Purple martins are secondary cavity nesters that historically used old woodpecker holes and, in the west, cliff crevices and saguaro cavities. In eastern North America that pattern has essentially vanished. Effectively the entire eastern breeding population now uses artificial housing, and the species there is dependent on a human activity for its nest sites.
The switch has deep roots. Indigenous peoples in the southeast hung hollowed gourds near settlements, and the practice was recorded by European observers, so the shift began before the twentieth-century hobbyist boom rather than being created by it. Competition from introduced house sparrows and European starlings for natural cavities pushed the dependence further, since managed housing can be defended against both.
The conservation implication is uncomfortable. A landlord network sustains the eastern population, and if it lapsed the birds would face a cavity shortage they no longer have the behavioural repertoire to solve quickly. The often-repeated claim that a martin colony controls mosquitoes is not supported; diet studies find martins take mostly larger daytime flying insects such as dragonflies, beetles and flies.