Update: This entry was started on April 4 and completed on May 17. In May we noticed that the nest had been destroyed.
So, which species of bird constructed this nest?
The Cornell Lab of Ornithology via its Nest Watch website provides clues to identify birds' nests based on including region, habitat, height above the ground, size, and nest material, but recommends a field guide to birds’ nests, too. Without such a guide on my bookshelves, I continued searching the web. Unfortunately, the search did not yield any i.d. clues, then.
Next, I did what I should have done from the start. I searched the catalogue of my local library for birds' nests i.d. books. I found "A Field Guide to the Nests, Eggs, and Nestlings of North American Birds" by Hal H. Harrison. However, for various reasons (general busyness), I have not borrowed the book from the library. I searched the internet once again and came across "Birds' Nests: An Introduction to the Science of Caliology" by Charles Dixon (1902). Dixon wrote that the Penduline titmouse builds pendulous nests. The local titmouse is the Oak titmouse:
4-5", year-round, grayish body, jaunty crest, light gray throat and underparts (Local Birds of Northern California Backyards & Trail Birds).
It is possible that the nest was built by a Wilson's warbler (4-5"; spring, summer, fall resident; olive above, bright yellow below; yellow face with black cap; longish tail) or a Bullock's oriole (7-8"; spring, summer resident; orange and yellow; black crown, eye line and throat; large white wing patch on dark gray wings).
Do you know?