Chestnut-backed Chickadees are a jewel of Oregon's coniferous forests, particularly common in the humid coastal and western regions. They are known for their distinctive calls, energetic foraging, and, as their name suggests, their unique coloration among North American chickadees.

However, when it comes to distinguishing males from females, it's the same story as with other chickadee species: male and female Chestnut-backed Chickadees are considered monomorphic, meaning they look virtually identical. There are no obvious differences in plumage, size, or coloration that a casual observer can use to tell them apart.

General Appearance (Both Sexes):
Size and Shape: They are small, compact birds, slightly smaller than a Black-capped Chickadee, with a relatively large head, short neck, and a small, pointed black bill.
Coloration: This is their most distinctive feature, making them easy to identify from other chickadees in Oregon:
Cap: A dark brown or sooty black cap on the head, extending down to just below the eye.
Cheeks: Bright white cheek patches, contrasting sharply with the dark cap and bib.
Bib: A small, neat black bib directly under the bill.
Back and Sides: This is where they get their name! A rich, deep rufous-chestnut color covers their back, rump, and often extends down their flanks (sides). This color can vary slightly in intensity, but it's always prominent. (Note: Some subspecies found further south in California may have grayer flanks, but in Oregon, the rich chestnut flanks are typical.)
Underparts: Whitish belly, sometimes with a very faint buffy wash.
Wings and Tail: Grayish-brown wings and tail, often with paler edgings on the flight feathers.
Bill, Legs, and Feet: All are black.
Vocalizations: Their calls are distinct from other chickadees, often described as a husky, rapid "chick-a-dee-dee-dee" that can sound wheezy or slightly buzzy. They do not have the clear, whistled "fee-bee" song of the Black-capped Chickadee.

Subtle Differences and Behavioral Cues (where distinctions might be observed):
Since visual identification is not reliable, observing their behavior, particularly in spring and early summer, is the best way to infer sex.
Size (Minimal Average Difference): Like other chickadees, males tend to be very slightly larger and heavier than females on average. This difference is negligible for field identification.
Behavior during Breeding Season (Most Reliable):
Pair Bonds: Chestnut-backed Chickadees form monogamous pair bonds, which can be long-lasting, sometimes year-round.
Nest Site Selection and Excavation: They are cavity nesters, often choosing holes in rotten wood, old woodpecker excavations, or nest boxes. The female often takes the lead in selecting the site and excavating or enlarging the cavity, though both may participate.
Nest Construction: The female primarily builds the nest itself within the cavity. They are known for using a substantial amount of animal fur or hair (sometimes up to half the nest material) for insulation, along with moss, lichens, and plant fibers. She also makes a thick fur "blanket" to cover the eggs when she leaves the nest.
Incubation: The female is the sole incubator of the eggs (typically 5-9, white with fine reddish-brown dots). This period lasts about 12-13 days.
Courtship Feeding/Feeding the Incubating Female: During incubation, the male diligently brings food to the female at the nest cavity. Observing this behavior is a strong indicator that the provisioning bird is the male and the recipient is the female.
Parental Care: Once the young hatch, the female initially remains with the nestlings, and the male continues to bring food for both her and the chicks. As the chicks grow (fledging around 16 days), both parents actively participate in feeding them.