This will be a quick review the following identification criteria, derived from numerous sources: head size and shape, bill size and shape, bib size and shape, nape colouration, cheek patch vs breast colouration, secondary and tertial edging, greater coverts base shade and edging, tail feather edging and tail length/wing chord ratio. Vocalizations are not covered here. Thoughts on potential hybrids are at the end.
Carolina Chickadee - Point Pelee - May 13, 2013
Head Size and Shape: A review of Black-capped and Carolina Chickadee photograph has shown that Carolina frequently appears to have a smaller and rounder head in contrast to Black-capped which frequently shows a proportionately larger head, appearing as a horizontal oval in shape.
Bill Size and Shape: This is difficult to properly quantify from photos or in the field, but the GISS of some Carolina's are of a smaller and shorter bill compared to Black-capped. Pyle (1997) lists the exposed culmen of Black-capped as measuring 7.6-10.5mm and of Carolina as 6.6-9.5mm. So there is overlap.
Bib Size and Shape: Some sources note that Carolina Chickadee has a smaller and more sharply defined bib than Black-capped Chickadee (National Geographic 2002). In my experience, some Carolina seem to show bibs small enough that they seem to be outside the normal variation of Black-capped, but presumably there is much overlap. Carolina also often appears neater with cleaner edges, but based on personal experience this should be assessed on birds that are more at rest (i.e., not twisting and turning during foraging).
Nape Colouration: Although this character is frequently referenced in field guides (e.g., Peterson 2008), I've had no luck assessing it in the field, with photos or on study skins. Perhaps it would be easier to assess on a live bird in-the-hand.
Cheek Patch vs Breast Colouration: Brought to my attention by Peter S. Burke who noted that Carolina seems to have a duller-gray breast when compared to the white cheek patch. This may be difficult to quantify through photographs but is likely a notable feature during field observations.
Flank Colouration: considered brighter peach in Black-capped vs. duller (to even colourless) in Carolina, my experience indicates that Black-capped (particularly in spring) can lose this and appear nearly colourless. This feature might be more reliable in fresh plumage (i.e., the fall).
Colourless Black-capped Chickadee - Burlington - May 2013
Rectrices: Pyle (1997) states that Black-capped Chickadee can be separated from Carolina Chickadee “by the [presence of] white edging to the outer rects”. I've personally tried to confirm the validity of this feature and discovered that many Carolina Chickadees from the central and northern parts of the species range show white edges on the rectrices (generally more white the further north in their range). Therefore, the presence of some white does not rule out Carolina Chickadee. You can just expect less white on Carolina than Black-capped.
Secondary and Tertial Edging: This feature ishvariable depending on time of year, angle or even camera settings (even with different views of the same individual). Carolina Chickadee is reported to show a more muted pattern, compared to Black-capped Chickadee (Sibley 2000).
Greater Covert Base Shade: A field mark rarely referenced is the base shade or colour of the centres of the greater primary and secondary coverts. It is reported to be gray in Carolina Chickadee and black in Black-capped (Crossley 2011). A study of 300 skins of both species at the Royal Ontario Muesum found that this feature is not reliable in direct comparison and assume it is simply a difference in impression, with Black-capped appearing more contrasting than the uniform gray of Carolina, especially when considering the difference in greater covert edging (see below).
Greater Covert Edging: Another field mark that is occasionally referenced is the contrasting white edges to the greater coverts of Black-capped Chickadee whereas Carolina shows a uniform gray edge. While it appears possible for Black-capped Chickadee to lose these white edges due to feather wear, especially in spring as chickadees do not do a pre-alternate molt (Pyle 1997), my experience is that having this happen is quite rare. In my opinion, there is no stronger "first impression" field mark for separating Black-capped from Carolina Chickadee than this one - just be careful of poor views, body feathers covering the greater coverts or misleading photos.
Above: bright white edges on the greater coverts of a Black-capped Chickadee
General Impression (GISS): A final thought, which is difficult to quantify, is that the above noted features do lead to a different GISS on Carolina vs. Black-capped. It's a classic birding conundrum, where if you try hard enough, you could turn any Black-capped into a possible Carolina, but when you see the real deal, you'll probably know you're on to something.
Hybrids: Hybrids occur regularly in the overlap zone and create significant identification challenges. My personal and unscientific approach is to call a spade a spade. If a bird outwardly shows nothing but Carolina Chickadee features, then why not consider it as such? Likewise, if anything appears intermediate or suggests Black-capped (even on an otherwise Carolina-like bird), a hybrid shouldn't be ruled out.
Carolina Chickadee - Point Pelee - May 13, 2013