Hi Dave and fellow VA-Birders,
I responded on Monday to this post from Dave Davis, but while I intended my
reply to go to the entire listserv I only sent it to Dave. So I am resending
it.
As you know, in general with a little careful study Black-capped and Carolina
chickadees can be distinguished by appearance using a combination of size,
proportion, and plumage characteristics. So during periodic invasion winters
when Blackcaps that have come down from more northerly portions of their range
are present in the contact area where the two species meet, and south into the
range of Carolina Chickadees, it is possible to tell them apart.
Where this ability to distinguish the two becomes problematic is at the contact
zone where the ranges of the two species meet. This is because clinal
variation across their range results in those Blackcaps residing closest to the
contact zone being most similar to the Carolinas there (for example being
smallest), just as the Carolinas likewise vary clinally (for example being
largest closest to the contact zone). This is not necessarily the result of
hybridization, but rather geographic variation, such as body size increasing to
the north in accordance with Bergmann's Rule. But it makes it hard to tell the
two species apart by appearance just where there is the most need to do so,
where both are found together at the contact zone.
To make matters worse, genetic studies have consistently found that as many as
50% or more of individuals at the center of this chickadee contact zone are
hybrids. These hybrids are not just first generation hybrids that would on
average fall intermediate between the two species in characteristics, but are
also later generation backcross hybrids, some of which can look essentially
like either of the two species. All of this means that it will not be possible
to reliably identify an individual chickadee by appearance as being of either
species or as a hybrid within a swath at least 10-20 miles in diameter where
their ranges meet and hybridization is occurring.
Of course the species' songs are distinctive, and song being determined
principally by learning, as opposed to being strictly inherited, is not so
susceptible to the influences of hybridization. However, the two species are
similar enough genetically that they can learn each other's songs even if they
are not hybrids. So song also cannot be relied on to identify individuals when
you are in the contact zone where they have the opportunity to hear each
other's songs during the song learning period. Finally, these chickadees can
be distinguished by birders vocally using the rate of their "chick-a-dee" call,
and songbird calls are typically not learned as songs are. However, this trait
then is going to be influenced by the birds' genetics, so that like size and
plumage, it will be an unreliable trait in the contact zone as a result of the
hybridization there.
In regards to the recent study of the contact zone's movement in Pennsylvania,
it is reasonable to consider whether any of its results might be compromised by
the use of eBird data submitted by birders who are not always aware of the
identification challenges with these chickadees in the vicinity of their hybrid
zone. I have not yet had the opportunity to read the article published in
Current Biology, and only have the summary posted by Joe Coleman to go by.
This statement is made in it, which is one I think has raised David's concerns,
"As a final step, the researchers overlaid temperature records on eBird
sightings of hybrid chickadees. They found a very close match: hybrids occurred
only in areas where the average winter low temperature was between 14 and 20
degrees Fahrenheit."
As I am confident that the authors of the study realize that birders are not
going to be able to identify individual chickadees as hybrids, what could be
meant is that the location of the hybrid zone was determined from eBird
sightings as birders reported chickadees as "Carolina/Blackcap", or "chickadee
sp." (unidentified chickadee species) as David does, knowing that he is in the
contact zone where hybridization is going on, or else birders are reporting a
mixture of both species in the area, based on vocalizations.
This seems more clearly to be what was done in the current study from the next
sentence in the summary,
"They also used eBird records to estimate where the hybrid zone had been a
decade earlier, and found the same relationship with temperature existed then,
too."
Birders' field observations are very helpful in reflecting where the hybrid
zone is at any time, assuming that many of the observations made will be based
on the songs the birds are singing. As only Blackcap songs will be heard on
one side of the contact zone (even if some of these birds fairly close to the
contact area are later generation hybrids that have learned only Blackcap songs
because that is all that is heard there), and only Carolina songs will be heard
on the other side (even if some of these birds fairly close to the contact area
are later generation hybrids that have learned only Carolina songs because that
is all that is heard there), with a mixture of both songs in between, this will
reliably pinpoint the location of the center of the hybrid zone at any given
time from the field reports of birders. I think that this is all that the
investigators were needing to know for this part of the study, whereas their
more detailed observations of the hybrid zone were based on birds that they
netted in the contact zone area and obtained blood samples from for genetic
analysis.
This is all assuming that birder's eBird reports are being made in awareness of
the presence of two species of chickadees in the area, with the determination
of which species is present being based on the vocalizations, especially songs,
heard in a given area. This is hopefully the case and probably a fairly safe
assumption for many/most birders who take the time and trouble to use eBird.
And there is a review process for eBird submissions that is designed to catch
reports of species that would not be expected in a given area. But this is a
good reminder to birders making observations anywhere in the vicinity of the
contact zone to be familiar with the songs of these two chickadees, and when a
bird is heard singing to take note of which song is heard, and to make note of
that in the eBird submission.
This process of birders tracking the position of the Blackcap/Carolina hybrid
zone when it is moving occurred here in Virginia decades ago. The hybrid zone
here has been essentially stationary on the western side of the Shenandoah
Valley since about the 1970s. Prior to that, beginning sometime around the
1930s, only Carolina Chickadees were found in Virginia. But as Blackcaps began
advancing into the state from West Virginia, apparently as a result of habitat
changes resulting from the regrowth of forest following extensive timbering
decades earlier (as opposed to climate change), birders documented that
invasion of Blackcaps (i.e. the movement of the hybrid zone) for the next 40
years. We did not have eBird, of course, so this documentation took place as
the result of articles published in The Raven, the journal of the Virginia
Society of Ornithology. Although not revealing the progress of the movement of
the hybrid zone in nearly the detail as is currently taking place in states
like Pennsylvania and Ohio because of the lower number of birders back then
compared to now, these reports in The Raven were essential in documenting this
hybrid zone movement. A paper in 1971 by Dr. David Johnston in the journal The
American Naturalist also reported on the movement of Black-capped Chickadees
into the Mountain Lake area in the 1960s as the hybrid zone's movement in
Virginia was winding down.
So I think that the investigators reporting now on the rate of the hybrid
zone's movement north in Pennsylvania have likely taken into account how robust
the data are that eBird provides us with, and we can see this as another
example of how valuable this reporting system for birders' field observations
are. The assumption all along has been that a gradual recent warming of the
climate has been responsible for the northward shift in distribution of not
just this chickadee hybrid zone, but of the range of a number of other species
as well. This study is one that makes us more confident of that conclusion,
along with providing an estimate of the rate of movement of the range interface
between these two species.
Incidentally, a couple of technical papers on the analysis of this chickadee
hybrid zone in Virginia have been posted at the VSO's website under
"Resources". Look on the far right and down a bit.
Good birding,
Gene Sattler
Lynchburg
-----Original Message-----
From: va-bird [mailto:va-bird-bounces+gsattler=liberty.edu@xxxxxxxxxxxxx] On ;
Behalf Of David Davis
Sent: Monday, March 10, 2014 11:48 AM
To: va-bird@xxxxxxxxxxxxx
Subject: [Va-bird] Regarding the article on shifting chickadee ranges
Thanks to Joe Coleman for posting the Cornell Lab press release on this study
of the shift of the Carolina-Black-cap overlap zone to the north with changing
winter temperatures. This is fascinating and important work, and studies like
it will help us all to better explain and understand climate change. However,
at the risk of being a bit of a wet blanket, I have to admit feeling a bit of a
shudder when I read that eBird data was a significant component of the
research. I am and have long been a major supporter of eBird, and I have over
3100 checklists in the data base. Over half of these checklists are from (or
near) our property in Shenandoah County, Virginia, specifically in the area
that was originally studied in describing the hybridization zone for these 2
species. Most of the early work was done by
Professor Gene Sattler, a frequent contributor to this list serv.
(Apologies if I spelled your name wrong; I didn't have a reference handy when I
typed this.).
Based upon my reading and communications with Prof. Sattler, I concluded that
it was not possible to reliably distinguish between
pure Carolinas, pure Black-caps, and hybrids in our reporting area.
Hence, almost all of our eBird and Feeder Watch records show "Carolina/
Black-capped Chickadee;" that is, a mix of the 3 possible genotypes.
The exception is a few reports of pure Black-caps during well- documented
irruption years for northern Black-caps, which are distinguishable based on
size and pattern. So, if the study is using our data for where the hybrids
are/were, and if other people are reporting similarly--that is, using the
"slash" type as the default reporting type--then I have to wonder about the
study's conclusions.
Put another way, we are still reporting our birds as Carolina/Black- capped
when, in fact, the introgression zone may have moved well beyond our site over
the years, and we may now have only Carolinas (expect for some winters). I
have considerable faith in the Cornell folks, so I can only assume that they
somehow took this into account, even if the press release left it ambiguous. I
certainly hope so, and I look forward to learning more about the study in the
coming months-- and to amending our reporting practice if that is called for.
Good birding and best wishes
Dave Davis
Arlington and Cedar Creek
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