Recently, there have been a number of postings concerning sightings of
Black-capped Chickadees in northwestern Virginia and some useful discussion
about
how to distinguish them in the field. Unfortunately--and I hate to rain on the
parade--but that region is a documented zone of hybridization between the
subject species and the Carolina Chickadee. Work by Gene Sattler and
associates,
most notably the paper published in The Auk in April 2000,* demonstrate that a
principal area of hybridization lies along the ridges that form the western
wall of the northern Shenandoah Valley. Based upon a series of transects,
especially one running northwest from Massanutten Mountain between Woodstock
and
Edinburg to a point just northeast of Liberty Furnace, Sattler and Michael
Braun found that..."Genetic data revealed that at least 58% of the birds in the
center of each transect were of mixed ancestry and that recombinant genotypes
predominated among hybrids, demonstrating that hybrids are fertile." In the
case of the transect described above, that center lies near Columbia Furnace
on/near Little North Mountain. Moreover, their work showed that..."patterns of
morphological variation were equivocal regarding introgression [hybridization]
across the hybrid zone." In other words, there was no useful correlation
between appearance--as shown in bird guides--and the genetic makeup of a given
bird. The same situation obtained with song. The bottom line, in
non-scientific
language, is that as you can't tell a book by its cover, in this region you
can't ID a chickadee by its appearance and/or song.
These field studies were carried out during the breeding season, so are most
pertinent for that period. There is strong reason to believe that pure
Black-capped Chickadees do drift south or downslope into northwestern Virginia
in the
winter as do siskins, Purple Finches, Red-breasted Nuthatches, and other
"winter birds." Accordingly, some of the chickadees we see in winter in that
area--and which look like black-caps--probably are indeed black-caps. The
problem
is that short of genetic analysis we really can't determine which are truly
black-caps and which are hybrids. Indeed, in that area we face the same
problem
with Carolinas, and some of the Carolina-looking birds we see in northwestern
Virginia are likely also hybrids.
My interest in this subject arose because I own property precisely in the
zone of greatest hybridization. That is the site where we participate in
Project
Feeder Watch (since 1989) and, more recently, routinely submit data to eBird.
Feeder Watch has provided a "mixed" category since the inception and eBird
has just this fall added both a hybrid and a mix category, both of which we use
all year for lack of a better option. However, if it comes down to a Life
Bird call, it appears that there is no good (or legal) solution.
Based on recent history on this web site, I expect that this information will
probably annoy or even provoke some readers. All I ask is that you bear in
mind that I am only the messenger and have no axe to grind here.
With best regards to all,
Dave Davis
Arlington and Cedar Creek
* Morphometric Variation as an Indicator of Genetic Interactions between
Black-capped and Carolina Chickadees at a Contact Zone in the Appalachian
Mountains. Gene D. Sattler and Michael J. Braun. The Auk 117(2):427-444, 2000.
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