Lori (in Kentfield, CA):
In February annually, Orange-crowned Warbler individuals arrive as the earliest wood-warbler migrant…….and they join a few over-wintering individuals……though it’s possible the latter may leave to nest elsewhere (See range map, below).
(Orange-crowned Warbler, above….courtesy of Cornell Lab of Ornithology)
A far larger pulse of this common nester throughout n. CA arrives on breeding grounds by March.
Interestingly, among all wood-warblers to complete their nesting cycle early, this ground-nesting one often does so….I’ve walked forested areas where hearing their song is a daily experience. Then, by mid-May, I never hear their trilling lyricism until I visit the higher altitudes in the state (such as the Sierra Madres mountain range three hours east of Marin County where I live and host the majority of my n. CA birding tours as a guide.
One amazing aspect of Orange-crowned Warblers’ behavior occurs post-breeding. Upon leaving a nesting area, many do not migrate south, but instead move to higher altitudes, including foothill areas east of Marin Co. and as high as 6,000+ feet. Bird banders have noticed these movements by recapturing banded individuals in the Sierra that were initially temporarily captured in nets in SF Bay Area counties, including Marin Co.
At the higher altitudes where periodic summer rainstorms occur (especially in the afternoon), larger abundance of insect fauna exist as prey base for Orange-crowned. By August and beyond into autumn, it’s thought these individual eventually migrate south to s. CA and as far as s. Mexico and portions of Belize (adjacent to Mexico).