Date: 1/22/19 3:00 pm From: Gregory Hanisek via CTBirds <ctbirds...> Subject: [CT Birds] more on chickadee songs, history etc
David provided some excellent information on chickadee songs. As someone who grew up in a state (NJ) crossed by the chickadee DMZ I can provide some historic perspective on the 2 species' geographic status. As someone who's pretty old I can take that perspective back to the 1950s.
While Carolina Chickadee is essentially a southern species, the historic dividing line crosses central NJ and southern PA. Because these aren't considered southern states (and information available on all aspects of bird study back then was a tiny fraction of what we have access to today) many Christmas Counts below the line in those states were assuming their birds were Black-capped Chickadees. It took awhile to sort that out.
Once the line was established (in NJ it roughly followed the Raritan R westward and crossed into PA near Trenton, but birds just north of the Raritan estuary at Sandy Hook were Black-capped) it became obvious there might as well have been a border wall - Carolina just never strayed northward. There was a small zone along the line where birds were occasionally heard singing the other's song. The learning David mentioned was probably at work there.
Black-capped Chickadees do stage some southward movements, so discerning birders would sometimes find them south of the line in winter. How often this happened seems to have been overstated, however. When CT's own David Sibley, still an aspiring field guide author, moved to Cape May he tried hard to find any well-documented Black-capped records.If I remember correctly he didn't find any. He didn't include it in the species lists in his The Birds of Cape May published in 1993.
When I moved to CT in 1992, the dividing line remained pretty static, although a few Carolinas started to leak up into southern Hunterdon County (north of Trenton). Since then Carolinas have spread north rapidly into northwestern NJ and northeastern PA. Hybrids have become regular to the extent they can be identified. Carolinas have reached the latitude of southern CT but as yet have not added an eastern component to their movement. They've staying west of the Hudson. This range is well-illustrated by eBird maps.
A 2014 article in the journal Current Biology titled "Climate-Mediated Movement in an Avian Hybrid Zone" postulates how the rate of northward movement of Carolina is tied to changes in winter temperatures - climate change in action.
So the possibility of finding a Carolina in CT is now theoretically on the table, but identifying one will be a challenge.