After my first post about this on 17 September 2015, I've continued to watch these White-tailed Kites come into roost in the evenings and leave the roost in the mornings.
I have lots of questions about this phenomenon.
Although the light levels are dim, when I've been able to see the birds well enough, it appears that many of the birds have orange markings on their breasts, indicating that they're juveniles. Is the roost site made up primarily of juveniles, or is it shared among different-aged birds?
There are at least five, and probably six, White-tailed Kites in the photo above. Two are obvious in the upper left; there's one in flight; one perched below the bird in flight; and there's one perched on the far right. I think there's also a hidden bird behind the foliage just to the right of the two birds perched up high.
The crows won't let these kites go by without many minutes of harassment (both entering and leaving the roost). Sometimes there are eight or more crows chasing one kite! (However, I've also noticed that some crows don't bother with the kites and fly directly to their own separate roost site.) It seems like being harassed every night and every morning could get tiring, but perhaps the kites are used to it? Why are the crows so concerned with the kites? Or is it more of a "game" to them?
It's hard to count the kites, as many of them come to the roost in a very short time period. I've seen them choosing a few different roost trees. Do the same individuals roost in the same trees? Or do they switch, and use different roost sites on different nights? Tonight many of them chose a roost site that I hadn't seen them use in weeks, and they all seemed to fly directly to it. How did they all know to use that particular roost site tonight?
How will the number of kites change through the fall/winter? Does weather affect the choice of roost site? Does the type of tree matter? Do the surroundings matter? How far are they coming from? Does a daylight cue trigger their arrival/departure to the roost?
They observed kites coming to roost on several different dates, e.g., Oct. 24 = 75 birds, Oct. 25 = 156 birds (!), Oct. 28 = 85 birds. Gordon Bolander and Mike Parmeter found the birds using a "walnut orchard" as a roost site. I don't know if there's still a walnut orchard here, or if the walnut orchard used to be close to where the kites are roosting now? Could this roost site be a traditional site, passed on through many generations? I'll have to ask Mike where that walnut orchard was.
P.S. The article cited above is: Bolander, G.L. and J.R. Arnold. 1965. An abundance of White-tailed Kites in Sonoma County, California. Condor 67: 446.