Purple Martin House Crescent Door
USE ONLY ON ENLARGED COMPARTMENTS. Because of the restricted size of the starling resistant entrance, the birds have little room to feed babies that try to stick their little heads out for food; if the martins were to use mud on the inside at the entrance, as they often do, they could easily barricade themselves inside; and some authorities feel it's too restrictive for the adult birds to go into a 6"x6" compartment through a starling resistant entrance — that additional room would be advantageous for the birds.
Ultimately it is up to the individual landlord as to what will work best at their colony site
6x6. Hole Opening is 2.75" at the widest point and 1.125" from top to bottom.