This was posted today on MOB Montana birders list. Not really an issue for
birders in Washington, but still an interesting issue.

Scott R

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For those of you trying to figure out if the warbler you observed last fall
was a MacGillivray's or Mourning Warbler, here is an article you may be
interested in:
Extensive hybridization in a contact zone between MacGillivray's warblers
Oporornistolmiei and mourning warblers O. philadelphia detected using
molecular and morphological analyses. Darren E. Irwin, Alan Brelsford, David
P. L. Toews, Christie MacDonald, Mark Phinney.
Journal of Avian Biology

Volume 40, Issue 5, pages 539?552, September 2009

There are many pairs of related western and eastern avian taxa in North
America, and for many of these, little is known about their interactions in
sympatry. One example is provided by MacGillivray's warblers Oporornis
tolmiei and Mourning Warblers, Oporornis philadelphia. There have been
occasional reports of range contact and hybridization between these forms,
but recent authors have doubted these reports. We show that these two
species do in fact come into extensive range contact in the southern Peace
Region of British Columbia, just east of the Rocky Mountains. We analyze
whether patterns of variation in morphometric traits, eye-arcs, a
mitochondrial DNA marker (COI), and a Z-chromosome marker (CHD1Z) are
consistent with reproductive isolation or hybridization in this contact
zone. Each trait shows strong differences between allopatric MacGillivray's
warblers and allopatric mourning warblers, yet in the contact zone there are
many birds with a combination of traits typical of both species. This is
clearly seen in the molecular markers, for which 18 of 50 birds genotyped in
the contact zone have both western and eastern alleles. These patterns
strongly indicate the presence of an extensive hybrid zone between
MacGillivray's and mourning warblers. Variation in each of the four traits
is explained well by a single sigmoidal cline, with a width of roughly
150?km (or 130?km based only on the molecular markers). This is only the
fourth hybrid zone known among North American wood-warblers (Parulidae).

John Carlson
Billings/Fort Peck, MT
jccarlson at surfbirder.com
www.prairieice.blogspot.com
http://www.pbase.com/mccownii
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