Update December 03, 2004:
added selected details of E. affinis published by Allan
R. Phillips in the Auk in the late 1940s.
These photos of Pine Flycatcher Empidonax
affinis were taken by Chris Wood in Colima,
Mexico in February 2004. I don't know how many individuals are
involved, but presumably at least two (one on each of Feb 15 and
Selected details from "THE RACES OF EMPIDONAX AFFINIS"
by Allan R. Phillips:
Phillips provides a key for separating six forms of Pine Flycatcher,
of which the two most likely to occur in the United States seem
to be: pulverius (Sierra Madre occidental; probably winters
mainly in southern part of range.) trepidus (mtns of Coahuila and SW Tamaulipas; winters
from central/northern Chiapas to northwestern Guatemala.)
By comparison to pulverius (which is likely the form shown
above,) trepidus (the form most likely to occur in south
Texas) is slightly smaller (by about 4mm in the wing; 2mm in the
tail), throat less grayish - more yellowish or whitish, and head
without any brownish tones - more grayish.
To quote Phillips:" Empidonax affinis trepidus Nelson.
This, the most (or only) migratory race of the species, is not
a mere intergrade between affinis and pulverius,
but is distinctly grayer than either."