This is a lovely photo Christina. It is definitely a Kingbird. I am no expert on SoCal birds, but I believe your choices are either Cassin's or Western. I lean towards Cassin's Kingbird because of the pale edges on the wing coverts, the dark head and white malar area, but the breast seems pale, so I am not sure.
That's a good thought, but it's larger than a mature western bluebird. So I'm not sure... I think it's some type of Kingbird, but the closest I can get is the Tropical Kingbird and those are very rare. Though this part of California has a lot of preserve areas, I'm not sure I would see it in such abundance if it was as rare as the Tropical is made out to be...
