I have Carolina Chickadees nesting in at least one of my nesting boxes, Bluebirds in one, and probably Prothonotary Warblers in one. The Chickadees usually compete with the Bluebirds for at least one box.
I did a little poking around, here’s what I found. (quoted)
They will have several different nest site locations excavated before selecting the one they use.
Should predators become a problem, the nesting pair will abandon the nest and build a new one some distance away.
From here
http://www.wild-bird-watching.com/Chickadees.html
That explains it then. A few years ago I had set up a homemade birdhouse in the tree behind my neighbor's house which chickadees had begun to nest in. Two evenings in a row I saw a starling trying to get into the nest and the chickadees abandoned.
I moved that house and attached it to the side of my house outside of the window and all that summer I kept having to empty the nesting crap that the house sparrows were trying to take it over.
That's when I got the idea to make the entrance hole smaller, even smaller than what the damn house sparrows can get into..........In that respect, there hasn't been any further attempt by either sparrows or starlings to invade the houses.
I get chickadees in winter. First snow they show up, snow gone they leave. I have a feeder in the front yard. I give them sunflower seeds and peanut butter.
It has always blown me away. It could be below zero up here and snowing hard and you can hear them singing. They hit the feeder even at below zero.
They are fun to watch. One of my fav birds up here. They cannot stay in once place to long. Plus I like their paint job.
That is a great link. I learned much more about Chickadees than I have been able to find elsewhere. Now I know how they survive up here below zero.