Warbler Guy, is the American Redstart a vagrant when seen in California during late summer/fall? American Redstarts in Point Reyes National Seashore are vagrant sightings?

Great question, Monica.

My opinion is "no" — given the Birds of North America account for this species states small populations of this species sometimes breed in far n. CA counties.....Thus, in migration, these populations may move south along the coast where, as you suggested, they are sometimes seen at this time of year at the Outer Point within Point Reyes National Seashore.

(Adult, male American Redstart, above)

That written, it's possible some of the American Redstarts seen from central to southern CA during late summer through fall arrive from as far north as southeastern Alaska and the northwest territories (as the northern most extension of its breeding range).

Observing one of these individuals would not qualify as a vagrant, I believe.

However, disoriented dispersing/migrating American Redstart from the East and Midwest would definitely be best termed vagrants, if seen on the West Coast.

Given the above information, a banded individual or so would need to be assessed to determine the precise answer to your question.