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).
Chialchia and Smith (2014) provide a detailed description of a hybrid between Cream-backed Woodpecker (Campephilus leucopogon) and Crimson-crested Woodpecker (C. melanoleucus), two species that co-occur along the banks of the Paraná River in Paraguay.
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). A genomic analyses of several hybrid zones showed that the tree species are clearly distinct with a small number of hybrids in each hybrid zone. This indicates that there is moderately strong reproductive isolation between them. There were no large regions of differentiation in the genome (so-called ‘genomic islands of differentiation’). However, the authors uncovered 19 small regions of differentiation, some of which were shared between species. One of those regions contained a candidate locus associated with plumage, which could contribute to reproductive isolation (Grossen et al., 2016).
Discordance between genetic data and song characteristics suggests that two subspecies of the Yellow-rumped Tinkerbird (P. bilineatus) are interbreeding (Nwankwo et al., 2017).
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