Do you have any guesses about which species of bird made this nest?
Here's a perfect description of the nest:
"Their adherent nests are composed of a mud shell lined with plant fibers, typically placed over water and plastered to a vertical wall within a few centimeters of a protective ceiling."
Did you guess swallow? If so, that's a good guess...but note that the nestlings in the picture above are black and they have fairly large bills, unlike swallows. Note that finding the bills (yellowish/pinkish in color) is the easiest way to count the nestlings. (There appear to be at least three nestlings, and perhaps more if some are hidden.)
Warning: The next picture will reveal the owner of the nest.
This adult Black Phoebe (Sayornis nigricans) had just caught a fly and was preparing to bring it to these nestlings.
Here are a few fun facts about Black Phoebe breeding behavior:
- It takes 1-3 weeks for the female to build the nest
- Building a nest close to a "protective ceiling" may help to discourage avian predators
- Clutch size varies between 1-6, with a mean of 4
- The female incubates the eggs for ~16 days
- Both parents feed the young in the nest for ~18 days
- Nestlings are primarily fed bees/wasps, flies, and butterflies/moths
- After leaving the nest, the young will stay with the parents for another 7-10 days
These pictures were taken near Falletti Park in Cotati on 31 May 2015.
P.S. Facts above from The Birds of North America account by B.O. Wolf.