-----Original Message-----
From: Kurt Gaskill [mailto:KurtCapt87@xxxxxxxxxxx] ;
Sent: Thursday, November 29, 2007 9:19 PM
To: Sattler, Gene D.
Subject: RE: [Va-bird] Chickadee ID in VA
Gene,
Can you state anything about species ID for birds displaying appearance
AND song of one species? Assuming the observation takes place in the
region of the contact zone. To be clear, this question refers to
observations of birds that display obvious BCCH (or CACH)
characteristics in both song AND plumage. And how does this answer
change as one one moves out of the contact zone?
Kurt
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Sorry for the delay in following up on this thread. I thought I would
respond via the listserv to Kurt's questions, as they are good ones that
some others will probably have an interest in. As I noted earlier, I
can not speak first-hand to the issue of identification based on
plumage, as my study only looked at wing length, tail length, and mass.
However, patterns and conclusions regarding variation in plumage versus
size and proportions will probably be parallel.
The question is whether combining morphology plus song makes
identification more reliable in this case. Technically I would have to
say no. To illustrate this point, some readers might be interested in
referring to an online copy of my morphology paper that has been
graciously posted on the VSO website (www.virginiabirds.net), with
permission from the AOU and The Auk, where it was published. This
online copy of the paper has the figures and tables included, which are
not accessible in all online versions of the paper.
Figure 3 shows six plots. The first two, A and B, analyze three
"parental" populations, one of blackcaps in PA and two of carolinas in
VA and OH. Fig. 3A has the birds identified by population, while Fig.
3B has the same individuals identified by their genetic status, based on
four genetic markers that we had available. Focusing just on PA and VA,
these two populations were comprised of pure blackcap and pure carolina
individuals respectively, based on our genetic markers. Basically, the
birds' morphology (wing length, tail length, and mass) was analyzed in a
way that combined these three dimensions into two dimensions that are
plotted on the X and Y axes. One can see that the two can be
distinguished using this method, with no overlap.
Then in Fig. 3C and Fig. 3D five different populations forming a
"transect" or a line across the hybrid zone are analyzed in the same
way. Fig. 1 shows roughly where these populations were located. The
dashed polygons of the two parental populations are also superimposed on
the plot. The essential point to see in Fig. 3D is that while blackcap
(atricapillus) individuals of the transect fell basically in the same
space as parental blackcaps, so did some hybrid individuals. Likewise,
while carolina (carolinensis) individuals of the transect fell basically
in the same space as parental carolinas, so did some hybrid individuals.
In other words, there is no way of telling for sure based on morphology
whether an individual in these populations is a blackcap, a carolina, or
a hybrid. And combining song as a second clue does not help. For
example, considering individuals of VA1 and VA2 in Fig 3C, virtually
every individual in these populations sang only blackcap song,
regardless of whether they were a pure blackcap or a hybrid. This is
because song is determined primarily by learning, and birds in both
these populations hear only blackcap song.
I need to point out an important point, however. Although three
individuals in VA1 were classified as hybrids, this does not mean that
they were "first generation" hybrids (F1s). They were not. They were
hybrids based on the fact that they carried one blackcap gene out of
several analyzed with our genetic markers. So they might be a fifth or
tenth or twentieth generation hybrid. But technically they are still
hybrids. And so in answer to Kurt's second question, again technically
it does not matter whether you are in the area of the contact zone (VA2
and VA3) or not (ex. VA1).
Now these results can seem to raise a real mess. That is because we
have found that even in VA1, which is in the Seneca Rocks area of West
Virginia, over 30 miles from the contact zone in Virginia, some birds
are technically hybrids. That means that even somewhere like Highland
County in Virginia, some birds are also likely technically late
generation hybrids, but hybrids none-the-less. So are we going to have
to stop calling chickadees in Highland County "blackcaps" because some
can be carrying carolina genes? I would answer, "No". As I noted in my
previous email of 11/29, in practice most birders probably identify
chickadees in the state by location, which is probably based in largest
part on the song heard in that area. And that is fine in practice.
The natural follow-up question to this is, "Are these really even two
species then, if there is that much genetic mixing between them, such
that carolina genes are found this deep into the range of blackcaps.?"
The answer to this would probably be given by most biologists as, "Yes,
these should still be considered two species based on the prevailing
definition of species, which uses reproductive isolation as the primary
criterion in defining species." This is because even though it would
seem that these two birds are not reproductively isolated, based on one
of our genetic markers, the other three genetic markers we used showed a
much narrower zone of genetic mixing, and reflected a strong genetic
barrier restricting widespread mixing of their genes. What the genetic
analysis of these chickadees really ends up suggesting is that nature is
not always as neat and tidy as our labels try to make it. While
Black-capped and Carolina chickadees will likely continue to be
considered two separate species, genetic studies such as this are
beginning to suggest that the genetic barriers between species are not
always as absolute as we have imagined them.
Good Birding,
Gene Sattler
Lynchburg
