HYBRIDIZATION IN CHICKADEES: MUCH TO LEARN FROM FAMILIAR BIRDS

HYBRIDIZATION HAS LONG fascinated ornithologists, as evidenced by both foundational and recent publications in evolutionary and avian biology (Mayr 1963; Grant and Grant 1992,1997; Gill 1998; Randier 2002), including frequent major contributions in The Auk. For example, work in the United States on the classic Great Plains hybrid zones involving orioles, flickers, buntings, and other taxa continues (e.g. Rising 1983, 1996; Moore and Koenig 1986; Moore and Price 1993; Baker and Johnson 1998; Allen and Omland 2003). Recent intensive work, taking advantage of the potential for direct measures of fitness that are possible when studying entire populations of Darwin's finches on small islands, is providing critical information about ecological and evolutionary consequences of introgressive hybridization (Grant and Grant 1994, 1996, 2002; Grant et al. 2005). Still other studies have focused on the potential for introgression to put species at risk of extinction (Rhymer and Simberloff 1996); research with this emphasis includes recent work, for example, on two pairs of North American wood warblers (Gill 1997, 2004; Rohwer and Wood 1998; Pearson 2000; Shapiro et al. 2004).

An especially productive subset of hybridization work examines contact zones, geographic regions where otherwise separate populations overlap and interbreed (Moore 1977, Barton and Hewitt 1989, Harrison 1993). The parapatrically distributed Black-capped Chickadee (Poecile atricapillus) and Carolina Chickadee (P. carolinensis) have long been known to hybridize-to the chagrin of generations of birders, who often have a tough enough time telling these superficially similar species apart-but their contact zone has only recently been sufficiently well studied to yield basic understanding of important patterns and processes. Bronson et al. (2005) represents the latest installment in this expanding body of work. The new study adds to others from the same research program (Bronson et al. 2003a, b); these papers, along with additional recent and ongoing research elsewhere, have greatly increased our understanding of the causes and consequences of hybridization in these chickadees, while leaving many exciting questions yet to be answered.

BIOLOGICAL CONTEXT OF CHICKADEE HYBRIDIZATION

Black-capped Chickadees inhabit most of Canada and the northern half of the United States, whereas Carolina Chickadees are restricted to the southeastern United States. The species come in contact along a narrow contact zone extending from Texas to New Jersey, with the Black-capped Chickadee's range occupying the higher elevations of the Appalachian Mountains to southwestern Virginia (extending also, formerly at least, to southwestern North Carolina; Tanner 1952), producing areas of contact on both flanks of the mountain chain and in both southwestern and southeastern Pennsylvania (Mostrom et al. 2002). Hybridization has been known or suspected in the zones of contact, on the basis of field observations or analysis of specimens from Kansas (Rising 1968), Missouri (Braun and Robbins 1986, Robbins et al. 1986), Illinois (Brewer 1963), Virginia (Johnston 1971, Sartler and Braun 2000), and Pennsylvania (Ward and Ward 1974).

Analysis of mitochondrial DNA (mtDNA) has shown that Black-capped and Carolina chickadees are not each other's closest relative (Gill et al. 1993, 2005). The sibling species of the Black-capped Chickadee is the Mountain Chickadee (P. gambeli) of western North America. The Carolina Chickadee is more distantly related, but the identity of its own closest relative remains unresolved: some analyses place P carolinensis closest to the atricapillusgambeli clade, whereas other results suggest that its sibling species is the Mexican Chickadee (P. sclateri). This phylogenetic issue is important as context for studies of chickadee hybridization, because its resolution would help us understand the extent to which Black-capped and Carolina chickadees have diverged, and this in turn would have implications for understanding how and why they continue to choose each other as mates. Regardless of the placement of the taxa on phylogenetic trees, it appears clear that the latter two species are evolutionarily distinct: their mtDNA gene sequences have diverged ~4% (Mack et al. 1986, Gill et al. 2005), a level of divergence comparable to that among clearly separable species in many avian genera. That conclusion contrasts with earlier suggestions that the two species might not merit recognition as separate species, based on observed interbreeding and similarities in proteins (Braun and Robbins 1986).

Hybridization between Black-capped and Carolina chickadees is not unique within the Paridae; indeed, hybridization -as at least an occasional event, but in some cases as a more regular phenomenon -has been a theme in studies of this family (Harrap and Quinn 1995). Most cases of parid hybridization have involved closely related species within clades that were considered until recently to be subgenera within the large genus Parus; those major groupings are now generally recognized as distinct genera (e.g. AOU 1998, based mainly on Slikas et al. 1996; see also Gill et al. 2005). Accordingly, analyses of hybridization have helped to determine species boundaries between two sibling pairs of North American titmice (Baeolophus spp.; Avise and Zink 1988, Dixon 1990, Cicero 1996, AOU 2002); between Coal Tit (Periparus ater) and Black-crested Tit (Pe. melanolophus) in Nepal (Löhrl 1994); and between Blue Tit (Cyanistes caeruleus), Azure Tit (C. cyanus), and Yellow-breasted Tit (C. flavipectus; reviewed by Harrap and Quinn 1995, Randier 2004; see also Kvist et al. 2004). Similarly, boundaries have been shown to be complex among subspecies of the Great Tit (Parus major) and related species, such as the Turkestan Tit (Pa. bokharensis; Harrap and Quinn 1995, Kvist et al. 2003).