Birds & berries

There is a tree in our front yard that turns dark red and produces berries each fall. I don't know what kind it is, but there are a few others in the neighborhood; it seems to be a popular sort. When the berries get "just right" the trees are swarmed by robins and cedar waxwings (see the photo at right); it would be easy to conjecture that the berries have fermented and the trees have become a local bar. But the birds don't seem to fly drunkenly (I've seen that and it's a strange sight indeed!) so probably they're just feeding and getting ready to fly south.  The robins should have gone already, because we've had snow flurries and it's gotten well below freezing.  But winter is close and we'll lose them all too soon now.