Anseriformes comprises about 150 species, divided over three families: Anhimidae (screamers), Anseranatidae (Magpie Goose) and Anatidae (ducks, geese and swans). Overviews of the incidence of hybridization among Anseriformes have been published (Johnsgard, 1960; Randler, 2000; Randler, 2008; Scherer & Hilsberg, 1982). Apart from a phylogenetic analysis, Gonzalez et al. (2009) also discuss patterns of hybridization. In waterfowl, the occurrence of hybridization can be explained by interspecific brood amalgamation and (to a lesser extent) forced extrapair copulations (Randler, 2005).
Geese (Anser and Branta)
Hybridization is common among geese (Ottenburghs et al., 2016b) and it has probably impacted their evolutionary history (Ottenburghs et al. 2016a; Ottenburghs, et al. 2017). The mitochondrial DNA of Anser species is closely related, but this could also be due to incomplete lineage sorting (Ruokonen, Kvist & Lumme, 2000). A few studies have compiled all occurrences (Delany, 1992; Kampe-Persson & Lerner, 2007; Randler, 2000; Randler, 2008).
Several species combinations have been studied. Hybrids between Snow Goose (A. caerulescens) and Ross’ Goose (A. rossii) have been reported (Hatch & Shortt, 1976; Trauger, Dzubin & Ryder, 1971) and experimental (Macinnes & Kerbes, 1987) and genetic (Weckstein et al., 2002) studies are known. Canada Goose (B. canadensis) hybrids with Snow Goose (Nelson, 1952; Prevett & Macinnes, 1973) and Greater White-fronted Goose (Craven & Westemeier, 1979) have only been documented, with some estimates of frequency.
The occurrence of wild hybrids between Greater and Lesser White-fronted Goose (A. erythropus) is certain (Nijman, Aliabadian & Roselaar, 2010), while the genetic purity of captive birds that were destined for a reintroduction programme remains a matter of debate (Ruokonen, Andersson & Tegelstrom, 2007).
A hybrid zone between Canada Goose and Cackling Goose (B. hutchinsii) is located in Northern Canada (Leafloor, Moore & Scribner, 2013).
Hybrids between Greylag Goose and domestic breeds have been confirmed by mtDNA analyses (Heikkinen et al., 2015).
The morphology of the following hybrids have been described in more detail: Bar-headed Goose x Snow Goose (Lehmhus & Gustavsson, 2014), Greylag Goose x Canada Goose (Gustavsson, 2011) and several hybrids between Barnacle Goose and Anser species (Gustavsson, 2009). In addition, some hybrids have coloured tail-coverts although all goose species have white tail-coverts (Gustavsson, 2010).
Honka et al. (2017) collected Bean Geese that were shot by Finnish hunters between 2010 and 2013. A genetic analysis, based on mitochondrial DNA (mtDNA) and microsatellites, revealed that the most shot geese belonged to the Taiga subspecies (A. f. fabalis). one individual carried mtDNA from a Pink-footed Goose (A. brachyrhynchus), while another individual had Greater White-fronted Goose (A. albifrons) DNA.
An ancient DNA study suggests that there could have been hybridization between domestic Greylag Geese and wild Taiga Bean Geese in Russia (Honka et al., 2018).
The behaviour of hybrids between Swan Goose (A. cygnoides) and Greylag Goose (A. anser), such as vigilance (Randler, 2003) and aggression (Randler, 2004), is no different from pure species. Also, the breeding times of Snow Goose x Greylag Goose hybrids are intermediate in a captive setting (Davies, Fischer & Gwinner, 1969).
Apart from these morphological, ecological and genetic studies, goose hybrids have also been extensively studied with regard to the meat industry (Kowalczyk, Adamski & Lukaszewicz, 2013; Kowalczyk & Lukaszewicz, 2012; Mazanowski & Bernacki, 2006).
The duck genus Anas has been studied extensively. Several studies provided clues for introgressive hybridization by studying several species (Kraus et al., 2012; Lavretsky, McCracken & Peters, 2014; Peters et al., 2005; Peters et al., 2014b).
Mallard (Anas platyrhynchos)
The key species in this group is the Mallard (A. platyrhynchos), which hybridizes with the majority of other duck species. Phylogeographic analysis of the Mallard revealed two mitochondrial haplotype clades, A and B (Avise, Ankney & Nelson, 1990; Kraus et al., 2011; Kulikova et al., 2005; Kulikova, Poysa & Zhuravlev, 2012). Worldwide, several monochromatic duck species evolved, with which the Mallard hybridizes extensively:
- Black Duck (A. superciliosa) in Australia and New Zealand (Braithwaite & Miller, 1975; Gillespie, 1985; Haddon, 1984; Rhymer, Williams & Braun, 1994; Taysom, Johnson & Guay, 2014)
- Hawaii Duck (A. wylvilliana) on the Hawaiian islands (Browne et al., 1993; Fowler, Eadie & Engilis, 2009)
- American Black Duck (A. rubripes) in North America (Johnsgard, 1967; Kirby, Sargeant & Shutler, 2004; Mank, Carlson & Brittingham, 2004)
- Spot-billed Duck (A. zonorhyncha) in the Russian Far East (Kulikova, Zhuravlev & McCracken, 2004)
- Mottled Duck (A. fulvigula) in North America (Bielefeld et al., 2016; Ford, Selman & Taylor, 2017; McCracken, Johnson & Sheldon, 2001; Peters et al., 2016; Peters et al., 2014a; Seyoum et al., 2012; Williams et al., 2005).
- Mexican Duck (A. diazi) in Mexico (Lavretsky et al., 2015a)
Experimental work on Mallard x Black Duck hybrids showed that hybrids were more similar to Black Ducks in terms of salt toleration (Barnes & Nudds, 1991). Hybrids also tended to have more Sarcocystis parasites compared to “pure” species (Mason & Clark, 1990).
The Hawaiian duck may represent a young hybrid species. Genetic analyses show that the contemporary Hawaiian duck is descended from an ancient hybridization event (dated around the Pleistocene-Holocene boundary) between Mallard and Laysan duck (A. laysanensis) (Lavretsky et al., 2015).
Other duck species
Besides the Mallard, other combinations of hybridizing duck species have been studied, such as Gadwalls (A. strepera) and Falcated ducks (A. falcata) in North America (Peters & Omland, 2007; Peters et al., 2007), Speckled Teal (A. flavirostris) and Yellow-billed Pintail (A. georgica) on the Falkland Islands (McCracken & Wilson, 2011), and Grey Teal (A. gracilis) and Chestnut Teal (A. castanea) in Australia (Joseph et al., 2009).
Genetic analyses show that hybridization between Grey Teal and Pacific Black Duck occurs bidirectionally (Guay et al., 2015).
Several subspecies of the Common Teal (A. crecca) also interbreed: there is continental gene flow between A. c. crecca and A. c. carolensis (Peters et al., 2012), and possible heteropatric speciation (Winker, 2006) between A. c. crecca and A. c. nimia (Winker et al., 2013).
In Japan, the behaviour of a A. penelope x falcata hybrid was recorded (Chiba & Honma, 2010).
In the Netherlands, a presumed hybrid between Northern Shoveler (A. clypeata) and Blue-winged Teal (A. discors) was reported. This hybrid has also been sighted in several other European countries (van Bemmelen et al. 2018).
Hybridization between Speckled Teal and Yellow-billed Pintail might have led to the introgression of hemoglobin molecular adapted to high altitude, an possible example of adaptive introgression in birds (Natarajan et al., 2015). In addition, there is gene flow of neutral loci between highland and lowland populations of Yellow-billed Pintail. Gene flow is restricted for hemoglobin genes and mtDNA, suggesting local adaptation (McCracken et al., 2009). A similar has been uncovered between subspecies of Cinnamon Teal (Anas cyanoptera) in South America. Gene flow of neutral loci is mainly from highlands to lowlands (Wilson et al., 2013).
One experimental study looked at sexual imprinting of a facultative brood parasite (A. americana) on his host (A. valisineria). They found that cross-fostered individuals approached and courted more towards their foster parents (Sorenson, Hauber & Derrickson, 2010).
Lavretsky et al. (2016) found evidence for gene flow from Lesser (A. affinis) into Greater Scaup (A. marila).
Bucephala and Mergus
Hybrids between species of this genus have only been described briefly, for instance Common Goldeneye (B. clangula) interbreeds with Barrow’s Goldeneye (B. islandica) (Martin & Dilabio, 1994), Smew (Mergus albellus) (Kovacovsky & Rychlik, 1999) and Bufflehead (B. albeola) (Finley & Huot, 2010).
Hybrids between Mute Swan (C. olor) and Whooper Swan (C. cygnus), and between Whooper Swan and Bewick’s Swan (C. columbianus bewickii) have been documented in the Baltic and are probably due to recent range expansions (Kampe-Persson & Boiko, 2011). Studies on hybrid swans are rare. One study compared the call characteristics of Trumpeter Swans (C. buccinators), Tundra Swans (C. columbianus) and their hybrids (Wood, Brooks & Sladen, 2002). Another study documented the behaviour of a hybrid between Whooper Swan and Mute Swan (Panov & Pavlova, 2010).
This genus is well known for the conservation issue of hybridization between the threatened White-headed Duck (O. leucocephala) and the invasive Ruddy Duck (O. jamaicensis) in Spain. The Ruddy Ducks are descendants of captive birds from the UK (Munoz-Fuentes et al., 2006), but a successful elimination program has prevented extensive introgression (Munoz-Fuentes et al., 2007). The situation is nicely summarized in Munoz-Fuentes et al. (2013). Both species and their hybrids use similar food sources (Sanchez, Green & Dolz, 2000).
The Colombian population of Ruddy Ducks (subspecies andina) is probably of hybrid origin, it received gene flow from the North American (jamaicensis) and Andean (ferruginea) subspecies (Lozano-Jaramillo et al., 2018).
One study describes a hybrid between Common Eider (S. mollissima) and King Eider (S. spectabilis) on the Kent Peninsula (Trefry, Dickson & Hoover, 2007).
Avise, J. C., Ankney, C. D. & Nelson, W. S. (1990). Mitochondrial Gene Trees and the Evolutionary Relationship of Mallard and Black Ducks. Evolution 44, 1109-1119.
Barnes, G. G. & Nudds, T. D. (1991). Salt Tolerance in American Black Ducks, Mallards, and Their F1-Hybrids. Auk 108, 89-98.
Bielefeld, R. R., Engilis, A., Feddersen, J. C., Eadie, J. M., Tringali, M. D. & Benedict, R. J. (2016). Is it a mottled duck? The key is in the feathers. Wildlife Society Bulletin 40, 446-455.
Braithwaite, L. & Miller, B. (1975). The mallard, Anas platyrhynchos, and mallard-black duck, Anas superciliosa rogersi, hybridization. Wildlife Research 2, 47-61.
Browne, R. A., Griffin, C. R., Chang, P. R., Hubley, M. & Martin, A. E. (1993). Genetic-Divergence among Populations of the Hawaiian Duck, Laysan Duck, and Mallard. Auk 110, 49-56.
Champagnon, J., Crochet, P. A., Kreisinger, J., Cizkova, D., Gauthier-Clerc, M., Massez, G., Soderquist, P., Albrecht, T. & Guillemain, M. (2013). Assessing the genetic impact of massive restocking on wild mallard. Animal Conservation 16, 295-305.
Chiba, A. & Honma, R. (2010). Morphological and behavioral traits of a wild hybrid Eurasian Wigeon x Falcated Duck male found at Hyo-ko Waterfowl Park, Niigata, Japan. Ornithological Science 9, 123-130.
Cizkova, D., Javurkova, V., Champagnon, J. & Kreisinger, J. (2012). Duck’s not dead: Does restocking with captive bred individuals affect the genetic integrity of wild mallard (Anas platyrhynchos) population? Biological Conservation 152, 231-240.
Craven, S. R. & Westemeier, R. L. (1979). PROBABLE CANADA GOOSE X WHITE-FRONTED GOOSE HYBRIDS. Wilson Bulletin 91, 628-629.
Davies, C., Fischer, H. & Gwinner, E. (1969). Die Brutzeiten einiger Gänsearten und ihrer Bastarde in identischen Bedingungen. Oecologia 3, 266-276.
Delany, S. N. (1992). Survey of Introduced Geese in Britain, Summer 1991: Provisional Results. Unpublished report to JNCC, Central Science Laboratory and National Trust. WWT, Slimbridge.
Finley, J. K. & Huot, S. (2010). Interspecific Mate Choice and Hybridism in the Bufflehead, Bucephala albeola. Canadian Field-Naturalist 124, 28-31.
Ford, R. J., Selman, W. & Taylor, S. S. (2017). Hybridization between Mottled Ducks (Anas fulvigula maculosa) and Mallards (A. platyrhynchos) in the western Gulf Coast region. The Condor 119, 683-696.
Fowler, A. C., Eadie, J. M. & Engilis, A. (2009). Identification of endangered Hawaiian ducks (Anas wyvilliana), introduced North American mallards (A. platyrhynchos) and their hybrids using multilocus genotypes. Conservation Genetics 10, 1747-1758.
Gillespie, G. D. (1985). Hybridization, Introgression, and Morphometric Differentiation between Mallard (Anas-Platyrhynchos) and Grey Duck (Anas-Superciliosa) in Otago, New-Zealand. Auk 102, 459-469.
Gonzalez, J., Duttmann, H. & Wink, M. (2009). Phylogenetic relationships based on two mitochondrial genes and hybridization patterns in Anatidae. Journal of Zoology 279, 310-318.
Guay, P.-J., Monie, L., Robinson, R. W. & van Dongen, W. F. (2015). What the direction of matings can tell us of hybridisation mechanisms in ducks. Emu.
Gustavsson, C. G. (2009). Images of Barnacle Goose Branta leucopsis hybrids–a photo documentation of some crosses with different Anser species. Ornis Svecica 19, 19-31.
Gustavsson, C. G. (2010). Coloured tail-coverts in Anser× Branta goose hybrids despite all-white coverts in both parent species. Ornis Svecica 20, 67-75.
Gustavsson, C. G. (2011). A white feathered area on the forehead is more common in hybrids of Greylag Goose Anser anser and Greater Canada Goose Branta canadensis than in the parent species. Ornis Svecica, 37-44.
Haddon, M. (1984). A Re-Analysis of Hybridization between Mallards and Grey Ducks in New-Zealand. Auk 101, 190-191.
Hatch, D. R. M. & Shortt, A. H. (1976). Possible Intermediate Ross Goose and Snow Goose in Manitoba. Auk 93, 391-392.
Heikkinen, M. E., Ruokonen, M., Alexander, M., Aspi, J., Pyhajarvi, T. & Searle, J. B. (2015). Relationship between wild greylag and European domestic geese based on mitochondrial DNA. Animal Genetics 46, 485-497.
Honka, J., Kvist, L., Heikkinen, M. E., Helle, P., Searle, J. B. & Aspi, J. (2017). Determining the subspecies composition of bean goose harvests in Finland using genetic methods. European Journal of Wildlife Research 63, 19.
Honka, J., Heino, M.T., Kvist, L., Askeyev, I.V., Shaymuratova, D.N., Askeyev, O.V., Askeyev, A.O., Heikkinen, M.E., Searle, J.B. & Aspi, J. (2018) Over a Thousand Years of Evolutionary History of Domestic Geese from Russian Archaeological Sites, Analysed Using Ancient DNA. Genes 9, 369.
Johnsgard, P. A. (1960). Hybridization in the Anatidae and its taxonomic implications. Condor, 25-33.
Johnsgard, P. A. (1967). Sympatry Changes and Hybridization Incidence in Mallards Black Ducks. American Midland Naturalist 77, 51-&.
Joseph, L., Adcock, G. J., Linde, C., Omland, K. E., Heinsohn, R., Chesser, R. T. & Roshier, D. (2009). A tangled tale of two teal: population history of the grey Anas gracilis and chestnut teal A-castanea of Australia. Journal of Avian Biology 40, 430-439.
Kampe-Persson, H. & Boiko, D. (2011). Occurrence of swan hybrids around the Baltic Sea – an outcome of range expansions. Ornis Svecica 21, 45-54.
Kampe-Persson, H. & Lerner, H. (2007). Occurrence of hybrid geese in Sweden – a conservation problem? Ornis Svecica, 154-186.
Kirby, R. E., Sargeant, G. A. & Shutler, D. (2004). Haldane’s rule and American black duck x mallard hybridization. Canadian Journal of Zoology-Revue Canadienne De Zoologie 82, 1827-1831.
Kovacovsky, P. & Rychlik, I. (1999). Observations of the smew (Mergus albellus) x golden eye (Bucephala clangula) hybrid on the Danube in Slovakia. BIOLOGIA 54, 219-221.
Kowalczyk, A., Adamski, M. & Lukaszewicz, E. (2013). Slaughter yield and meat quality of hybrid derived from crosses between Canada Goose (Branta canadensis L.) males and White Koluda (Anser anser L.) goose females. Archiv Fur Geflugelkunde 77, 66-71.
Kowalczyk, A. & Lukaszewicz, E. (2012). The possibility of obtaining intergeneric hybrids via White Koluda (Anser anser L.) goose insemination with fresh and frozen-thawed Canada goose (Branta canadensis L.) gander semen. Theriogenology 77, 507-513.
Kraus, R. H. S., Kerstens, H. H. D., van Hooft, P., Megens, H. J., Elmberg, J., Tsvey, A., Sartakov, D., Soloviev, S. A., Crooijmans, R. P. M. A., Groenen, M. A. M., Ydenberg, R. C. & Prins, H. H. T. (2012). Widespread horizontal genomic exchange does not erode species barriers among sympatric ducks. Bmc Evolutionary Biology 12.
Kraus, R. H. S., Zeddeman, A., van Hooft, P., Sartakov, D., Soloviev, S. A., Ydenberg, R. C. & Prins, H. H. T. (2011). Evolution and connectivity in the world-wide migration system of the mallard: Inferences from mitochondrial DNA. Bmc Genetics 12.
Kulikova, I. V., Drovetski, S. V., Gibson, D. D., Harrigan, R. J., Rohwer, S., Sorenson, M. D., Winker, K., Zhuravlev, Y. N. & McCracken, K. G. (2005). Phylogeography of the Mallard (Anas platyrhynchos): Hybridization, dispersal, and lineage sorting contribute to complex geographic structure. Auk 122, 949-965.
Kulikova, I. V., Poysa, H. & Zhuravlev, Y. N. (2012). Phylogeography of the mallard Anas platyrhynchos from Eurasia inferred from sequencing of the mtDNA control region. Russian Journal of Genetics 48, 705-712.
Kulikova, I. V., Zhuravlev, Y. N. & McCracken, K. G. (2004). Asymmetric hybridization and sex-biased gene flow between Eastern Spot-billed Ducks (Anas zonorhyncha) and Mallards (A-platyrhynchos) in the Russian Far East. Auk 121, 930-949.
Lavretsky, P., Dacosta, J. M., Hernandez-Banos, B. E., Engilis, A., Sorenson, M. D. & Peters, J. L. (2015a). Speciation genomics and a role for the Z chromosome in the early stages of divergence between Mexican ducks and mallards. Molecular Ecology 24, 5364-5378.
Lavretsky, P., McCracken, K. G. & Peters, J. L. (2014). Phylogenetics of a recent radiation in the mallards and allies (Aves: Anas): Inferences from a genomic transect and the multispecies coalescent. Molecular Phylogenetics and Evolution 70, 402-411.
Lavretsky, P., Engilis, A., Eadie, J. M. & Peters, J. L. (2015). Genetic admixture supports an ancient hybrid origin of the endangered Hawaiian duck. Journal of Evolutionary Biology 28, 1005-1015.
Lavretsky, P., Peters, J. L., Winker, K., Bahn, V., Kulikova, I., Zhuravlev, Y. N., Wilson, R. E., Barger, C., Gurney, K. & McCracken, K. G. (2016). Becoming pure: identifying generational classes of admixed individuals within lesser and greater scaup populations. Molecular Ecology.
Leafloor, J. O., Moore, J. A. & Scribner, K. T. (2013). A Hybrid Zone between Canada Geese (Branta Canadensis) and Cackling Geese (B. Hutchinsii). Auk 130, 487-500.
Lehmhus, J. & Gustavsson, C. G. (2014). Hybrids between Bar-headed Goose Anser indicus and Snow Goose Anser caerulescens. Ornis Scvecica, 147-163.
Lozano-Jaramillo, M., McCracken, K.G. & Cadena, C.D. (2018) Neutral and functionally important genes shed light on phylogeography and the history of high-altitude colonization in a widespread New Worlds duck. Ecology and Evolution 8, 6515-6528.
Macinnes, C. D. & Kerbes, R. H. (1987). Growth of the Snow Goose, Chen-Caerulescens, Colony at Mcconnell River, Northwest-Territories – 1940-1980. Canadian Field-Naturalist 101, 33-39.
Mank, J. E., Carlson, J. E. & Brittingham, M. C. (2004). A century of hybridization: Decreasing genetic distance between American black ducks and mallards. Conservation Genetics 5, 395-403.
Martin, P. R. & Dilabio, B. M. (1994). Natural Hybrids between the Common Goldeneye Bucephala-Clangula, and Barrows Goldeneye, B-Islandica. Canadian Field-Naturalist 108, 195-198.
Mason, J. R. & Clark, L. (1990). Sarcosporidiosis Observed More Frequently in Hybrids of Mallards and American Black Ducks. Wilson Bulletin 102, 160-162.
Mazanowski, A. & Bernacki, Z. (2006). Characteristics of reproductive traits and egg traits in Graylag goose (Anser anser L.) crossbreds. Archiv fur Geflugelkunde 70, 56-63.
McCracken, K. G., Johnson, W. P. & Sheldon, F. H. (2001). Molecular population genetics, phylogeography, and conservation biology of the mottled duck (Anas fulvigula). Conservation Genetics 2, 87-102.
McCracken, K.G., Bulgarella, M., Johnson, K.P., Kuhner, M.K., Trucco, J., Valqui, T.H., Wilson, R.E., & Peters, J.L. (2009) Gene Flow in the Face of Countervailing Selection: Adaptation to High-Altitude Hypoxia in the βA Hemoglobin Subunit of Yellow-Billed Pintails in the Andes. Molecular Biology and Evolution 26(4), 815–827.
McCracken, K. G. & Wilson, R. E. (2011). Gene Flow and Hybridization between Numerically Imbalanced Populations of Two Duck Species in the Falkland Islands. Plos One 6.
Munoz-Fuentes, V., Green, A. J. & Negro, J. J. (2013). Genetic studies facilitated management decisions on the invasion of the ruddy duck in Europe. Biological Invasions 15, 723-728.
Munoz-Fuentes, V., Green, A. J., Sorenson, M. D., Negro, J. J. & Vila, C. (2006). The ruddy duck Oxyura jamaicensis in Europe: natural colonization or human introduction? Molecular Ecology 15, 1441-1453.
Munoz-Fuentes, V., Vila, C., Green, A. J., Negro, J. J. & Sorenson, M. D. (2007). Hybridization between white-headed ducks and introduced ruddy ducks in Spain. Molecular Ecology 16, 629-638.
Natarajan, C., Projecto-Garcia, J., Moriyama, H., Weber, R. E., Munoz-Fuentes, V., Green, A. J., Kopuchian, C., Tubaro, P. L., Alza, L., Bulgarella, M., Smith, M. M., Wilson, R. E., Fago, A., McCracken, K. G. & Storz, J. F. (2015). Convergent Evolution of Hemoglobin Function in High-Altitude Andean Waterfowl Involves Limited Parallelism at the Molecular Sequence Level. PLoS Genet 11, e1005681.
Nelson, H. K. (1952). Hybridization of Canada Geese with Blue Geese in the wild. The Auk, 425-428.
Nijman, V., Aliabadian, M. & Roselaar, C. S. (2010). Wild hybrids of Lesser White-fronted Goose (Anser erythropus) x Greater White-fronted Goose (A. albifrons) (Aves: Anseriformes) from the European migratory flyway. Zoologischer Anzeiger 248, 265-271.
Ottenburghs, J., Megens, H. J., Kraus, R. H., Madsen, O., van Hooft, P., van Wieren, S. E., Crooijmans, R. P., Ydenberg, R. C., Groenen, M. A. & Prins, H. H. (2016a). A tree of geese: A phylogenomic perspective on the evolutionary history of True Geese. Molecular Phylogenetics and Evolution 101, 303-313.
Ottenburghs, J., van Hooft, P., van Wieren, S. E., Ydenberg, R. C. & Prins, H. H. T. (2016b). Hybridization in geese: a review. Frontiers in Zoology 13, 1-9.
Ottenburghs J., Megens H.-J., Kraus R.H.S., van Hooft P., van Wieren S.E., Crooijmans R.P.M.A., Ydenberg R.C., Groenen M.A.M., Prins H.H.T. (2017_. A history of hybrids? Genomic patterns of introgression in the True Geese. BMC Evolutionary Biology 17:201.
Panov, E. N. & Pavlova, E. Y. (2010). Observations on a Hybrid between the Whooper Swan (Cygnus cygnus) and the Mute Swan (Cygnus olor). Biology Bulletin 37, 759-767.
Peters, J. L., Lavretsky, P., DaCosta, J. M., Bielefeld, R. R., Feddersen, J. C. & Sorenson, M. D. (2016). Population genomic data delineate conservation units in mottled ducks (Anas fulvigula). Biological Conservation 203, 272-281.
Peters, J. L., Mccracken, K. G., Pruett, C. L., Rohwer, S., Drovetski, S. V., Zhuravlev, Y. N., Kulikova, I., Gibson, D. D. & Winker, K. (2012). A parapatric propensity for breeding precludes the completion of speciation in common teal (Anas crecca, sensu lato). Molecular Ecology 21, 4563-4577.
Peters, J. L., McCracken, K. G., Zhuravlev, Y. N., Lu, Y., Wilson, R. E., Johnson, K. P. & Omland, K. E. (2005). Phylogenetics of wigeons and allies (Anatidae : Anas): the importance of sampling multiple loci and multiple individuals. Molecular Phylogenetics and Evolution 35, 209-224.
Peters, J. L. & Omland, K. E. (2007). Population structure and mitochondrial polyphyly in North American Gadwalls (Anas strepera). Auk 124, 444-462.
Peters, J. L., Sonsthagen, S. A., Lavretsky, P., Rezsutek, M., Johnson, W. P. & McCracken, K. G. (2014a). Interspecific hybridization contributes to high genetic diversity and apparent effective population size in an endemic population of mottled ducks (Anas fulvigula maculosa). Conservation Genetics 15, 509-520.
Peters, J. L., Winker, K., Millam, K. C., Lavretsky, P., Kulikova, I., Wilson, R. E., Zhuravlev, Y. N. & McCracken, K. G. (2014b). Mito-nuclear discord in six congeneric lineages of Holarctic ducks (genus Anas). Molecular Ecology 23, 2961-2974.
Peters, J. L., Zhuravlev, Y., Fefelov, I., Logie, A. & Omland, K. E. (2007). Nuclear loci and coalescent methods support ancient hybridization as cause of mitochondrial paraphyly between gadwall and falcated duck (Anas spp.). Evolution 61, 1992-2006.
Prevett, J. P. & Macinnes, C. D. (1973). Observations of Wild Hybrids between Canada and Blue Geese. Condor 75, 124-125.
Randler, C. (2000). Wasservogelhybriden (Anseriformes) im westlichen Mitteleuropa – Verbreitung, Auftreten und Ursachen. Ökologie der Vögel 22, 1-106.
Randler, C. (2003). Vigilance in urban Swan Geese and their hybrids. Waterbirds 26, 257-260.
Randler, C. (2004). Aggressive interactions in Swan Geese Anser cygnoides and their hybrids. Acta Ornithologica 39, 147-153.
Randler, C. (2005). Do forced extrapair copulations and interspecific brood amalgamation facilitate natural hybridisation in wildfowl? Behaviour 142, 477-488.
Randler, C. (2008). Hybrid wildfowl in Central Europe – an overview. Waterbirds 31, 143-146.
Rhymer, J. M., Williams, M. J. & Braun, M. J. (1994). Mitochondrial Analysis of Gene Flow between New-Zealand Mallards (Anas-Platyrhynchos) and Grey-Ducks (a-Superciliosa). Auk 111, 970-978.
Ruokonen, M., Andersson, A. C. & Tegelstrom, H. (2007). Using historical captive stocks in conservation. The case of the lesser white-fronted goose. Conservation Genetics 8, 197-207.
Ruokonen, M., Kvist, L. & Lumme, J. (2000). Close relatedness between mitochondrial DNA from seven Anser goose species. Journal of Evolutionary Biology 13, 532-540.
Sanchez, M. I., Green, A. J. & Dolz, J. C. (2000). The diets of the White-headed duck Oxyura leucocephala, Ruddy Duck O-jamaicensis and their hybrids from Spain. Bird Study 47, 275-284.
Scherer, S. & Hilsberg, T. (1982). Hybridisierung und Verwandtschaftsgrade innerhalb der Anatidae-eine systematische und evolutionstheoretische Betrachtung. Journal of Ornithology 123, 357-380.
Seyoum, S., Tringali, M. D., Bielefeld, R. R., Feddersen, J. C., Benedict, R. J., Fanning, A. T., Barthel, B. L., Curtis, C., Puchulutegui, C., Roberts, A. C. M., Villanova, V. L. & Tucker, E. C. (2012). Fifty-nine microsatellite markers for hybrid classification studies involving endemic Florida Mottled Duck (Anas fulvigula fulvigula) and invasive Mallards (A. platyrhynchos). Conservation Genetics Resources 4, 681-687.
Söderquist, P., Elmberg, J., Gunnarsson, G., Thulin, C. G., Champagnon, J., Guillemain, M., Kreisinger, J., Prins, H. H.T., Crooijmans, R. P.M.A., Kraus, R. H.S. (2017) Admixture between released and wild game birds: a changing genetic landscape in European mallards (Anas platyrhynchos). European Journal of Wildlife Research 63, 98.
Sorenson, M. D., Hauber, M. E. & Derrickson, S. R. (2010). Sexual imprinting misguides species recognition in a facultative interspecific brood parasite. Proceedings of the Royal Society B-Biological Sciences 277, 3079-3085.
Taysom, A., Johnson, J. & Guay, P. J. (2014). Establishing a genetic system to distinguish between domestic Mallards, Pacific Black Ducks and their hybrids. Conservation Genetics Resources 6, 197-199.
Trauger, D. L., Dzubin, A. & Ryder, J. P. (1971). White Geese Intermediate between Ross’ Geese and Lesser Snow Geese. Auk 88, 856-&.
Trefry, S. A., Dickson, D. L. & Hoover, A. K. (2007). A common eider x king eider hybrid captured on the Kent Peninsula, Nunavut. Arctic 60, 251-254.
van Bemmelen, R.S.A., Lehmhus, J. & Mlodinow, S.G. (2018) Hybrid Northern Shoveler x Blue-winged Teal on Schiermonnikoog, Netherlands, in May 2014, and identification and WP occurrence. Dutch Birding, 40:71-81.
Weckstein, J. D., Afton, A. D., Zink, R. M. & Alisauskas, R. T. (2002). Hybridization and population subdivision within and between Ross’s Geese and Lesser Snow Geese: A molecular perspective. Condor 104, 432-436.
Williams, C. L., Brust, R. C., Fendley, T. T., Tiller, G. R. & Rhodes, O. E. (2005). A comparison of hybridization between mottled ducks (Anas fulvigula) and mallards (A-Platyrhynchos) in Florida and South Carolina using microsatellite DNA analysis. Conservation Genetics 6, 445-453.
Wilson, R.E., Peters, J.L. & McCracken, K.G. (2013) Genetic and phenotypic divergence between low‐and high‐altitude populations of two recently diverged cinnamon teal subspecies. Evolution 67(1), 170-184.
Winker, K. (2006). On the origin of species through heteropatric differentiation. Journal of Ornithology 147, 273-273.
Winker, K., McCracken, K. G., Gibson, D. D. & Peters, J. L. (2013). Heteropatric speciation in a duck, Anas crecca. Molecular Ecology 22, 5922-5935.
Wood, T. C., Brooks, T. O. & Sladen, W. L. (2002). Vocal characteristics of Trumpeter and Tundra Swans and their hybrid offspring. Waterbirds 25, 360-362.