Chestnut-backed Chickadee

The chestnut-backed chickadee (Poecile rufescens) is a small passerine bird in the tit family, native to western North America. == Taxonomy == In the early 20th century, Joseph Grinnell hypothesized that the chestnut-backed chickadee diverged from the boreal chickadee (Poecile hudsonicus), because both species inhabited similar coniferous forest environments. Grinnell noted that the main differences between the boreal chickadee and the chestnut-backed were in the shade and tone of their respective brown coloration. He drew parallels between the varied chickadee characteristics using the fact that some bird species become smaller and more vibrantly brown as their habitat becomes more humid. Modern molecular phylogenetic studies have confirmed that the chestnut-backed chickadee is sister to the boreal chickadee since there were strong genetic similarities in mitochondrial DNA. More recent research regarding the population distribution of the chestnut-backed chickadee suggests that the genetic fragmentation of the chestnut-backed chickadee from the boreal chickadee was due to the changing glacial landscapes of the Pleistocene era. After this species divergence, the chestnut-backed chickadee migrated south to inhabit the range described above. === Subspecies === There are three subspecies, with the flanks being grayer and less rufous further south: Poecile rufescens rufescens (Townsend, 1837). Nominate subspecies; Alaska…