Anza-Borrego Desert SP
Also their nonprofit partner: http://theabf.org/
So it turns out that it takes WAY WAY longer to get to Anza-Borrego than I expected. It's really remote! My parents picked me up from the Orange County airport at 9:30AM and with a stop for lunch in Julian we didn't get to the park until 2PM! If you plan a day trip, you need to leave early in the morning... and unless you REALLY love apple pie, I suggest you bring a sandwich for lunch.
Even in the winter, bring big sunglasses, sunscreen, and more water than you can imagine drinking.
The Swainson's hawk thing starts at 3:30PM during the migration season, but I think the hawks don't really show up until an hour before sunset... so be prepared to wait by the side of the road for a couple hours.