A multi-locus phylogeny suggests an ancient hybridization event between Campephilus and melanerpine woodpeckers (Melanerpes and Sphyrapicus) during the colonization of the New World (Fuchs et al., 2013).
Hybridization between Yellow-shafted Flicker (C. auratus auratus) and Red-shafted Flicker (C. a. cafer) was discovered and discussed early on (Allen, 1892; Anderson, 1971; Deakin, 1936; Eaton, 1893; Rhoads, 1892). The situation was thoroughly described by Short (1965). Pairing among subspecies was found to be random (Bock, 1971; Moore, 1987). Genetic analyses showed a clear structure in mtDNA, but none in nuclear markers (allozymes), which pointed towards gene flow (Fletcher & Moore, 1992; Grudzien & Moore, 1986; Grudzien et al., 1987; Moore, Graham & Price, 1991). The hybrid zone was considered a “hybrid superiority zone” because the zone remained stable (Moore & Buchanan, 1985) and there was no difference in reproductive success between hybrids and pure individuals (Grudzien & Moore, 1986). However, certain populations did not follow these patterns. In Alberta, the hybrid zone was expanding (Mcgillivray & Biermann, 1987). And in the northern populations mating was non-random (Wiebe, 2000; Wiebe & Bortolotti, 2001). This raised the question whether phenotypic variation was related to survival. Further studies showed that carotenoid colour was not correlated with reproduction or survival (Wiebe & Bortolotti, 2002). Instead survival seemed to depend on large-scale weather patterns (Flockhart & Wiebe, 2007; Flockhart & Wiebe, 2008). Possibly, annual changes in selection pressures could mask fitness differences between phenotypes (Flockhart & Wiebe, 2009).
In addition to this well-studied hybrid zone, other Colaptes species are known to hybridize. For example, Red-shafted Flicker (C. a. cafer) and Gilded Flicker (C. chrysoides) in North America (Short, 1965) and several (sub)species in South America (Short, 1972).
Miller (1955) describes a hybrid between Hairy Woodpecker (D. villosus) and Ladder-backed Woodpecker (D. scalaris). And to study introgression between Great Spotted Woodpecker (D. major) and Syrian Woodpecker (D. syriacus) in Poland multiple molecular markers were developed (Gorman, 1997; Michalczuk et al., 2014).
A hybrid between Pale-headed Woodpecker (G. grantia) and Bamboo Woodpecker (G. viridis) from Thailand was described morphologically (Round et al., 2012).
This genus comprises four species: Red-naped Sapsucker (S. nuchalis), Red-Breasted Sapsucker (S. ruber), Williamson’s Sapsucker (S. thyroideus) and Yellow-bellied Sapsucker (S. varius), that all occur in North America (Howell, 1952). Hybrids between several of these species have been reported (Browning, 1977; Scott, Ankney & Jarosch, 1976; Short & Morony, 1970; Weisser, 1973). Red-naped and Red-breasted Sapsucker display low genetic distances, based on nuclear (Johnson & Zink, 1983) and mitochondrial markers (Cicero & Johnson, 1995). Hybrid zone analyses indicate selection against hybrids (Johnson & Zink, 1983; Seneviratne et al., 2016). Similarly, genetic and morphological analyses of the hybrid zone between Red-breasted and Yellow-bellied Sapsucker show signs of selection against hybrids (Seneviratne et al., 2016; Seneviratne et al., 2012).
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