Chestnut-backed Chickadee
Poecile rufescens
It is a small chickadee, 11.5–12.5 cm (4.5–4.9 in) long with a weight of 8.5–12.6 g (0.30–0.44 oz). The head is dark blackish-brown with white cheeks, the mantle is bright rufous-brown, the wing feathers are dark gray with paler fringes. The underparts are white to pale grayish-white, with rufous or pale gray flanks. It is often considered the most vibrant of all chickadees. Joseph Grinnell was one of the first to provide a significantly detailed description of this variation. Voice: Chestnut-backed chickadees can be identified by their “chick-a-dee” call, with “dee” repeated a few times. Adults and older nestlings can also be identified by their hissing noise, which mimics a snake while protecting their nests. They rarely use their “gargle” calls.
ID text from Wikipedia, CC BY-SA 4.0
Mountain Chickadee
Poecile gambeli
Adults of both sexes have a black cap joining a black postocular stripe behind distinctive white eyebrows. Their backs and flanks are gray and they have paler gray underparts; they have a short black bill, and a black bib. The typical adult wingspan is 7.5 in (19 cm), and the overall length is 5–6 in (13–15 cm). The Mountain Chickadee is one of 55 species of chickadees and tits. Voice: Their call is a throaty chick-adee-dee-dee, while their song is a 3- or 4-note descending whistle fee-bee-bay or fee-bee-fee-bee. Their song sounds like it is whistling the word "Cheeseburger". They travel in pairs or small groups, and may join multi-species feeding flocks after breeding season.
ID text from Wikipedia, CC BY-SA 4.0
Where you might meet both
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