The Old World Flycatchers are a large passerine family mostly restricted to Europe, Asia and Africa. Several genera display hybridization and some have been studied in greater detail.
The phylogeography of Magpie-robins (genera Copsychus and Trichixos) is well studied. On Borneo and Java, black-bellied and white-bellied populations of Oriental Magpie-robin (C. saularis) have come into contact and are interbreeding (Sheldon et al., 2009). A similar process can also be observed between subspecies of the White-rumped Shama (C. malabaricus) on Borneo (Lim et al., 2010).
Hybrids between Chorister Robin-Chat (C. dichroa) and Red-capped Robin-Chat (C. natalensis) have been recorded in South Africa (Davies et al., 2011).
Hybridization between Collared Flycatcher (F. albicollis) and Pied Flycatcher (F. hypoleuca) has been studied extensively on the Baltic Islands, Gotland and Öland (Alatalo, Gustafsson & Lundberg, 1982; Alerstam et al., 1978; Vallin et al., 2012a), and in a hybrid across Central Europe (Saetre et al., 1999). The results from these studies have been reviewed elsewhere (Qvarnstrom, Rice & Ellegren, 2010; Saetre & Saether, 2010), but I will nonetheless give an overview of the main findings.
Hybrids are perfectly viable and a translocation experiment found difference in survival between hybrids and pure nestlings (Vallin et al., 2013). Hybrid fertility follows Haldane’s Rule, females are sterile while males are able to reproduce (Gelter, Tegelstrom & Gustafsson, 1992). There is however sexual selection against males in combination with genetic incompatibilities (Svedin et al., 2008), which can disrupt spermatogenesis (Alund et al., 2013). The effects of postzygotic selection increase in later generations (Wiley et al., 2009a). There might be selection against hybrids if they follow intermediate migration routes, but isotope analyses showed that hybrids use the same wintering areas as their parental species with a preference for Pied Flycatcher (Veen et al., 2014; Veen et al., 2007). Hybrids might also suffer from reduced immune function, but the prevalence of haemosporidians was intermediate in hybrids (Wiley, Qvarnstrom & Gustafsson, 2009b).
Hybridization can sometimes be beneficial, which has been termed adaptive mate choice. Benefits include more male offspring and siring of offspring by other conspecifics (Veen et al., 2001), possible heterotic effects (Merila, Sheldon & Griffith, 2003), and access to better territories (Wiley et al., 2007).
On the island of Öland, both species have advanced their timing of breeding in response to increasing spring temperatures. However, Pied Flycatchers showed a slower response compared to Collared Flycatchers, possibly leading to the build-up of temporal isolation (Sirkiä et al., 2018). In addition, competition has led to habitat isolation. High quality habitats are associated with a high risk of hybridization for Pied Flycatchers, leading to a selection pressure for immigrants to settle in lower quality habitats (Rybinksi et al. 2016).
In allopatry, the songs of both species are different (Wallin, 1986). In sympatry, however, the songs converge (Gelter, 1987; Haavie et al., 2004). Males singing mixed songs (caused by heterospecific copying, not hybridization) attract more females, which increases the risk of hybridization (Qvarnstrom et al., 2006).
Flycatchers display female mimicry (males resemble females) due to delayed maturation (Wiley et al., 2005). If these males resemble heterospecifics, this might increase the risk of maladaptive hybridization. Based on this reasoning, Saetre et al. (1993) hypothesized that light plumage colour in sympatry is driven by reinforcement. But subsequent studies showed that the occurrence of light plumage can be attributed to interspecific effects, such as aggression avoidance (Alatalo, Gustafsson & Lundberg, 1994). Reinforcement does not explain the light plumage colour, it could nonetheless explain divergence in plumage colour (Saetre et al., 1997). However, it is also possible that divergent males are simply more successful (Vallin et al., 2012b). Several major plumage traits have diverged in sympatry, but increase gradually in conspicuousness with distance to the sympatric zone. This suggests a cline maintained by gene flow (Laaksonen et al., 2015).
Tegelstrom and Gelter (1990) compared divergence at mitochondrial and nuclear loci and discovered that mtDNA was highly divergent in contrast to nDNA. They explained this pattern by sex-biased gene flow. Fertile males mitigate gene flow of nDNA, while sterile females prevent gene flow of mtDNA. This pattern can also be explained by a rapid mutational saturation of mtDNA (Saetre et al., 2001). Further studies also indicated strong selection against gene flow on the sex chromosomes (especially Z-chromosome) and suggested that male plumage is sex-linked (Saetre et al., 2003). This suggestion led to intensive studies of the Z-chromosome. Borge et al. (2005) found reduced variation on the Z-chromosome, which can be attributed to recurrent selective sweeps (by sexual selection?) or complex demographic history, such as a population bottleneck. Cross-fostering experiments indicated that species recognition genes are inherited on the Z-chromosome, and probably physically linked to genes of male plumage traits and genes causing low hybrid fitness (Saether et al., 2007). A candidate gene approach uncovered seven divergent loci of which two were associated with plumage (Backstrom et al., 2010a). There is however no evidence for a role for rearrangements of the Z-chromosome in reproductive isolation (Backstrom et al., 2010b). The inclusion of more species and the availability of whole genome sequences confirmed that Z-chromosomes are more divergent than autosomal loci (Ellegren et al., 2012; Hogner et al., 2012b). In addition, the genomic landscape of Flycatchers consists of several “islands of divergence”, which indicates that different genomic regions are at different stages of speciation (Backstrom, Saetre & Ellegren, 2013). Linkage disequilibrium is also highest in these “genomic islands” (Kawakami et al., 2014). Genomic analyses also provided strong evidence for gene flow among the flycatcher species, with distinct patterns of reduced introgression on the Z chromosome (Nater et al., 2015).
The population genetic structure of Pied Flycatcher has also been mapped. Populations in Norway and Czech Republic are indistinguishable, possibly due to gene flow, while the Spanish birds are clearly divergent (Haavie, Saetre & Moum, 2000).
Common Nightingale (L. megarhynchos) and Thrush Nightingale (L. luscinia) interbreed regularly (Becker, 1995; Becker, 2007; Kverek, 1998; Kverek, 2002; Kverek et al., 2008; Nöhring, 1943; Reifová, Kverek & Reif, 2011). Several individuals have been documented that produce songs of both species (Lille, 1988; Sorjonen, 1986), but a genetic analysis showed that this need not be the result of hybridization; many mixed singers did not possess genetic material from the other species (Vokurkova et al., 2013). Cultural transmission of song it thus an important mechanism. In addition, mixed songs might facilitate the coexistence of these two species by establishing interspecific territoriality. This concept is known as convergent agonistic character displacement (Souriau et al., 2018).
Hybrids can be detected based on morphological characteristics (Kovats, Vegvari & Varga, 2013). But despite hybridization, these species still diverge in certain traits, such as bill size (Reifova et al., 2011). This is probably due to differences in habitat use that are driven by interspecific competition (Sottas et al. 2018).
Both species differ in sperm morphology. However, sperm morphology is more different in sympatry than in allopatry, which suggests reinforcement might be operating (Albrecht et al., 2018).
Genomic analysis showed reduced introgression on the Z-chromosome compared to autosomes (Storchova, Reif & Nachman, 2010), which supports the idea that the Z-chromosome plays an important role in reproductive isolation (female hybrids are sterile). Female sterility is probably due to incompatible genes involved in meiosis that are located in genomic islands of differentiation (Morkovsky et al. 2018). The gene flow patterns on the Z-chromosome can be explained by higher levels of genetic drift in the Common Nightingale, leading to faster accumulation of hybrid incompatibilities (Janoušek et al., 2018).
The distribution of two haplotypes in the Japanese Robin (L. akahige) can be best explained by past introgression (Seki, Nishiumi & Saitoh, 2012). As similar situation occurs in Russia, where a genetic analysis of the Siberian Rubythroat (L. calliope) revealed two haplotypes that are shared by certain populations. This may indicate the existence of a hybrid zone (Spiridonova et al., 2013).
Randler et al. (2012) evaluated the status of the Cyprus Wheatear (O. cypriaca), which is closely related to Pied Wheatear (O. pleschanka) and Black-eared Wheatear (O. hispanica). This close relationship can be explained if O. cypriaca is a young taxon or if there is ongoing hybridization among these three species. A genomic analysis of this species complex revealed that Cyprus wheatear is more closely related to Eastern Black-eared Wheatear, followed by Pied Wheatear and Western Black-eared Wheatear. Moreover, there is extensive gene flow between Pied and Eastern Black-eared Wheatear (Schweizer et al., 2019a; Schweizer et al., 2019b). Indeed, a hybrid zone between these species has been described (Grabovsky, Panov & Rubtsov, 1992; Panov, 1986). More detailed studies of this hybrid zone showed song convergence (Grabovsky & Panov, 1992) and no differences in survival between hybrids and pure males (Rubtsov, 1995).
Geographical variation in Eastern Pied Wheatear (O. pictata) can be explained by hybridization dynamics between three morphological races (pictata, opistholeuca and capistrata) (Panov, Grabovsky & Ljubustchenko, 1993).
Hybridization between Kurdish Wheatear (O. xanthoprymna) and Persian Wheatear (O. chrysopygia) has been documented based on morphological data and the occurrence of intermediate colour variants (Chamani et al., 2010).
European Common Redstart (P. phoenicurus) and Black Redstard (P. ochruros) interbreed (Landmann, 1987). Experimental work indicated no differences between these species and their hybrids in prey-handling time and efficiency (Grosch, 2003) and habitat selection (Grosch, 2004). However, in an urban environment both species do use different habitats and interact aggressively, this behaviour could act as a reproductive barrier (Sedlacek, Fuchs & Exnerova, 2004). Genetic analysis of Common Redstart revealed two divergent mitochondrial haplotypes that appear to coexist (Hogner et al., 2012a). Intergeneric hybridization between Common Redstart and Winchat (Saxicola rubetra) has been confirmed by molecular analysis (Hogner et al., 2015).
Different subspecies of captive-bred Stonechats (S. torquata) have been used extensively in experimental settings. In most cases, hybrids showed intermediate outcomes in parameters, such as moult schedule (Gwinner & Neusser, 1985; Helm & Gwinner, 1999), clutch size (Gwinner, Konig & Haley, 1995), growth (Starck, Konig & Gwinner, 1995), circannual behaviour (Helm, 2009; Helm, Schwabl & Gwinner, 2009). However, the regulation of Basal Metabolic Rate (BMR) is more complicated. Hybrids showed differential BMR, which suggests that mitochondrial and nuclear genes are involved (Tieleman et al., 2009; Versteegh et al., 2012). Also, environmental factors and phenotypic plasticity may play an important role. The same is true for immune function (Versteegh et al., 2014).
Alatalo, R. V., Gustafsson, L. & Lundberg, A. (1982). Hybridization and Breeding Success of Collared and Pied Flycatchers on the Island of Gotland. Auk 99, 285-291.
Alatalo, R. V., Gustafsson, L. & Lundberg, A. (1994). Male Coloration and Species Recognition in Sympatric Flycatchers. Proceedings of the Royal Society B-Biological Sciences 256, 113-118.
Albrecht, T., Opletalová, K., Reif, J., Janoušek, V., Piálek, L., Cramer, E.R.A., Johnsen, A. & Reifová, R. (2018) Sperm divergence in a passerine contact zone: Indication of reinforcement at the gametic level. Evolution 73(2):202-213.
Alerstam, T., Ebenman, B., Sylven, M., Tamm, S. & Ulfstrand, S. (1978). Hybridization as an Agent of Competition between 2 Bird Allospecies – Ficedula-Albicollis and F-Hypoleuca on the Island of Gotland in the Baltic. Oikos 31, 326-331.
Alund, M., Immler, S., Rice, A. M. & Qvarnstrom, A. (2013). Low fertility of wild hybrid male flycatchers despite recent divergence. Biology Letters 9.
Backstrom, N., Lindell, J., Zhang, Y., Palkopoulou, E., Qvarnstrom, A., Saetre, G. P. & Ellegren, H. (2010a). A High-Density Scan of the Z Chromosome in Ficedula Flycatchers Reveals Candidate Loci for Diversifying Selection. Evolution 64, 3461-3475.
Backstrom, N., Palkopoulou, E., Qvarnstrom, A. & Ellegren, H. (2010b). No evidence for Z-chromosome rearrangements between the pied flycatcher and the collared flycatcher as judged by gene-based comparative genetic maps. Molecular Ecology 19, 3394-3405.
Backstrom, N., Saetre, G. P. & Ellegren, H. (2013). Inferring the demographic history of European Ficedula flycatcher populations. BMC Evolutionary Biology 13.
Becker, J. (1995). Sympatrisches Vorkommen und Hybridisierung von Sprosser Luscinia luscinia und Nachtigall L. megarhynchos bei Frankfurt (Oder), Brandenburg. Vogelwelt 116, 109-118.
Becker, J. (2007). About Nightingales (Luscinia megarhynchos), Trush Nightingales (Luscinia luscinia) and their hybrids-further results of an investigation via bird ringing in the Frankfurt (Oder) area. Vogelwarte 45, 15-26.
Borge, T., Webster, M. T., Andersson, G. & Saetre, G. P. (2005). Contrasting patterns of polymorphism and divergence on the Z chromosome and autosomes in two Ficedula flycatcher species. Genetics 171, 1861-1873.
Chamani, A., Kaboli, M., Karami, M., Aliabadian, M., Pasquet, E. & Prodon, R. (2010). Morphological consequences of hybridization in two interbreeding taxa: Kurdish Wheatear (Oenanthe xanthoprymna) and Persian Wheatear (O. chrysopygia) in western Iran. African Journal of Biotechnology 9, 7817-7824.
Davies, G. B. P., Symes, C. T., Boon, R. G. C. & Campbell, H. A. (2011). Inferred hybridisation, sympatry and movements of Chorister Robin-Chat Cossypha dichroa and Red-capped Robin-Chat C. natalensis. Ostrich 82, 231-241.
Ellegren, H., Smeds, L., Burri, R., Olason, P. I., Backstrom, N., Kawakami, T., Kunstner, A., Makinen, H., Nadachowska-Brzyska, K., Qvarnstrom, A., Uebbing, S. & Wolf, J. B. W. (2012). The genomic landscape of species divergence in Ficedula flycatchers. Science, 491(7426), 756.
Gelter, H. P. (1987). Song Differences between the Pied Flycatcher Ficedula hypoleuca, the Collared Flycatcher Ficedula albicollis, and Their Hybrids. Ornis Scandinavica 18, 205-215.
Gelter, H. P., Tegelstrom, H. & Gustafsson, L. (1992). Evidence from Hatching Success and DNA Fingerprinting for the Fertility of Hybrid Pied X Collared Flycatchers Ficedula hypoleuca x albicollis. Ibis 134, 62-68.
Grabovsky, V. I. & Panov, E. N. (1992). Song Convergence in Pied Wheatear (Oenanthe-Pleschanka) and Black-Eared Wheatear (O-Hispanica) in Secondary Contact Zones. Zoologichesky Zhurnal 71, 75-84.
Grabovsky, V. I., Panov, E. N. & Rubtsov, A. G. (1992). Phenotypic Composition and Reproductive Success in a Hybrid Population of Pied Wheatears Oenanthe-Pleschanka and Black-Eared Wheatears O-Hispanica. Zoologichesky Zhurnal 71, 109-121.
Grosch, K. (2003). Hybridization between two insectivorous bird species and the effect on prey-handling efficiency. Evolutionary Ecology 17, 1-17.
Grosch, K. (2004). Hybridization between redstart Phoenicurus phoenicurus and black redstart P-ochruros, and the effect on habitat exploitation. Journal of Avian Biology 35, 217-223.
Gwinner, E., Konig, S. & Haley, C. S. (1995). Genetic and environmental factors influencing clutch size in equatorial and temperate zone stonechats (Saxicola torquata axillaris and S-T-rubicola): An experimental study. Auk 112, 748-755.
Gwinner, E. & Neusser, V. (1985). Post-Juvenile Molt of European and African Stonechats (Saxicola-Torquata-Rubicula and Saxicola-Torquata-Axillaris) and Their F1-Hybrids. Journal Fur Ornithologie 126, 219-220.
Haavie, J., Borge, T., Bures, S., Garamszegi, L. Z., Lampe, H. M., Moreno, J., Qvarnstrom, A., Torok, J. & Saetre, G. P. (2004). Flycatcher song in allopatry and sympatry – convergence, divergence and reinforcement. Journal of Evolutionary Biology 17, 227-237.
Haavie, J., Saetre, G. P. & Moum, T. (2000). Discrepancies in population differentiation at microsatellites, mitochondrial DNA and plumage colour in the pied flycatcher – inferring evolutionary processes. Molecular Ecology 9, 1137-1148.
Helm, B. (2009). Geographically distinct reproductive schedules in a changing world: Costly implications in captive Stonechats. Integrative and Comparative Biology 49, 563-579.
Helm, B. & Gwinner, E. (1999). Timing of postjuvenal molt in African (Saxicola torquata axillaris) and European (Saxicola torquata rubicola) Stonechats: Effects of genetic and environmental factors. Auk 116, 589-603.
Helm, B., Schwabl, I. & Gwinner, E. (2009). Circannual basis of geographically distinct bird schedules. Journal of Experimental Biology 212, 1259-1269.
Hogner, S., Laskemoen, T., Lifjeld, J. T., Porkert, J., Kleven, O., Albayrak, T., Kabasakal, B. & Johnsen, A. (2012a). Deep sympatric mitochondrial divergence without reproductive isolation in the common redstart Phoenicurus phoenicurus. Ecology and Evolution 2, 2974-2988.
Hogner, S., Saether, S. A., Borge, T., Bruvik, T., Johnsen, A. & Saetre, G. P. (2012b). Increased divergence but reduced variation on the Z chromosome relative to autosomes in Ficedula flycatchers: differential introgression or the faster-Z effect? Ecology and Evolution 2, 379-396.
Hogner, S., Riera, A. B., Wold, M., Lifjeld, J. T. & Johnsen, A. (2015). Intergeneric hybridization between Common Redstart Phoenicurus phoenicurus and Whinchat Saxicola rubetra revealed by molecular analyses. Journal of Ornithology, 1-8.
Janoušek, V., Fischerová, J., Mořkovský, L., Reif, J., Antczak, M., Albrecht, T. & Reifová, R. (2018) Postcopulatory sexual selection reduces Z-linked genetic variation and might contribute to the large Z effect in passerine birds. Heredity
Kawakami, T., Backstrom, N., Burri, R., Husby, A., Olason, P., Rice, A. M., Alund, M., Qvarnstrom, A. & Ellegren, H. (2014). Estimation of linkage disequilibrium and interspecific gene flow in Ficedula flycatchers by a newly developed 50k single-nucleotide polymorphism array. Molecular Ecology Resources 14, 1248-1260.
Kovats, D., Vegvari, Z. & Varga, Z. (2013). Morphological Patterns of a Nightingale Population in a Contact Zone of Luscinia Megarhynchos and L-Luscinia. Acta Zoologica Academiae Scientiarum Hungaricae 59, 157-170.
Kverek, P. (1998). A crossing of nightingale (Luscinia megarhynchos) and the thrush nightingale (Luscinia luscinia) occurs. Panurus 9, 99-102.
Kverek, P. (2002). Another nightingale (Luscinia megarhynchos) and the thrush nightingale (Luscinia luscinia) hybrid found. Sylvia 38, 125-128.
Kverek, P., Storchová, R., Reif, J. & Nachman, M. (2008). Occurrence of a hybrid between the Common Nightingale (Luscinia megarhynchos) and the Thrush Nightingale (Luscinia luscinia) in the Czech Republic confirmed by genetic analysis. Sylvia 44, 17-26.
Laaksonen, T., Sirkia, P. M., Calhim, S., Brommer, J. E., Leskinen, P. K., Primmer, C. R., Adamik, P., Artemyev, A. V., Belskii, E., Both, C., Bures, S., Burgess, M. D., Doligez, B., Forsman, J. T., Grinkov, V., Hoffmann, U., Ivankina, E., Kral, M., Krams, I., Lampe, H. M., Moreno, J., Magi, M., Nord, A., Potti, J., Ravussin, P. A. & Sokolov, L. (2015). Sympatric divergence and clinal variation in multiple coloration traits of Ficedula flycatchers. Journal of Evolutionary Biology 28, 779-790.
Landmann, A. (1987). Über Bastardierung und Mischbruten zwischen Gartenrotschwanz Phoenicurus phoenicurus und Hausrotschwanz P. ochruros. Ökol. Vögel 9, 97-106.
Lille, R. (1988). Species-specific song and mixed singing of Nightingale and Thrush Nightingale (Luscinia megarhynchos, L. luscinia). Journal für Ornithologie 129, 133-159.
Lim, H. C., Zou, F. S., Taylor, S. S., Marks, B. D., Moyle, R. G., Voelker, G. & Sheldon, F. H. (2010). Phylogeny of magpie-robins and shamas (Aves: Turdidae: Copsychus and Trichixos): implications for island biogeography in Southeast Asia. Journal of Biogeography 37, 1894-1906.
Merila, J., Sheldon, B. C. & Griffith, S. C. (2003). Heterotic effects on fitness in a wild bird population. Annales Zoologici Fennici 40, 269-280.
Morkovsky, L., Janousek, V., Reif, J., Ridl, J., Paces, J., Choleva, L., Janko, K., Nachman, M.W. & Reifova, R. (2018) Genomic islands of differentiation in two songbird species reveal candidate genes for hybrid female sterility. Molecular Ecology, 27:949-958.
Nater, A., Burri, R., Kawakami, T., Smeds, L. & Ellegren, H. (2015). Resolving Evolutionary Relationships in Closely Related Species with Whole-Genome Sequencing Data. Systematic Biology 64, 1000-1017.
Nöhring, R. (1943). Ueber die Bastardierung von Luscinia luscinia und Luscinia megarhynchos. Ornithologische Monatsberichte 51: 1 4.
Panov, E. N. (1986). New Data on Hybridization of Oenanthe-Pleschanka and Oenanthe-Hispanica. Zoologichesky Zhurnal 65, 1675-1683.
Panov, E. N., Grabovsky, V. I. & Ljubustchenko, S. V. (1993). Divergence and Hybrid Polymorphism in the Complex Eastern Pied Wheatears, Oenanthe-Picata. Zoologichesky Zhurnal 72, 80-96.
Qvarnstrom, A., Haavie, J., Saether, S. A., Eriksson, D. & Part, T. (2006). Song similarity predicts hybridization in flycatchers. Journal of Evolutionary Biology 19, 1202-1209.
Qvarnstrom, A., Rice, A. M. & Ellegren, H. (2010). Speciation in Ficedula flycatchers. Philosophical Transactions of the Royal Society B-Biological Sciences 365, 1841-1852.
Randler, C., Forschler, M. I., Gonzalez, J., Aliabadian, M., Bairlein, F. & Wink, M. (2012). Phylogeography, pre-zygotic isolation and taxonomic status in the endemic Cyprus Wheatear Oenanthe cypriaca. Journal of Ornithology 153, 303-312.
Reifová, R., Kverek, P. & Reif, J. (2011). The first record of a female hybrid between the Common Nightingale (Luscinia megarhynchos) and the Thrush Nightingale (Luscinia luscinia) in nature. Journal of Ornithology 152, 1063-1068.
Reifova, R., Reif, J., Antczak, M. & Nachman, M. W. (2011). Ecological character displacement in the face of gene flow: Evidence from two species of nightingales. BMC Evolutionary Biology 11.
Rubtsov, A. S. (1995). The Mean-Life Span of Males in a Hybrid Population of Oenanthe Hispanica and O-Pleschanka. Zoologichesky Zhurnal 74, 112-121.
Rybinski, J., Sirkiä, P., McFarlane, S., Vallin, N., Wheatcroft, D., Ålund, M., & Qvarnström A. (2016) Competition‐driven build‐up of habitat isolation and selection favoring modified dispersal patterns in a young avian hybrid zone. Evolution 70(10), 2226-2238.
Saether, S. A., Saetre, G. P., Borge, T., Wiley, C., Svedin, N., Andersson, G., Veen, T., Haavie, J., Servedio, M. R., Bures, S., Kral, M., Hjernquist, M. B., Gustafsson, L., Traff, J. & Qvarnstrom, A. (2007). Sex chromosome-linked species recognition and evolution of reproductive isolation in flycatchers. Science 318, 95-97.
Saetre, G. P., Borge, T., Lindell, J., Moum, T., Primmer, C. R., Sheldon, B. C., Haavie, J., Johnsen, A. & Ellegren, H. (2001). Speciation, introgressive hybridization and nonlinear rate of molecular evolution in flycatchers. Molecular Ecology 10, 737-749.
Saetre, G. P., Borge, T., Lindroos, K., Haavie, J., Sheldon, B. C., Primmer, C. & Syvanen, A. C. (2003). Sex chromosome evolution and speciation in Ficedula flycatchers. Proceedings of the Royal Society B-Biological Sciences 270, 53-59.
Saetre, G. P., Kral, M. & Bicik, V. (1993). Experimental-Evidence for Interspecific Female Mimicry in Sympatric Ficedula Flycatchers. Evolution 47, 939-945.
Saetre, G. P., Kral, M., Bures, S. & Ims, R. A. (1999). Dynamics of a clinal hybrid zone and a comparison with island hybrid zones of flycatchers (Ficedula hypoleuca and F-albicollis). Journal of Zoology 247, 53-64.
Saetre, G. P., Moum, T., Bures, S., Kral, M., Adamjan, M. & Moreno, J. (1997). A sexually selected character displacement in flycatchers reinforces premating isolation. Nature 387, 589-592.
Saetre, G. P. & Saether, S. A. (2010). Ecology and genetics of speciation in Ficedula flycatchers. Molecular Ecology 19, 1091-1106.
Schweizer, M., Warmuth, V., Kakhki, N.A., Aliabadian, M., Forschler, M., Shirihai, H., Suh, A. & Burri, R. (2019a) Parallel plumage colour evolution and introgressive hybridization in wheatears. Journal of Evolutionary Biology. 32(1), 100-110
Schweizer, M., Warmuth, V. M., Kakhki, N. A., Aliabadian, M., Förschler, M., Shirihai, H., Ewels, P., Guselius, J., Olden, R.-A., Schielzeth, H., Suh, A. & Burri, R. (2019b). Genome-wide evidence supports mitochondrial relationships and pervasive parallel phenotypic evolution in open-habitat chats. Molecular Phylogenetics and Evolution, 139, 106568.
Sedlacek, O., Fuchs, R. & Exnerova, A. (2004). Redstart Phoenicurus phoenicurus and black redstart P-ochruros in a mosaic urban environment: neighbours or rivals? Journal of Avian Biology 35, 336-343.
Seki, S. I., Nishiumi, I. & Saitoh, T. (2012). Distribution of Two Distinctive Mitochondrial DNA Lineages of the Japanese Robin Luscinia akahige Across Its Breeding Range Around the Japanese Islands. Zoological Science 29, 681-689.
Sheldon, F. H., Lohman, D. J., Lim, H. C., Zou, F., Goodman, S. M., Prawiradilaga, D. M., Winker, K., Braile, T. M. & Moyle, R. G. (2009). Phylogeography of the magpie-robin species complex (Aves: Turdidae: Copsychus) reveals a Philippine species, an interesting isolating barrier and unusual dispersal patterns in the Indian Ocean and Southeast Asia. Journal of Biogeography 36, 1070-1083.
Sirkiä, P.M., McFarlane, S.E., Jones, W., Wheatcroft, D., Ålund, M., Rybinski, J. & Qvarnström, A. (2018) Climate‐driven build‐up of temporal isolation within a recently formed avian hybrid zone. Evolution 72, 363-374.
Sorjonen, J. (1986). Mixed singing and interspecific territoriality-consequences of secondary contact of two ecologically and morphologically similar nightingale species in Europe. Ornis Scandinavica, 53-67.
Sottas, C., Reif, J., Kuczynski, L. & Reifova, R. (2018) Interspecific competition promotes habitat and morphological divergence in a secondary contact zone between two hybridizing songbirds. Journal of Evolutionary Biology.
Souriau, A., Kohoutová, H., Reif, J., Vokurková, J., Petrusek, A., Reifová, R. & Petrusková, T. (2018) Can mixed singing facilitate coexistence of closely related nightingale species? Behavioral Ecology 29(4), 925–932.
Spiridonova, L. N., Val’chuk, O. P., Belov, P. S. & Maslovsky, K. S. (2013). Intraspecific Genetic Differentiation of the Siberian Rubythroat (Luscinia calliope): Data of Sequencing the mtDNA Cytochrome b Gene. Russian Journal of Genetics 49, 638-644.
Starck, J. M., Konig, S. & Gwinner, E. (1995). Growth of Stonechats Saxicola-Torquata from Africa and Europe – an Analysis of Genetic and Environmental Components. Ibis 137, 519-531.
Storchova, R., Reif, J. & Nachman, M. W. (2010). Female Heterogamety and Speciation: Reduced Introgression of the Z Chromosome between Two Species of Nightingales. Evolution 64, 456-471.
Svedin, N., Wiley, C., Veen, T., Gustafsson, L. & Qvarnstrom, A. (2008). Natural and sexual selection against hybrid flycatchers. Proceedings of the Royal Society B-Biological Sciences 275, 735-744.
Tegelstrom, H. & Gelter, H. P. (1990). Haldane Rule and Sex Biased Gene Flow between 2 Hybridizing Flycatcher Species (Ficedula albicollis and F. hypoleuca, Aves, Muscicapidae). Evolution 44, 2012-2021.
Tieleman, B. I., Versteegh, M. A., Fries, A., Helm, B., Dingemanse, N. J., Gibbs, H. L. & Williams, J. B. (2009). Genetic modulation of energy metabolism in birds through mitochondrial function. Proceedings of the Royal Society B-Biological Sciences 276, 1685-1693.
Vallin, N., Nonaka, Y., Feng, J. & Qvarnstrom, A. (2013). Relative performance of hybrid nestlings in Ficedula flycatchers: a translocation experiment. Ecology and Evolution 3, 356-364.
Vallin, N., Rice, A. M., Arntsen, H., Kulma, K. & Qvarnstrom, A. (2012a). Combined effects of interspecific competition and hybridization impede local coexistence of Ficedula flycatchers. Evolutionary Ecology 26, 927-942.
Vallin, N., Rice, A. M., Bailey, R. I., Husby, A. & Qvarnstrom, A. (2012b). Positive Feedback between Ecological and Reproductive Character Displacement in a Young Avian Hybrid Zone. Evolution 66, 1167-1179.
Veen, T., Borge, T., Griffith, S. C., Saetre, G. P., Bures, S., Gustafsson, L. & Sheldon, B. C. (2001). Hybridization and adaptive mate choice in flycatchers. Nature 411, 45-50.
Veen, T., Hjernquist, M. B., Van Wilgenburg, S. L., Hobson, K. A., Folmer, E., Font, L. & Klaassen, M. (2014). Identifying the African Wintering Grounds of Hybrid Flycatchers Using a Multi-Isotope (delta H-2, delta C-13, delta N-15) Assignment Approach. Plos One 9.
Veen, T., Svedin, N., Forsman, J. T., Hjernquist, M. B., Qvarnstrom, A., Hjernquist, K. A. T., Traff, J. & Klaassen, M. (2007). Does migration of hybrids contribute to post-zygotic isolation in flycatchers? Proceedings of the Royal Society B-Biological Sciences 274, 707-712.
Versteegh, M. A., Helm, B., Gwinner, E. & Tieleman, B. I. (2012). Annual cycles of metabolic rate are genetically determined but can be shifted by phenotypic flexibility. Journal of Experimental Biology 215, 3459-3466.
Versteegh, M. A., Helm, B., Kleynhans, E. J., Gwinner, E. & Tieleman, B. I. (2014). Genetic and phenotypically flexible components of seasonal variation in immune function. Journal of Experimental Biology 217, 1510-1518.
Vokurkova, J., Petruskova, T., Reifova, R., Kozman, A., Morkovsky, L., Kipper, S., Weiss, M., Reif, J., Dolata, P. T. & Petrusek, A. (2013). The Causes and Evolutionary Consequences of Mixed Singing in Two Hybridizing Songbird Species (Luscinia spp.). Plos One 8.
Wallin, L. (1986). Divergent Character Displacement in the Song of 2 Allospecies – the Pied Flycatcher Ficedula hypoleuca, and the Collared Flycatcher Ficedula albicollis. Ibis 128, 251-259.
Wiley, C., Bengtson, J. M., Svedin, N. & Qvarnstrom, A. (2005). Hybridization cost of delayed maturation of secondary sexual traits in the collared flycatcher. Evolution 59, 2711-2716.
Wiley, C., Fogelberg, N., Saether, S. A., Veen, T., Svedin, N., Kehlenbeck, J. V. & Qvarnstrom, A. (2007). Direct benefits and costs for hybridizing Ficedula flycatchers. Journal of Evolutionary Biology 20, 854-864.
Wiley, C., Qvarnstrom, A., Andersson, G., Borge, T. & Saetre, G. P. (2009a). Postzygotic Isolation over Multiple Generations of Hybrid Descendents in a Natural Hybrid Zone: How Well Do Single-Generation Estimates Reflect Reproductive Isolation? Evolution 63, 1731-1739.
Wiley, C., Qvarnstrom, A. & Gustafsson, L. (2009b). Effects of hybridization on the immunity of collared Ficedula albicollis and pied flycatchers F-hypoleuca, and their infection by haemosporidians. Journal of Avian Biology 40, 352-357.
* Ficedula text reviewed by Hans Ellegren (Uppsala Universitet, Sweden)