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.

Greg Hanisek
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