Cassin's Kingbird, Tyrannus vociferans

Cassin's Kingbird
Cassin's Kingbirds nest in various outlying parts of the SF Bay Area, which is at the
northern edge of their regular breeding range. They are distinguished from Western Kingbirds
by three features seen in the picture below: a stronger contrast between a narrow
white throat patch and a darker gray breast; light-colored tips to the tail feathers; and
white wing bars formed by white tips to the wing coverts. (Westerns have dark
undertails all the way to the end, white outer tail feathers, lighter gray head and chest,
and solid-colored wing coverts.) Tropical Kingbirds, which are relatively rare visitors
to Northern California in winter, have much longer bills.

Cassin's Kingbird
The bird pictured above, and the one at the bottom of the page, were photographed
at La Paz, near the southern end of Baja California in Mexico, where Cassin's
Kingbirds are common.

Cassin's Kingbird
This dorsal view clearly shows the white linings of the wing coverts.

Cassin's Kingbird
The guidebooks all say that Cassin's and Western Kingbirds can be hard to tell apart, and this one illustrates that;
I'd be hard-pressed to say which species it was if it appeared somewhere they both are found. But the picture was
taken in La Paz on the Baja California peninsula, where Cassin's are common and Westerns do not appear.