|Black-capped Chickadee MDC|
The appearance of the two species is very similar to most observers. They are hard to identify even with side-by-side pictures, such as those at Tricky Bird IDs: Black-capped Chickadee and Carolina Chickadee, a site which has a good description of their differences including songs, drawings and photos.
A defining characteristic of a true member of the GOAS is apparently knowing that Carolina Chickadees are in our region while Black-capped Chickadees occur just a little further north. There is only a very narrow overlap zone between the populations where both may be found. As described by an MDC site:
"Black-capped: generally northern Missouri, occasionally moving southward in winter. Carolina: generally southern Missouri and seldom wander north of their range. Where the ranges overlap, the birds sometimes hybridize and sing intermediate songs."
|Map from Birdsource|
Back to parapatry in the title. Parapatry describes when two similar species ranges meet with little or no overlap. A study in Eurekalert.org. describes the range of two closely related millipede species in Tasmania, Australia. The mixing zone where they meet is 140 miles long but only 100 meters wide. You might expect some geographical or environmental features to define this territory, but there are none.
In a world in which we humans create, and fight over, artificial boundaries, these millipedes have apparently peacefully established their distinct boundaries just like our chickadees.