Knock, knock, who's there?

My front door is to the right, and, ahem...someone else's to the left. This is the almost brand new facade my husband spent hours making to cover up an existing colonial turned wood street lamp that might have looked great with a New England saltbox, but did not suit my southwest location or sensibility. The copper shade was a wedding gift from some friends, made by artisans in the mountains of Mexico.

I think this hole was made by one of our many woodpeckers, but I see the carpenter ants are also coming and going. I do feel for the plight of cavity dwellers, and for that reason we have left the majority of the property in it's natural state, and even provided a bluebird nest box in the short grass prairie overlooking a small pond. It is curently inhabited by a bluebird. The opening to it was somewhat enlarged last year by some other bird to look as convex as this mysterious opening.

Too bad our house guest did not choose one of the other three sides for construction.

Here's the nest box. I can tell it is a bluebird nest from the construction materials: native mosses. I am fascinated by nest construction, just as I am of architecture in general. What makes one species of bird choose moss over sticks, leaves, or lichen?