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 Russia and China (Melville, 1997; Kulikova et al., 2004; Wang et al., 2018)
- 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).
Eastern Spot-billed Ducks and Indian Spot-billed Ducks (A. poecilorhyncha) form mixed pairs in China (Leader, 2006)
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).
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