In addition to subspecies identification, the pronounced
emargination/notching of the 3 outer primaries that Christian
pointed out also provides confirmation that the bird is a male.
According to both Pyle's
Identification Guide to North American
Birds, Part 1 and Howell's
Rare Birds of North America,
females of ssp.
savana have tapered/pointed outer primaries
that are not strongly notched. Here's a figure from Howell's book
showing the distinction:
On a related note, a relevant research paper on Fork-tailed
Flycatcher wing morphology was recently published in the journal
Evolutionary
Ecology: Provinciato, I.C.C., M.S. Auaujo & A.E. Jahn.
2018. Drivers of wing shape in a widespread Neotropical bird: a duel
role of sex-specific and migration-related functions.
Evolutionary
Ecology 32: 379-393.
In the paper, the authors suggest that wing shape differences
between males and females, and between the migratory ssp.
savana
and the non-migratory ssp.
monachus, are the product of
a mix of selection pressures, with some (e.g., efficiency in
foraging, predator avoidance, and migration) acting on both males
and females, while others are sex-specific (e.g., male courtship
displays).
David Steingraeber
Fort Collins