5/8" is sufficient to exclude house sparrows, but allow American goldfinches, lesser goldfinches, house finches, pines siskins, and other small birds. 1/2" excludes house finches, but allows very small birds like lesser goldfinches. I'm not sure if American goldfinches would be able to fit on these perches since they are larger. We don't have them in my area. I use a Dremmel with a 409 cut-off wheel to trim metal perches. I have found this to be an extremely effective way to exclude larger birds from eating all my Nyjer!