The Piciformes includes woodpeckers and their allies, divived over nine families.
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.
Benz and Robbins (2011) documented an introgression event in this genus, in which paraphyly of Chestnut Woodpecker (C. elegans) was probably the consequence of mitochondrial introgression from Pale-crested Woodpecker (C. lugubris).
Scaly-breasted Woodpecker (C. grammicus) and Waved Woodpecker (C. undatus) diverged very recently – estimated about 50,000 years ago – and might still be exchanging genes (Sampaio et al., 2018). They should be considered the same species.
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). It has been challenging to discriminate genetically between Red-shafted, Yellow-shafted and Gilded Flicker, but a recent genomic study was able to do so (Aguillon et al., 2018).
Miller (1955) describes a hybrid between Hairy Woodpecker (D. villosus) and Ladder-backed Woodpecker (D. scalaris).
Great Spotted Woodpecker (D. major) and Syrian Woodpecker (D. syriacus) interbreed in Poland (Gorman 1997; Figarski and Kajtoch 2018) and hybrids can be detected genetically (Michalczuk, McDevitt et al. 2014).
In North America, Nuttall’s Woodpecker (D. nuttallii) and Ladder-backed Woodpecker (D. scalaris) occasionally hybridize (Short, 1971). Although they can hybridize, little of the genomic content moves between species (Manthey et al., 2019).
A hybrid between Pale-headed Woodpecker (G. grantia) and Bamboo Woodpecker (G. viridis) from Thailand was described morphologically (Round et al., 2012).
Geographical patterns of mtDNA and plumage patterns in the Red-bellied Woodpecker (M. carolinus) indicate a hybrid zone between population in Florida (subpecies perplexus) and the remainder of the populations (Barrowclough et al., 2018).
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) and positive assortative mating based on plumage (Billerman et al., 2019).
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 three 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).
Hybrids between Grey-headed (P. canus) and Eurasian Green Woodpecker (P. viridis) have been described in Poland (Ławicki, Cofta et al. 2015).
In the Pyrenees, Eurasian Green Woodpecker and Iberian Green Woodpecker (P. sharpei) hybridize. There is no gene flow outside of this hybrid zone, suggesting some selection against hybrids (Pons et al., 2019).
A phylogenetic study uncovered michondrial capture between bronzy jacamar (G. leucogastra) and purplish jacamar (G. chalcothorax) in Amazonia (Ferreira et al., 2018).
Discordance between genetic data and song characteristics suggests that two subspecies of the Yellow-rumped Tinkerbird (P. bilineatus) are interbreeding (Nwankwo et al., 2017).
A genetic study uncovered rampant introgression between Yellow-fronted Tinkerbird (Pogoniulus chrysoconus extoni) and Red-fronted Tinkerbird (P. pusillus pusillus) in Southern Africa (Nwankwo et al., 2019). Hybridization was suspected due to the presence of orange-fronted individuals (Ross, 1970).
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