Hi stuh,
A trip to Southern California during late March sounds like a dream. I would highly recommend it, and have only been south of San Fransisco in the middle of winter.
Recommendations?
1. I would advise you to get a copy of the ABA Guide to Southern California. The book is awesome, and will give you a good range of locations and what is possible to see there. Contact me via a "personal message" by clicking my name and I can arrange to have you borrow mine for the next few months and save the cost of purchasing one. In other words, save your money for fuel
2. I would also suggest that of the 4 days of birding, you consider spending 2 along the coast and 2 in the mountains/desert. The habitats and birds are very different, and you could get quite a list that way! It depends on how much travelling your group wants to do, of course.
Egrets?
Great Egret - guaranteed sightings right along the hwy.
Snowy Egret -guaranteed sightings right along the hwy.
Cattle Egret - should see, 100's if you make it to Salton Sea.
Reddish Egret - possible, but would be on the RBA
Woodpeckers?
Acorn Woodpecker - guaranteed, probably 20+ if you hit a few suggested spots in the guide.
Gila Woodpecker - you have to be more east. I had my life birds just south of Salton Sea in Brawley (2 hours east of San Diego on I8 - probably too far).
Williamson's Sapsucker - no guarantees. I think I was lucky on this last trip to get this bird in the Cuyamaca Mountains.
Ladder-backed Woodpecker - You should get it if you are east of the mountains. (Borrego Springs, Brawley, etc)
Nuttall's Woodpecker - anywhere. Kind of like finding a Downy in Chilliwack. Also on the coast.
White-headed Woodpecker - I have not found it yet. I looked in the Cuyamaca Mountains this year for it and found Williamson's Sapsucker instead, which is much more rare in the mountains there. That's birding for you!
Hummers?
Anna's Hummingbird - most abundant, will see.
Costa's Hummingbird - little difficulty locating.
Allen's Hummingbird - should be there, but good luck seperating from Rufous Hummingbird which is just as likely in March!
Owls?
Long-eared Owl - should be nesting at Tamarisk Grove near Borrego Springs if it is a normal year.
Top 2 Locations?:
San Elijo Lagoon is a great place to start. Probably 30-45minutes south of Disneyland, between the I5 and 101. California Towhee, Wrentit, Cassin's Kingbird, etc... A nice trail system to get used to South. California birds. Mission Bay is a bit farther south and will have Black Skimmers, Royal Terns, etc waiting for you.
Anza-Borrego Desert State Park is a great spot for a sampling of desert birds. California Gnatcatcher, Greater Roadrunner, Thrashers, Cactus Wren, Verdin, Ladder-backed Woodie, Phainopepla, Gambel's Quail, Black-throated Sparrow should all be pretty easy to get.
Once I know where you plan on traveling/staying, I can try to give better advice.
Hope this helps!
Dave Beeke