Here's a species that really needs help, and if you have some space, and the desire to provide housing, the Purple Martin really can make use of it here in Wisconsin.
The trend map above (top) shows one way of representing decline here in the upper Midwest (using BBS data). The second figure above is a graph of ~37 years of BBS data for this species on Wisconsin BBS routes. It's clearly declining.
Lots of people have martin houses, but lots of those houses are not being used. There's abundant information to help correct this. Go to the website of the Purple Martin Conservation Association (PMCA) to learn how to properly put up and maintain a martin house, and sustain a group of martins over a long period. It does require some effort and time - it IS a commitment. But as you can see from the map and graph above, this species is in decline in this state, and can really use the help. Bluebirds have greatly benefited from human efforts to rebuild their populations. Martins need the same kind of advocacy! One critical factor is the location of a martin house, another is caring for it in winter to exclude house sparrows or starlings. Martins can't use the house if either of the other two species is already in possession. It's easy to do this right - follow the info at this link. PMCA will help show you where to locate a house, what to do in winter to exclude exotic species, and how to have success with helping this species over the long haul.