Short answers, pulled from the story.
The black-capped chickadee survives extreme cold by entering a state of torpor and dropping its body temperature to just above freezing to conserve energy. This physiological adaptation allows the bird to endure the harsh winters of North America and northern Europe where food is scarce and the cold is relentless.
The name chickadee is an onomatopoeia derived directly from the bird's distinctive alarm call. This vocalization is a sophisticated communication tool that conveys specific information about the level of threat posed by a predator through the number of dee syllables added to the end of the call.
For much of the 20th century the large genus Parus served as a catch-all for nearly all tits but modern genetic analysis has split this group into multiple distinct genera including Poecile Cyanistes and Baeolophus. This reclassification was driven by molecular studies such as the 2013 analysis by Ulf Johansson and collaborators which showed that the traditional grouping was paraphyletic.
Most tree-nesting tits such as the great tit and the blue tit dig their nests into dead wood or soft tree trunks to create a safe haven for their large clutches of eggs. The blue tit can lay as many as 19 eggs in a single clutch while the ground tit of the Tibetan Plateau builds its nest on the earth.
The ancestors of the Baeolophus genus settled North America during the Early-Mid Pliocene. Chickadees arrived somewhat later and the family contains 62 species in 13 genera with some species such as the fire-capped tit and the yellow-browed tit occupying unique ecological niches.