Mountain birds have invaded the Colo plains

There is a phenomena that occurs every so many (??) years during which birds that are permanent residents of Colorado's foothills and mountains invade the lower elevation plains and that has been occurring both in SE Colorado and other areas of the state. Though there is always a lot of speculation about why this is occurring there is no scientific answer.

I noticed it about a month ago when 2 Mountain Chickadees showed up in my backyard, feeding on some peanuts I had in one of my feeders. Now I do get 1-2 Mountain Chickadees every years or so but I don't usually see the numbers of this species as I have seen on the Canon City Riverwalk in the past several weeks. And several days ago I found a Cassin's Sparrow on one of the large sunflowers in my yard, feasting on the ripened seeds.

Others have reported not only occurences of these species but also lots of Red-breasted Nuthatches and other mountain species being found all the way to near the Kansas border. The Mountain Chickadee in these two photos was foraging on the Riverwalk and flew so close, only a few feet from me, that I it was too close to photograph for awhile.