The Tits, Chickadees and Titmice a large passerine family which occurs across the Northern Hemisphere and in Africa. Hybridization has been observed in most genera.
Oak Titmouse (B. inornatus) and Juniper Titmouse (B. ridgwayi) interbreed across northern California. Patterns of molecular variation are congruent with morphological and bioclimatic data. Oak Titmouse is able to expand into the range of Juniper Titmouse, which results in asymmetric introgression (Cicero, 2004).
In Texas and south-western Oklahoma Black-crested Titmouse (B. atricristatus) and Tufted Titmouse (B. bicolor) hybridize (Braun, Kitto & Braun, 1984; Dixon, 1955; Dixon, 1990). Morphological analysis of this hybrid zone indicates selection against hybrids (Curry & Patten, 2014).
The Great Tit (P. major) complex is divided into four groups, each containing several subspecies. Mitochondrial DNA analysis showed that these groups are monophyletic and that hybridization between members of these groups are thus the result of secondary contact (Kvist et al., 2003a). The Great Tit complex is not a ring species, a conclusion that was further supported by new genetic data and song analysis (Packert et al., 2005). The hybrid zone between Great Tit and Japanese Tit (P. minor) has been well studied (Fedorov et al., 2006; Fedorov et al., 2009; Kvist & Rytkonen, 2006; Nazarenko, Valchuk & Martens, 1999) and even led to the discovery of paternal leakage of mtDNA (Kvist et al., 2003b).
Gene flow between island and mainland populations of Great Tits has led to differences in clutch sizes. Immigrants bring in genes for bigger clutch sizes. When gene flow is low (due to low immigration rates and selection against immigrants) clutch sizes are small, an adaptation to the island environment. In other areas, where gene flow is higher, clutch sizes are also bigger (Postma & van Noordwijk, 2005).
A cross-fostering experiment led to heterospecific pairings between Great Tit and Blue Tit (P. caeruleus), although all fledgling were Blue Tits (Slagsvold et al., 2002).
French populations of Blue Tit exhibit a mitochondrial polymorphism that could be the result of hybridization with Azure Tit (P. cyaneus) (Taberlet, Meyer & Bouvet, 1992).
Hybridization between Siberian Tit (P. cinctus) and Willow Tit (P. montanus) has been documented on multiple occasions (Järvinen, Ylimaunu & Hannila, 1985; Järvinen, 1987; Järvinen, 1989; Järvinen, 1997).
A genetic study of the European Coal Tit (P. ater) indicated that German populations are a mixture of northern and southern populations, extending over a broad zone of intergradation (Tritsch et al. 2018).
Morphological and ecological similarities between Black-capped Chickadee (P. atricapillus) and Carolina Chickadee (P. carolensis) raised the suspicion that these species might be interbreeding (Brewer, 1963; Tanner, 1952). Hybridization was later confirmed by morphological (Johnston, 1971; Rising, 1968; Robbins, Braun & Tobey, 1986) and genetic analyses (Braun & Robbins, 1986; Sattler & Braun, 2000). Hybrids showed decreased reproductive success, indicating endogenous selection (Bronson, Grubb & Braun, 2003a; Bronson et al., 2005). The situation was summarized by Curry (2005). Hybridization has led to introgression, which was lower at highly divergent loci that might be involved in reproductive isolation (Taylor et al., 2014a). One population of Black-capped Chickadees is isolated in the Appalachian Mountains and has experienced less introgression compared to other populations near the hybrid zone (Davidson et al., 2013).
Other genetic studies showed that the hybrid zone is moving north and becoming wider (Reudink et al., 2007), suggesting a selective advantage for the Caroline Chickadee. Indeed, experimental work confirmed that females have a preference for Carolina-like birds (Bronson et al., 2003b) and patterns of extrapair paternity (EPP) led to a similar conclusion (Reudink, Mech & Curry, 2006). The precise characteristics that provide Caroline Chickadees were unknown. Sexual selection might be involved, but song turned out to be a poor indicator for hybridization (Enstrom & Bollinger, 2009; Sattler, Sawaya & Braun, 2007). However, winter temperatures seem to limit the distribution of Caroline Chickadees. The northward expansion (and subsequent hybridization) may be connected to recent climate change (Taylor et al., 2014b), although another study did not find a significant relation between lower air temperatures and basal metabolic rates (BMR) of Caroline Chickadees (Olson et al., 2010).
Hybrids perform worse on associative learning and problem-solving tasks. This lower intelligence could act as a postzygotic isolation mechanisms (McQuillan et al., 2018).
Black-capped Chickadees also hybridize with Boreal Chickadee (P. hudsonicus) (Lait, Lauff & Burg, 2012) and Mountain Chickadee (P. gambeli) (Grava et al., 2012). In the latter case, hybridization might have led to character displacement in song (Grava et al., 2013). Populations of Mountain Chickadee that have been diverging since the Pleistocene exhibit different rates of gene flow among each other (Spellman, Riddle & Klicka, 2007). While southern populations continue to diverge, northern populations might converge and form a “hybrid” species (Manthey, Klicka & Spellman, 2012). Although there are phenotypic differences along an altitudinal gradient, there is no genetic differentiation between high and low elevation populations. Phenotypic variation might be maintained by strong selection despite gene flow (Branch et al. 2017).
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