I made a bunch years ago ,the martins never moved in.
I think I am just out of their flight path and they never found the gourds.?
May be just as well. I hear they eat bees, and I keep bees !
Make sure they are dry befor making holes.
A hole saw really helps
Then pressure wash inside ,let dry ,and paint .painted ones lasted longest.
Pouring paint inside to coat the bottom ,and let in drip out the drain holes
Now I use them for other small birds in the orchard.
Seems like if they are hanging by a wire ,swinging around, some birds won’t nest in them, iam guessing they think it’s going to fall down.
So I just put a small branch through the hole were the wire would go.
And leave them on the tree