Every year I open the birdhouses after nesting season, once I've made sure they're no longer occupied, and clean them out. The cleaning part isn't all that interesting. What's interesting is seeing what's inside.
From the outside, most of the boxes look about the same. Then you open one.
Some are packed full of grass, feathers, and whatever else a bird decided belonged in there. So much material that you wonder how they managed to carry it all through that little entrance hole. Others have surprisingly little inside. Just a thin layer of grass, a few feathers, and not much else.
Every now and then I'll find layers of material pressed together. Layer on top of layer, packed down underneath. It's hard to tell where one ends and another begins.
Then sometimes I open one and find absolutely nothing. Those always make me stop for a second. The box looked occupied all year in my mind. Then I open it and realize I was telling myself a story that never happened.
It's also funny how different they can be. Some nests look like they were built by a bird that planned every detail. Others look like they were thrown together in an afternoon.
Eventually I stop looking and get back to cleaning. I pull the old material out, make sure everything is dry, check that nothing has worked loose, and put the house back together.
Truth is, I usually spend more time looking than cleaning. The cleaning takes only a few minutes. The wondering lasts a little longer.