Looks like several of the "unidentified genera" depicted in Bugguide, such as this one:
Seems like the only realistic candidate, though young leaves.
This bird looks "off" to me. It may just be a 1st winter WTSP, though in the field I wondered whether it may be a hybrid White-throated x White-crowned, though the photos seem to indicate a White-throated with seemingly little tangible evidence of hybridization.
While songs are generally thought to be learned among birds, the calls, which are not believed to be learned, were also like Anna's in this individual despite the overall appearance of a Costa's. The tail of the Costa's typically does not exceed the tip of the primaries this much (and all primaries present - count them). Gorget and crown feathering clearly reddish, and always medium purple in Costa's. Costa's bill is typically curved noticeably downward, while this bird's is nearly straight
Video of this individual that contains the vocalizations can be seen here: