The chickadee ID issue in Virginia comes up periodically here because of
its confusing nature, so my thanks also to Dave Davis for addressing it.
I thought that some additional words of clarification might be useful.
As he summarized well, genetic analysis has revealed that in the center
of the hybrid zone where it has been examined in Virginia and West
Virginia during the breeding season, over 50% of the birds examined were
hybrids. Based on morphological study of these same birds, which was
only based on wing length, tail length, and mass (i.e. size), hybrids
and birds that appear genetically to be pure Blackcaps and Carolinas can
not be distinguished. Of course there are significant size differences
between the two chickadee species far from their hybrid zone. But a
problem arises in part because the Blackcaps become smaller as you move
south through their range towards and hybrid zone, and Carolinas
likewise become larger as you move north. This is not because of
hybridization, but because of ecological variation in the size of the
two species; it is advantageous to be larger and have less body surface
for heat loss in colder climates (Bergmann's Rule). So as a result of
both the natural size variation in the two species, and their
hybridization where their ranges meet, the two species and hybrids can
not be distinguished at the contact zone. While this conclusion has not
rigorously been tested for other morphological features such as plumage
coloration, there is no reason to believe that it would not also hold
true there because of the extensive genetic mixing.
Likewise, song can not be relied on for identification in the contact
zone because the two species are similar enough that they can learn each
others' songs. Occasionally you might hear an abnormal "intermediate"
or "hybrid" song in the contact zone. But this individual could
potentially be either a pure Carolina, a pure Blackcap, or a hybrid, any
one of which could have heard both species' songs at a critical stage of
song learning. Most often, however, in the vicinity of the contact
zone, you hear a bird singing only one of the species' songs or the
other. But at least some of these individuals will be bilingual, and
capable of singing good renditions of either song. This can often be
elicited by doing a playback to them of the alternate species' song that
it is not currently singing. This stimulus will sometimes cause them to
switch their song to that of the other species that they are now
hearing, something known as "song-matching" or "matched countersinging".
The chickadee ID issue has come up here again because of some recent
reports of Blackcaps outside of their normal range in areas of northern
Virginia (Frederick, Clarke, Warren, Loudoun, and Shenandoah Counties,
as well as Harrisonburg). While caution is always in order for reports
of chickadees outside of their normal range, I would not want the above
mentioned research to be taken to imply that all such reports should be
dismissed because of identification issues. The "Gold Book" notes
records for the Coastal Plain and out of the normal range in the
Piedmont, primarily occurring in "flight years". Black-capped
Chickadees appearing out of range in these invasion years (and in
smaller numbers in other years) appear to be coming from populations a
good distance to the north, away from the contact zone, where they
differ enough from Carolinas that there are detectable differences in
both size and plumage. And when the two species are showing up at
feeders together and providing side by side comparisons, the task is no
doubt made easier.
So the bottom line is that if you are talking about the period when
northerly Blackcaps might make an incursion some distance into the range
of Carolinas (basically November-March) then you might take the
possibility of an out of range Blackcap into consideration, with
caution.
But when you are dealing with identifying chickadees in the range of
contact where hybrids are found (which at a minimum is at least several
miles wide), especially outside the period when Blackcap incursions
occur, then you need to realize that these birds are not technically
identifiable without genetic sampling. If you wanted to be absolutely
accurate under those conditions, then, you could identify all of the
birds as "Chickadee sp." (unidentified as to species).
In practice, however, most birders probably identify the birds by
location, which is likely based primarily on the song that is heard sung
in that area. As long as you realize that many of these birds could
actually be hybrids, there is not necessarily any harm in this. It is
basically how we have arrived at our current understanding of the range
of the two species in Virginia, as most recently reflected in the range
maps published from the Virginia Breeding Bird Atlas Project that ran
from 1985-1989. And if you are noting what the predominant song is in a
given area, and identify chickadees based on that, then if the hybrid
zone moves over time, which it has done in Virginia in the past, and
appears to be doing currently in some other states, then your
observations can help track that. Song does identify accurately the
position of the center of the chickadee contact zone. What you do not
want to do, however, is think that either appearance or song is as
diagnostic around the contact zone as it is far from it.
Hope this helps.
Gene Sattler
Lynchburg
