The purple martin is a cavity nester. Historically it nested in such places as woodpecker holes, natural cavities in trees and even in the crevices of cliffs. Purple martins nesting in the western United States still utilize these traditional nesting sites. However, since sometime prior to 1900, such is not the case in the eastern United States. Here our largest swallow appears to have completely abandoned naturally occurring nesting sites.
In all of my years working as a wildlife biologist, I never encountered purple martins nesting in a natural cavity or woodpecker hole. However, years ago I found a pair of purple martins nesting in broken light fixture at a convenience store.
As I pumped gas into my vehicle, I watched a purple martin bringing food to its young housed beneath the plastic cover over a fluorescent light illuminating the fueling area.
If you have ever seen martins nesting in anything other than nest boxes or gourds, please let me know.