Purple martin housing is exposed to the harshest weather conditions. I have used just about every style of housing available, and all are vulnerable to getting wet inside. I developed ChirpyNest so the nest is not subject to water that might enter the cavity. The nest sits above the floor of the cavity, and is built on a screen surface. This screen surface keeps the nest from wicking moisture from the floor of the cavity, as it does in most other types of housing.
Because the ChirpyNest cavity is slanted, there is no chance for water that may enter to stick around. The cedar threshold at the entrance directs any rain that may enter the cavity, to the bottom where it flows, without contact with the nest, to the back of the cavity and out. Drainage holes in gourds become blocked with nesting material, after the nest is built, becoming virtually useless for allowing drainage and ventilation. The cavity wide opening, slanted design and screen bottom nest tray, along with gravity keep the ChirpyNest nest dry. A dry nest is a more productive nest.
When a purple martin landlord first examines the ChirpyNest many want to suggest adding a rain canopy, in order to shield the opening from potential rain. I too have experimented with this option. After trial and contemplation I find a canopy to be unnecessary for two reasons. The number one reason is that the design of the house prevents the problem of rain soaking the nest. The threshold may get wet but the nest does not. Martins are wild birds and they have no problem with rain. Water that condenses inside a cavity is likely to be more of a problem in martin housing than rain. The ChirpyNest prevents both condensation and rain from reaching the nest. The second reason not to have a canopy is that a canopy would hinder visibility for the martins as they exit the house. Sitting on the porch under a canopy could make a martin vulnerable to attack by a hawk by blocking visibility.