celata) on the Kenai Peninsula, Alaska. Wilson Journal of Ornithology 127, 29-42.
Gill, F. B. (1980). Historical Aspects of Hybridization between Blue-Winged and Golden-Winged Warblers. Auk 97, 1-18.
Gill, F. B. (1987). Allozymes and Genetic Similarity of Blue-Winged and Golden-Winged Warblers. Auk 104, 444-449.
Gill, F. B. (1997). Local cytonuclear extinction of the golden-winged warbler. Evolution 51, 519-525.
Gill, F. B. (2004). Blue-Winged Warblers (Vermivora pinus) versus Golden-Winged Warblers (V chrysoptera). Auk 121, 1014-1018.
Gill, F. B. & Murray, B. G. (1972a). Discrimination Behavior and Hybridization of Blue-Winged and Golden Winged Warblers. Evolution 26, 282-&.
Gill, F. B. & Murray, B. G. (1972b). Song Variation in Sympatric Blue-Winged and Golden-Winged Warblers. Auk 89, 625-&.
Graves, G. R. (1988). Evaluation of Vermivora X Oporornis Hybrid Wood-Warblers. Wilson Bulletin 100, 285-289.
Hall, G. A. (1979). Hybridization between Mourning and MacGillivray’s warblers. Bird-Banding, 101-107.
Hubbard, J. P. (1969). Relationships and Evolution of Dendroica Coronata Complex. Auk 86, 393-&.
Hubbard, J. P. (1970). Geographic Variation in Dendroica-Coronata Complex. Wilson Bulletin 82, 355-&.
Irwin, D. E., Brelsford, A., Toews, D. P. L., MacDonald, C. & Phinney, M. (2009). Extensive hybridization in a contact zone between MacGillivray’s warblers Oporornis tolmiei and mourning warblers O-philadelphia detected using molecular and morphological analyses. Journal of Avian Biology 40, 539-552.
Irwin, D.E., Mila, B., Toews, D.P.L., Brelsford, A., Kenyon, H.L., Porter, A.N., Grossen, C., Delmore, K.E., Alcaide, M. & Irwin, J.H. (2018) A comparison of genomic islands of differentiation across three young avian species pairs. Molecular Ecology.
Jewett, S. G. (1944). Hybridization of hermit and Townsend warblers. Condor 46, 23-24.
Kenyon, H. L., Toews, D. P. L. & Irwin, D. E. (2011). Can Song Discriminate between Macgillivray’s and Mourning Warblers in a Narrow Hybrid Zone? Condor 113, 655-663.
Kenyon, H. L., Alcaide, M., Toews, D. P. L., & Irwin, D. E. (2017). Cultural isolation is greater than genetic isolation across an avian hybrid zone. Journal of Evolutionary Biology, 30(1), 81-95.
Krosby, M. & Rohwer, S. (2009). A 2000 km genetic wake yields evidence for northern glacial refugia and hybrid zone movement in a pair of songbirds. Proceedings of the Royal Society B-Biological Sciences 276, 615-621.
Krosby, M. & Rohwer, S. (2010). Ongoing Movement of the Hermit Warbler X Townsend’s Warbler Hybrid Zone. Plos One 5.
Latta, S. C. & Parkes, K. C. (2001). A possible Dendroica kirtlandii hybrid from Hispaniola. Wilson Bulletin 113, 378-383.
Latta, S. C., Parkes, K. C. & Wunderle, J. M. (1998). A new intrageneric Dendroica hybrid from Hispaniola. Auk 115, 533-537.
Leichty, E. R. & Grier, J. W. (2006). Importance of facial pattern to sexual selection in golden-winged Warbler (Vermivora chrysoptera). Auk 123, 962-966.
Lovette, I. J. (2004). Molecular phylogeny and plumage signal evolution in a trans Andean and circum Amazoman avian species complex. Molecular Phylogenetics and Evolution 32, 512-523.
Lovette, I. J. & Bermingham, E. (1999). Explosive speciation in the New World Dendroica warblers. Proceedings of the Royal Society B-Biological Sciences 266, 1629-1636.
Lovette, I. J., Bermingham, E., Rohwer, S. & Wood, C. (1999). Mitochondrial restriction fragment length polymorphism (RFLP) and sequence variation among closely related avian species and the genetic characterization of hybrid Dendroica warblers. Molecular Ecology 8, 1431-1441.
Meeker, J. C. (1906). A Male Golden-Winged Warbler (Helminthophila chrysoptera) Mated with a Female Blue-Winged Warbler (Helminthophila pinus) at Bethel, Conn. The Auk, 104-104.
Mila, B., Smith, T. B. & Wayne, R. K. (2007). Speciation and rapid phenotypic differentiation in the yellow-rumped warbler Dendroica coronata complex. Molecular Ecology 16, 159-173.
Mila, B., Toews, D. P. L., Smith, T. B. & Wayne, R. K. (2011). A cryptic contact zone between divergent mitochondrial DNA lineages in southwestern North America supports past introgressive hybridization in the yellow-rumped warbler complex (Aves: Dendroica coronata). Biological Journal of the Linnean Society 103, 696-706.
Morrison, M. L. & Hardy, J. W. (1983). Hybridization between Hermit and Townsend’s warblers. The Murrelet, 65-72.
Moulton, L.L., Vallender, R., Artuso, C. & Koper, N. (2017). The final frontier: early-stage genetic introgression and hybrid habitat use in the northwestern extent of the Golden-winged Warbler breeding range, Conservation Genetics: 1-7.
Murray Jr, B. G. & Gill, F. B. (1976). Behavioral interactions of Blue-winged and Golden-winged Warblers. The Wilson Bulletin, 231-254.
Neville, K. J., Vallender, R. & Robertson, R. J. (2008). Nestling sex ratio of golden-winged warblers Vermivora chrysoptera in an introgressed population. Journal of Avian Biology 39, 599-604.
Owen-Ashley, N. T. & Butler, L. K. (2004). Androgens, interspecific competition and species replacement in hybridizing warblers. Proceedings of the Royal Society B-Biological Sciences 271, S498-S500.
Palmer, W. & Sage, J.H. (1885). Occurrence of Helminthophila leucobronchialis in Virginia. The Auk 2, 304-304.
Parkes, K. C. (1951). The genetics of the Goldenwinged x Blue-winged warbler complex. Wilson Bull 63, 5-15.
Parkes, K. C. (1978). Still Another Parulid Intergeneric Hybrid (Mniotilta X Dendroica) and Its Taxonomic and Evolutionary Implications. Auk 95, 682-690.
Parkes, K. (1983). Three additional hybrid combinations in North American birds. In Proceedings of the 101st meeting of the American Ornithologists’ Union, New York, pp. 26-30.
Parkes, K. C. (1995). Reinterpretation of the probable parentage of a hybrid wood-warbler (Seiurus x Dendroica). Auk 112, 510-511.
Parkes, K. (1996). Nashville x Tennessee warbler hybrids. Ontario Birds 14, 110-116.
Patti, S. T. & Myers, M. L. (1976). A probable mourning× MacGillivray’s warbler hybrid. The Wilson Bulletin, 490-491.
Pearson, S. F. (2000). Behavioral asymmetries in a moving hybrid zone. Behavioral Ecology 11, 84-92.
Pearson, S. F. & Manuwal, D. A. (2000). Influence of niche overlap and territoriality on hybridization between Hermit Warblers and Townsend’s Warblers. Auk 117, 175-183.
Pearson, S. F. & Rohwer, S. (1998). Influence of breeding phenology and clutch size on hybridization between Hermit and Townsend’s Warblers. Auk 115, 739-745.
Pearson, S. F. & Rohwer, S. (2000). Asymmetries in male aggression across an avian hybrid zone. Behavioral Ecology 11, 93-101.
Ralston, J., Ermacor, D. A. & Kirchman, J. J. (2015). A new wood warbler hybrid (Oreothlypis celata x O. ruficapilla) from the Adirondack mountains, New York. The Wilson Journal of Ornithology 127, 21-28.
Rohwer, S. (1994). 2 New Hybrid Dendroica Warblers and New Methodology for Inferring Parental Species. Auk 111, 441-449.
Rohwer, S., Bermingham, E. & Wood, C. (2001). Plumage and mitochondrial DNA haplotype variation across a moving hybrid zone. Evolution 55, 405-422.
Rohwer, S. & Martin, P. R. (2007). Time since contact and gene flow may explain variation in hybrid frequencies among three Dendroica townsendi x D-occidentalis (Parulidae) hybrid zones. Auk 124, 1347-1358.
Rohwer, S. & Wood, C. (1998). Three hybrid zones between Hermit and Townsend’s Warblers in Washington and Oregon. Auk 115, 284-310.
Rohwer, S., Wood, C. & Bermingham, E. (2000). A new hybrid warbler (Dendroica nigrescens x D-occidentalis) and diagnosis of similar D-townsendi x D-occidentalis recombinants. Condor 102, 713-718.
Sage, J. H. (1889). The interbreeding of Helminthophila pinus and H. chrysoptera. The Auk, 279-279.
Sage, J. H. (1893). Notes on Helminthophila chrysoptera, pinus, leucobronchialis and lawrencei in Connecticut. The Auk, 10(2) 208-209.
Shapiro, L. H., Canterbury, R. A., Stover, D. M. & Fleischer, R. C. (2004). Reciprocal introgression between Golden-Winged Warblers (Vermivora chrysoptera) and Blue-Winged Warblers (V-pinus) in eastern North America. Auk 121, 1019-1030.
Short, L. (1963). Hybridization in the wood warblers Vermivora pinus and V. chrysoptera. In Proceedings of the XIII International Ornithological Congress, pp. 147-160.
Short, L. L. & Robbins, C. S. (1967). An Intergeneric Hybrid Wood Warbler (Seiurus X Dendroica). Auk 84, 534-&.
Smith, C. E. & Rohwer, S. (2000). A phenotypic test of Haldane’s rule in an avian hybrid zone. Auk 117, 578-585.
Toews, D. P. L., Brelsford, A. & Irwin, D. E. (2011). Hybridization between Townsend’s Dendroica townsendi and black-throated green warblers D. virens in an avian suture zone. Journal of Avian Biology 42, 434-446.
Toews, D. P. L., Brelsford, A. & Irwin, D. E. (2014a). Isotopic variation across the Audubon’s-myrtle warbler hybrid zone. Journal of Evolutionary Biology 27, 1179-1191.
Toews, D. P. L., Mandic, M., Richards, J. G. & Irwin, D. E. (2014b). Migration, Mitochondria, and the Yellow-Rumped Warbler. Evolution 68, 241-255.
Toews, D. P., S. A. Taylor, R. Vallender, A. Brelsford, B. G. Butcher, P. W. Messer and I. J. Lovette (2016). Plumage genes and little else distinguish the genomes of hybridizing warblers. Current Biology 26(17): 2313-2318.
Toews, D.P.L., Lovette, I.J., Irwin, D.E. & Brelsford, A. (2018) Similar hybrid composition among different age and sex classes in the Myrtle-Audubon’s warbler hybrid zone. The Auk, 135: 1133-1145.
Toews, D.P.L., Streby, H.M., Burket, L. & Taylor, S.A. (2018) A wood-warbler produced through both interspecific and intergeneric hybridization. Biology Letters, 14(11): 20180557.
Toews, D. P., Taylor, S. A., Streby, H. M., Kramer, G. R., & Lovette, I. J. (2019). Selection on VPS13A linked to migration in a songbird. Proceedings of the National Academy of Sciences, 116(37), 18272-18274.
Vallender, R., Friesen, V. L. & Robertson, R. J. (2007a). Paternity and performance of golden-winged warblers (Vermivora chrysoptera) and golden-winged X blue-winged warbler (V-pinus) hybrids at the leading edge of a hybrid zone. Behavioral Ecology and Sociobiology 61, 1797-1807.
Vallender, R., Robertson, R. J., Friesen, V. L. & Lovette, I. J. (2007b). Complex hybridization dynamics between golden-winged and blue-winged warblers (Vermivora chrysoptera and Vermivora pinus) revealed by AFLP, microsatellite, intron and mtDNA markers. Molecular Ecology 16, 2017-2029.
Vallender, R., Gagnon, J. P. & Lovette, I. (2009a). An Intergeneric Wood-Warbler Hybrid (Mniotilta Varia X Dendroica Coronata) and Use of Multilocus DNA Analyses to Diagnose Avian Hybrid Origins. Wilson Journal of Ornithology 121, 298-305.
Vallender, R., Van Wilgenburg, S. L., Bulluck, L. P., Roth, A., Canterbury, R., Larkin, J., Fowlds, R. M. & Lovette, I. J. (2009b). Extensive Rangewide Mitochondrial Introgression Indicates Substantial Cryptic Hybridization in the Golden-winged Warbler (Vermivora chrysoptera). Avian Conservation and Ecology 4.
Vallender, R., Bull, R. D., Moulton, L. L. & Robertson, R. J. (2012). Blood Parasite Infection and Heterozygosity in Pure and Genetic-Hybrid Golden-Winged Warblers (Vermivora Chrysoptera) across Canada. Auk 129, 716-724.
Wang, S., Rohwer, S., Delmore, K., & Irwin, D. E. (2019). Cross‐decades stability of an avian hybrid zone. Journal of Evolutionary Biology.
Wang, S., Rohwer, S., de Zwaan, D. R., Toews, D. P., Lovette, I. J., Mackenzie, J., & Irwin, D. (2020). Selection on a small genomic region underpins differentiation in multiple color traits between two warbler species. Evolution Letters, 4(6), 502-515.
Willis, P. M., Symula, R. E. & Lovette, I. J. (2014). Ecology, song similarity and phylogeny predict natural hybridization in an avian family. Evolutionary Ecology 28, 299-322.
Wood, E. M., Barker Swarthout, S. E., Hochachka, W. M., Larkin, J. L., Rohrbaugh, R. W., Rosenberg, K. V. & Rodewald, A. D. (2016). Intermediate habitat associations by hybrids may facilitate genetic introgression in a songbird. Journal of Avian Biology.