I used to keep many gourds in my pecan trees for bluebirds. They are territorial, so they occupy only one about every 100'. I had them spaced all over the little farm and over a few years got a large crowd of birds.
I cut my holes with a 1.25" hole saw on a drill. Then I put a hole in the top for a hangar, fished the insides and seed out with a piece of stiff wire, then drilled 4-6 drain holes 0.25" in the bottom.
Every time I painted one, no birds would even look it over. My solution was to heat a large pot of paraffin wax and submerge the gourds one at a time in the paraffin wax till it stopped bubbling. At that point the water had boiled out, and the paraffin had penetrated through and through the shell. That can be a dangerous job. One time I was lifting a gourd out of the wax, and let it slip and fall back into the pot. Instantly I had a bad kitchen fire, but I had a fire extinguisher about two steps from the stove. I bought a new stove for my wife.
Every winter I pulled the gourds down and cleaned the nests out then reboiled them in paraffin wax. Gourds maintained like that last several years, and the birds love them.
That is a lot of trouble, but it is nature friendly, and each year the birds have a nesting place that is free of diseases.
