My Dad loved little "Jenny wrens". He used a dime to gauge the size of the entry hole. About 12-14 years ago, I made some wren houses, and helped my granddaughter hang them up. A woodpecker adopted one, and remodeled the hole to something bigger than a quarter. We always has gold finches (wild canaries) and black-cap chickadees here, but haven't seen any in a couple of years.