When I decided to make a few birdhouses, I kept running across the word kerfs. I didn't know much about them at the time, so I did what most people do. I read a little, watched a little, and tried to figure out why they mattered.
Turns out it was pretty simple.
A few shallow cuts on the inside of the front panel, just below the entrance hole, give young birds something to grip when it's time to climb out. Basically, a small ladder cut into the wood. It seemed to make perfect sense.
So I began cutting them from the start. It only takes a minute or two, and then it's done.
I keep the cuts shallow, about ⅛ inch wide and ⅛ inch deep, spaced roughly ½ inch apart. Those are just the measurements I use, not some hard-and-fast rule. I don't worry much about making them perfect. Cedar is soft, and it doesn't take much to leave a little texture behind. Most days I grab a Japanese pull saw because it's nearby. Other times I'll use a small router if I already have it out.
Either way, the result is about the same. The birds just need a little something to grip as they climb toward the entrance hole.
If the inside surface is already rough enough for young birds to get a foothold, the extra cuts might not be necessary. But the cedar boards I use are usually pretty smooth, so I go ahead and add them.
Once the box is together, I don't really think about the kerfs anymore. They're hidden inside, and most people would never know they're there unless they opened the house up and looked.
They're a little thing, but I figure if they help a young bird make its way out of the house, they're worth the couple of minutes it takes to cut them.