Phylogeography of chestnut-backed chickadees in western North America

North American chickadees and titmice are believed to be descendents of Eurasian lineages that crossed the Bering land bridge in the late Pliocene 3.5 to 4 million years ago (Gill et al. 2005; see also Chapters 1 and 2). The North American chickadees are divided into two groups based on phenotypic differences (references in Gill et al. 1989). The black-capped group contains four species: black-capped, Carolina, Mexican, and mountain chickadees. As their name suggests, these have a black-cap and morphologically they are most similar to the Eurasian species in the Poecile clade, particularly the willow tit. The brown-capped group contains three species: boreal, chestnut backed chickadees, and the Siberian tit. In North America, Siberian tits are only found in the northwestern Arctic and are considered by some to be a distinct subspecies (gray-headed chickadee, Sibley 2000). Birds of the brown-capped group have brownish head feathers and a rusty colored back. Several studies attempted to determine the phylo-using allozymes tentatively concluded that the brown-capped and black-capped groups were more similar to each other than to any of the Eurasian lineages, and chestnut-backed and boreal chickadees were closely related as were Mexican and mountain chickadees. A chromosomal rearrangement found in both Mexican and mountain chickadees adds support to the allozyme data (unpublished data cited in Gill et al. 1989). The two subsequent phylogenies employed mitochondrial DNA (mtDNA) (Gill et al. 1993, 2005—phylogeny reprinted earlier in this book, see Fig. 2.1). The mtDNA restriction data placed the Mexican chickadee within the brown-capped group while phylogenies using mtDNA sequence data showed the three brown-capped species as a mono-phyletic clade nested within the black-capped clade. Based on standard mtDNA divergence rates of 2%/MY (million years), it appears that speci-ation in the North American chickadees occurred during the late Pliocene or early Pleistocene (Gill et al. 1993, 2005). While any such dates are approximate, and debated, the timing corresponds with Pleistocene divergence of other avian taxa, especially boreal species (e.g. The Pleistocene glaciations were influential in shaping the population structure of many species, particularly those now inhabiting temperate and polar regions. Most of Canada and parts of the northern United States were covered under thick sheets of ice (Pielou 1991). Three main ice sheets were present during the Pleistocene: Cordilleran (west of Rocky Mountains), Innuitian (Arctic) and Laurentide (east of Rocky Mountains: Pielou 1991; Dyke et al. 2002). The glaciers expanded and contracted during the alternating …