Date:   4/6/2000 3:59:30 AM Eastern Daylight Time
From:   jwcoffey@tricon.net (Valley Birds)
To: jwcoffey@tricon.net (Valley Birds)

From: Gene Sattler
Subject: RE: Chickadee hybrids
Date: Wed, 5 Apr 2000 15:08:59 -0400


Look for a paper to come out this month in The Auk on the subject of
chickadee hybridization.  The reference is: Sattler, G. D., and M. J. Braun.
2000. Morphometric variation as an indicator of genetic interactions between
Black-capped and Carolina chickadees at a contact zone in the Appalachian
Mountains.  Auk 117(2):427-444.  Estimates of the proportion of hybrids from
twelve population samples along two transects crossing the hybrid zone in
Virginia and West Virginia are given.  Hybridization exceeded 58% of birds
at the center of both contact areas.  These estimates were based on four
diagnostic genetic markers, three DNA-based and one protein-based. A
multivariate analysis of morphology was based on three measurements: mass,
wing chord, and tail length.  It could distinguish the two species when they
were sampled far from the contact zone. However, at the contact zone the
morphology of the hybrids overlapped broadly with those individuals
diagnosed genetically as either species.  So the identity of most
individuals in some populations close to the contact zone was open to
question based on these morphological characters.

Song is not addressed in this paper, but will be in a follow-up paper
treating the same populations.  Basically, however, at those two transects
studied, at the center of each contact zone, some birds were bilingual, and
sang what sounded like good songs of both species.  A multivariate analysis
of several song characters confirmed that most songs sampled in the hybrid
zone, even of bilingual birds, did not differ from the typical species'
songs of the two chickadees.  That is, few songs were of an intermediate
nature, even those of bilingual birds.  Use of the same four genetic markers
used in the morphology study also showed that bilingual birds were not
necessarily hybrids.  And the correlation between a song's characteristics
and the singing bird's genetics was very weak.  This is not surprising,
given the strong role of learning in the development of song in many
songbirds.  So song is going to be even less reliable than morphology in
identifying these chickadees when you are in an area of contact.
Chickadee calls will not be analyzed in this paper.

A earlier paper that showed the capability of Black-capped and Carolina
chickadees to do a reasonable job of learning each others' songs in the
laboratory is: Kroodsma, D. E., D. J. Albano, P. W. Houlihan, and J. W.
Wells. 1995. Song development by Black-capped Chickadees (Parus
atricapillus) and Carolina Chickadees (P. carolinensis). Auk 112:29-43.

Bottom line: We should accept the fact that where the ranges of the two
chickadees meet there is a narrow region where the species boundary is
fuzzy, and field identifications don't count for much. Realistically a
chickadee there is a "chickadee sp.", and is as likely to be a hybrid as
anything else. (How narrow is "narrow?" At least five miles wide, but the
figure is probably going to vary at different locations.)  *These
conclusions do not necessarily apply to birds encountered in winter that
could be individuals that have moved down from far north of the contact
zone.  Such birds are considerably larger, and are not necessarily hard to
identify based on measurements, or even sight ID or rate of calling,
especially when other birds are available for comparison, such as at a
feeder.  Caution is always warranted, however.

Finally, a natural question that is likely to be raised is, "Are these two
birds one species or two?"  If you use the definition of a species based on
reproductive isolation (biological species concept), which seems
straightforward, the answer is not necessarily that clear.  Many hybrid
chickadees were present, and most are probably not sterile, because the four
genetic markers show most of the chickadees to be not first-generation
hybrids, but later-generation hybrids (backcrosses, etc.).  But for three of
the four genetic markers, the area of genetic mixing between the two
chickadees was less than 25 miles wide.  This indicates some kind of a
genetic barrier, and argues for two species status.  However, the fourth
genetic marker showed evidence of long-range introgression or genetic
mixing, and would argue for one species status.  Which type of marker is
more representative?  It's not easy to determine.  There is a natural bias
towards the use of markers that are likely to show little genetic mixing,
because these are the type of markers that will be diagnostic in identifying
each species without question.  These chickadees could be in the early
stages of blending together, or they could over time have all genetic mixing
cease.  But just as likely, this could represent a stable hybrid zone that
will maintain a fuzzy boundary between the two forms for a long, long time.
If this last scenario is the case, then they could maintain their basic
genetic integrity, but also have the capability of exchanging some genes,
especially if that trait is a useful one, under positive selection.

Bottom line: We don't know, and the question may not really have an answer
(unless you're partial to the kind that are simple, and usually wrong).  But
based on these results, I wouldn't expect the two-species status to change.
So if you're a lister, you can probably relax ;)

Gene Sattler
Liberty University
Lynchburg, VA

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