Parrots!

Red-masked parakeets, actually. I'd seen them at this same location the first day after I moved in to Caltech and have been on the lookout for them ever since.

I often hear other parrot-like birds flying around (they have a pretty distinctive call) and have seen larger parrots/parakeets a couple times in trees around the area, such as these yellow-headed parrots by Annenberg (photo from 11/28):

and these red-crowned parrots on a giant tree along Wilson Ave (photo from 11/28):

but these guys were close enough for me to take a video and some decent photos and be pretty confident in my identification.

They are super messy eaters; I think most of what they chew on ends up on the ground rather than in their mouths. Standing there, you can just hear the discarded bits of little berry-sized fruits falling around you while the parakeets softly chatter amongst themselves. They also have a loud, abrasive call that was unleashed on me when I got too close to the tree. Occasionally some more parakeets fly in or out of the group or go to other trees (one of them flew very very close to my face; I like to think he would have hit me if I hadn't ducked).

Anyways, they are pretty cool! It was quite a shock the first time I saw them in the wild. The oft-repeated origin myth is that they escaped from a pet store/nursery that caught fire in 1959. There may also be a more scientific explanation out there.