Answer:

If we look closely at the bird’s bill, we see that it is thin, pointed and relatively long. This bill shape is characteristic of warblers, so we can conclude that this bird is a member of that family.

There are many warblers with yellowish plumage, some common in LA County, some rare. The bird in question has very bright yellow plumage, which eliminates the common Orange-crowned Warbler as a possibility. Also, the face and belly appear to have no markings. From this, we have just a few possibilities: a female Yellow Warbler, an immature Hooded Warbler, a Prothonotary Warbler, or a Wilson’s Warbler.

Fortunately, all of these four species have different tail patterns. The bird in the photo has yellow undertail coverts and a dark tail. A Yellow Warbler would have yellow all the way to the tip of the tail, so this bird cannot be a Yellow Warbler. Hooded and Prothonotary Warblers can also be eliminated because their tails are white from below. This leaves Wilson’s as the only possibility, and the tail pattern of that species matches that of the bird in the photo. Thus, the bird is a Wilson’s Warbler.

This was a difficult one. Congratulations to Lily, who figured out Wilson’s Warbler!