Why Hole Size Matters
After building and hanging a few birdhouses, I started to see how much the entrance hole influences who shows up. It's a small detail, but it changes the whole feel of a house.
When I first started, I didn't think much about it.
A hole was a hole.
Then I started noticing that certain birds seemed to prefer certain size openings.
Most of the houses I build have an entrance somewhere between one and one-quarter and one and three-eighths inches, depending on what I'm building the house for. For bluebirds, I use one and one-half inches.
Those are the sizes I've settled into using around my yard.
The size of the opening also determines which larger birds can get inside. That's one reason I don't make the entrance any bigger than it needs to be. A properly sized opening gives the smaller cavity-nesting birds a better chance of having the house to themselves.
From what I've seen, if the opening is too small, birds will inspect it, pause for a second, and move on. It doesn't take them long to make up their minds.
These days I stick with what's worked.
When I drill the openings, I use a Forstner bit because it leaves a clean, smooth opening. Cedar cuts easily, and I like the finished edge it gives me.
Every now and then I'll watch a bird land, look inside, and leave. A day or two later, a different bird may show up and settle in without hesitation. Sometimes one of the same kinds of birds ends up using that house again the following season. Whether it's the same bird or not, I really couldn't say.
For something that takes less than a minute to drill, that little hole ends up making a lot of decisions.
Most of them just aren't mine.
A Note on Sizes
Entrance-hole recommendations vary by species. I use the sizes described here based on what I build and the birds around my yard. For species-specific nest-box dimensions and recommendations, Cornell Lab's NestWatch is a good resource.