This bird order contains small to medium birds. Most of the about 350 species occur near water. They were formerly divided into three suborders: the Charadrii (waders), Lari (gulls) and Alcae (auks). Hybridization occurs in many genera.
An apparent hybrid between Dunlin (C. alpina) and White-rumped Sandpiper (C. fuscicollis) was documented in Wisconsin (Anich, 2013).
In Norway, a hybrid between Great Snipe (G. media) and Common Snipe (G. gallinago) was described, based on behaviour, morphology and genetic data (Höglund et al., 2015).
Hybridization between the endemic Black Stilt (H. novaezelandiae) and the Pied Stilt (H. himantopus leucocephalus) is of major conservation concern (Wallis, 1999). The situation has been studied from a morphological (Pierce, 1984) and genetic (Greene, 1999; Steeves et al., 2010) perspective. The most recent study found evidence for extensive hybridization, but not for introgression (Steeves et al., 2010). In addition, a hybrid between American Avocet (Recurvirostra americana) and Black-necked Stilt (H. mexicanus) is known (Principe, 1977).
One study gives a morphological description of a Northern Jacana (J. spinosa) x Wattled Jacana (J. jacana) hybrid (Betts, 1973). A study of the Panamanian hybrid zone between both species revealed introgression from J. jacana into J. spinosa (Miller et al., 2014b). Also, the complete mitochondrial genome of both species is known (Miller et al., 2014a).
Hybridization is widespread among gulls (Lönnberg, 1919; Smith, 1966). Due to the high frequency of hybridization and the rapid speciation in this bird group, they are genetically closely related (Snell, 1991a), which hampers the construction of phylogenetic trees (Crochet et al., 2003; Crochet, Lebreton & Bonhomme, 2002; Sonsthagen et al., 2016) and leads to the lack of a genetic population structure (Sonsthagen et al., 2012). Gulls were once seen as a classical example of a ring species, but this has been proven incorrect (Liebers, de Knijff & Helbig, 2004).
The Herring Gull complex (Larus argentatus – cachinnans – fuscus) is circumpolar and has been studied extensively in terms of breeding area distribution (Adriaens, Vercruijsse & Stienen, 2012; Filchagov, 1994; Yesou, 1991) and genetic structure (Liebers, Helbig & De Knijff, 2001). Most studies have focused on certain species combinations, always involving the Herring Gull (L. argentatus). Gull species hybridizing with Herring Gull include Yellow-legged Gull (L. michahellis) in the Mediterranean (Pons et al., 2004), Great Black-backed Gull (L. marinus) in North America (Andrle, 1972; Jehl, 1960), Lesser Black-backed Gull (L. fuscus) in Europe (Brown, 1967; Harris, 1970; Harris, Morley & Green, 1978) and North America (Ellis et al., 2014), and Glaucous-winged Gull (L. glaucescens) in North America (Mlodinow; Patten & Weisbrod, 1974; Williamson & Peyton, 1963).
The Iceland Gull complex (L. glaucoides – kumlieni – thayeri) consists of three (sub)species that all interbreed (Gaston & Decker, 1985; McGowan & Kitchener, 2001; Snell, 1989; Weir, Kitchener & McGowan, 2000). Other species combinations that have been studied were Red-billed Gull (L .novaehollandiae) x Black-billed Gull (L. bulleri) in New Zealand (Gurr, 1967) and American Herring Gull (L. smithsonianus) x Great Black-backed Gull (L. marinus) in North America (Pons et al., 2014). A few species combinations have received more attention.
Herring Gull (L. argentatus) x Caspian Gull (L. cachinnans)
There are hybrid zones between these species in Poland (Faber et al., 2001; Neubauer et al., 2006) and eastern Europe (Panov & Monzikov, 1999). Genetic studies revealed asymmetric introgression (Gay et al., 2007) and selection on plumage melanism and bare-parts coloration (Gay et al., 2009). The former can be explained by latitude (i.e., Gloger’s Rule), while the latter may be due to sexual selection. Behavioural observations revealed that there is premating isolation due to different breeding phenology and assortative mating according to bare-parts coloration (Neubauer et al., 2009). There is no strong postmating isolation: breeding performance did not differ between pure species and hybrids (Zagalska-Neubauer & Neubauer, 2012). However, female hybrids do seem to have a lower survival rate (Neubauer, Nowicki & Zagalska-Neubauer, 2014).
Herring Gull (L. argentatus) x Glaucous Gull (L. hyperboreus)
Numerous of these hybrids have been reported (Andrle, 1980; Jehl, 1971; Jehl & Frohling, 1965; Spear, 1987). Ingolfsson (1970; 1987) described the hybrid zone on Iceland. However, Snell (1991b) argued that the intermediate plumage was not due to hybridization, but a consequence of light-winged founders in the 1920s. Detailed genetic (and morphological) analyses settled the debate and indicated widespread hybridization (Palsson, Vigfusdottir & Ingolfsson, 2009; Sternkopf et al., 2010; Vigfusdottir, Palsson & Ingolfsson, 2008).
Western Gull (L. occidentalis) x Glaucous-winged Gull (L. glaucescens)
The hybrid zones between these species is located at the coast of Washington and Oregon (Scott, 1971) and has been described from a morphological and genetic perspective (Bell, 1996; Hoffman, Wiens & Scott, 1978). Cline analysis of the hybrid zone detected introgression (Gay et al., 2008), but also (sexual) selection on bare-parts coloration (Gay et al., 2009). The majority of literature on this hybrid zone focuses on breeding success. Bell (1997) found that L. occidentalis has a higher breeding success compared to L. glaucescens and mixed pairs, while Good et al. (2000) documented hybrid superiority. The breeding success of the hybrids was mainly influenced by nest predation, which decreased with increasing vegetation structure around the nest (Good, 2002). However, recent studies did not detect any differences in breeding performance (Megna et al., 2014; Moncrieff et al., 2013).
While studying the speciation process of American Golden-plover (P. dominica) and Pacific Golden-Plover (P. fulva), Withrow and Winker (2014) detected one hybrid individual. Analyses showed that gene flow was close to zero.
Numerous hybrids between various species have been recorded and described (Chen & He, 2011; Cox & Close, 1977; Dies & Dies, 1998; Ewins, 1987; Hays, 1975; Robbins, 1974; Ross, Egan & Priddel, 1999; Steele & McGuigan, 1989; Verroken, 1990; Whittam, 1998; Zingo, Church & Spendelow, 1994), but without any further studies.
Several observations of hybrids (and backcrosses) between Common Murre (U. aalge) and Thick-billed Murre (U. lomvia) have been reported (Birkhead, Johnson & Nettleship, 1986; Cairns & deYoung, 1981). Reports of hybrids based on morphological characters are often questioned (Cairns, 1983; Sluys, 1983), but a backcross was successfully identified based on molecular data (Friesen et al., 1993). Furthermore, a detailed study detected cryptic introgression between the two species (Taylor et al., 2012). Also, a possible hybrid between Common Murre and Razorbill (Alca torda) has been reported (Wilhelm et al., 2001).
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Friesen, V. L., Barrett, R. T., Montevecchi, W. A. & Davidson, W. S. (1993). Molecular-Identification of a Backcross between a Female Common Murre X Thick-Billed Murre Hybrid and a Male Common Murre. Canadian Journal of Zoology-Revue Canadienne De Zoologie 71, 1474-1477.
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* The Larus text has been reviewed by Grzegorz Neubauer (Polish Academy of Sciences, Warsaw)