To provide an update: With input from Sheri Williams, the author of the Peterson Field guide to Hummingbirds of North America (thanks Rob!), my report of Ruby-throated Hummingbird will be accepted on eBird and represents the first record for Alameda County and 22nd record for California, if accepted by the CBRC. Thanks to Noah Arthur for his keen-eyed assistance and others with supportive emails.
From Sherri: "It's a juvenile male Ruby-throated. The mask is darker than usual in juvenile male Black-chinneds, and the tail notch is quite deep, exposing the black tips of R3-4, and the third photo shows the diagnostic elongated oval tip on P5 and the sharply pointed P6."
As noted in my post from Sunday I went back to look for this bird and didn't relocate it.
Now to prepare the CBRC report.