I have a bird house at my dock it is 10 feet up on a pole. I have 13 martins in it right now. They need to have a 2 inch hole at least that’s what they prefer
Very few people use 2" round holes anymore. There are several "Starling Resistant Entrance Holes" on the market now. They can be cut into a natural gourd too. I have so many Starlings here that I would never be able to have a colony with round holes.