On one of the branches in these trees, a piece of bark has partially peeled away, making a small cup-shaped hollow beneath the branch that faces my window. One late afternoon in January, just as it was starting to get dark, I happened to see a little Black-Capped Chickadee fly straight into this hollow and burrow its face down into the crevice at the back. I checked again later that day (before it got completely dark), and the chickadee was still there -- as far as I could tell, it stayed there all night, bundled up against the cold. I seem to be able to see into a perfect little bird-bedroom!
After that, I checked back with the hiding hole whenever I remembered, and without fail, by around 4:30 there would be a chickadee in residence. Then, one afternoon in February, I happened to look in, and there was no chickadee to be found, but a Tufted Titmouse instead -- an interloper perhaps? The titmouse was safe and secure in the chickadee's hole, and for a little while the chickadee was even sitting right there, just outside, waiting patiently to be let in. (I say "the" chickadee, but of course I have no way of knowing if this was actually the same individual.)
For a while after that, the hole's occupancy alternated between chickadee and titmouse (whoever got there first, I guess), but about a week ago that changed, when a pushy White-Breasted Nuthatch took over. I saw this nuthatch actually chasing chickadees away from the branch, and he or she even hides in there sometimes during the day as well.
Every evening now, as the Sun is starting to set, the nuthatch is in the hiding hole. And how interesting that nuthatches apparently prefer to sleep with their heads facing outwards and chickadees and titmice choose the opposite position.
So now I know where birds sleep at night, or at least one place where a maximum of one bird can sleep at a time. I'll be keeping an eye on this hiding spot from now on -- what bird will take it over next? Oh, the drama!
4/5/11 - UPDATE: For the past couple of weeks, this piece of bark had been looking more and more tenuously connected to the tree, and today when I checked, it was gone completely. So no more bird bedroom outside of my window, but it was fun while it lasted. As far as I can tell, the nuthatch had it until the end.