Answer:

This passerine-shaped bird has a lemon-yellow belly and is gray above, narrowing this species down to the flycatcher family, more specifically, the kingbirds. More conspicuous than many other flycatchers, they often perch out in the open and glare at you, like this one. In Los Angeles County, we have two regularly occurring species in this genus, Cassin’s and Western, with four others reported as vagrants: Tropical, Thick-billed, and Eastern Kingbirds, plus Scissor-tailed Flycatcher (which is also a member of the Tyrannus genus despite not having ‘kingbird’ in its name.)

Scissor-tailed can easily be ruled out due to the lack of its namesake tail, as can Eastern, as the bird in the photo is gray above and yellow below, rather than black above and white below. While Thick-billed does have yellow below, it is black or dark brown above, with a very heavy bill as its name implies, which our bird lacks. Tropical does have this gray and yellow general color scheme, but has a longer and heavier bill, a greenish back, and a notched tail–all of which are hard to see from this angle. From this angle, this bird’s chest is gray, while a Tropical Kingbird would have the yellow belly extend up until the white throat, but blend into a more greenish color.

This leaves Cassin’s and Western. Cassin’s is dark gray above, with a highly contrasting white throat, while Western is a frostier gray color above that makes the chin stand out less. In flight or whenever the tail is splayed, Cassin’s will have a whitish tip to the edge of the tail, while Western will have white outer rectrices (tail feathers), similar to a junco, but that doesn’t really help us with this bird. The best field mark to look at here is the contrast between this bird’s black tail and frosty gray upper back. On Cassin’s, the dark gray head and back blend into the dark tail without much contrast, not like this bird. This Western Kingbird was photographed at Franklin Park in Redondo Beach on March 26, 2023.

There was another hint to ID this bird: the clue in the prompt said that this is a species that “commonly migrates” into the county in March. Cassin’s Kingbirds are a year-round species in LA, while Western Kingbirds are only here to breed from March-September, rarely overwintering.

Congratulations to Charles, Brandon, Brendan, Cadeo, Adrian, and someone who calls themselves “Red-flanked Bluetail” (nice bird!) for correctly figuring out Western Kingbird!