Here in CA we have Anna's Hummingbirds all year round. I wouldn't worry about Hummers in Sonoma. We are farther North then you, in the N. Sacramento Valley. We keep feeders up year round. I figure they need the extra food more when it's cold.
Donna

In Portland, Oregon, just in the past three years, we've had Anna's winter over. We never used to have them year-round, but we do now! I wouldn't fret too much. I know people in your area who are experiencing the same thing and actually did last year as well.

This morning at 7AM, we had 7 Anna's at our feeder. They have been mobbing it all day long. The rain we had this weekend didn't slow them down a bit, they were at the feeder all day long.
Donna

I live in Washington, north of Seattle, and we have plenty of Anna's Hummingbirds now. They stay here all year round.

Having not spent any time in Ca those different climates you have boggles my mind. Sure wouldnt worry about anything in zone 9.

I also have the pp209 feeders and $9.75 is the best price Ive seen I usually see them discounted at mayble $12.

These little creatures are a lot tougher than you would imagine, I noticed three the first day of rough weather last week where it stayed below freezing and put out a feeder, bringing it in at night, mixing the sugar/water ratio a little stronger so it wouldn't freeze as quickly.

Storm passed on Thanksgiving day finally (4 days) and they are still here a week later. I wish I knew where they had gone at night when it was 20F, but apparently they found someplace suitable.