Parapatry for Beginners

Some years ago on Bull Creek, I made the mistake of identifying a Black-capped Chickadee to a group on an Audubon (GOAS) field trip and heard a resounding chorus of "They don't occur here- that is a Carolina Chickadee."  I was unaware of parapatry, a lesson that was burned into my brain that day.


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

The appearance of the two species is very similar to most observers.  They are hard to identify even with side-by-side pictures. So to add further confusion, hybridization occurs in the overlap zone, making it even more difficult to separate the two species.  As if this weren't complicated enough, the overlap zone is creeping northward in recent years as climate change moves the average warm temperatures to the north.

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.