According to the All About Birds page for Yellow-bellieds, they drill 2 kinds of holes: deep round holes into which they insert their bill to extract sap, and shallow rectangular holes which must be constantly maintained in order for the sap to keep flowing.
Their preferred nesting habitat is aspen and birch trees, both of which surround the house.
I observed several individuals, but had a hard time pinning down sex and age. The one in the middle seems to be a male (ID'd by the red on the throat), but the others could be female, or juveniles. I did see some juveniles being fed by a parent, but didn't get any photos of that. Leaves on the trees and poor lighting kept me from getting excellent views of them, even with binoculars.
One thing I was able to see pretty clearly, though, when the light shone on the tree just right, was the abundance of bugs that were attracted to the sap flowing from those holes. A little while later, I had my aha! moment:
Bugs attracted by sap means food for the birds! Not rocket science, I know, but I don't think I would have necessarily made the connection if I had not seen it with my own eyes. Just goes to show how powerful of a learning (and remembering) tool personal observation can be.
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