I agree with Kimball that at this time of year the flock is most likely to be white-throated swifts. As the nesting season ends and the temperatures get cooler the white-throated swifts that overwinter in California congregate in larger groups in communal roosts. These roosts can range from just a few birds into the 100's. For anyone who finds a large group of swifts circling at a likely roost site near sunset, I recommend sticking around and watching what happens next. Their evening entry into crack in a cliff, eaves of a building, or freeway-bridge drainage hole is one of the most spectacular things you will see in nature. At some roosts over 100 birds will enter per minute at what appears to be full speed.
I have been informally keeping track of these roosts as they are reported. I would like to request that anyone who sees a large group or knows of a winter roost, please let me know off-line at wtswift@...
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