On the west coast, Purple Martins will nest in natural cavities, but for whatever reason (the type of habitat which was taken over, perhaps?) on the East, they tend to nest almost exclusively in birdhouses.
This, of course, makes for nice photographic opportunity. You can get good pictures of these birds at their houses without causing real trouble. This one was taken at Parker River Wildlife Refuge.
Another cool fact about the Purple Martin (both these cool facts were taken from Cornell's absolutely awesome bird guide site) is that Purple Martins will nest in communities and not only defend their own nests, but defend each others' as well.