What are these tiny shelters along the side of the highway?

While driving NE on Highway 401 between Windsor and London, Ontario a couple weeks ago, I would notice a tiny shelter along the side of the highway about every couple of miles. At least I think it’s a shelter? Sometimes there would be two or three of them right next to each other. Are they for some kind of animal? I found one in street view:

Artificial habitats for barn swallows.

CBC article

PDF with more pictures and explanations.

Three minutes for the win! Well done.

When I finally get my place I’ll have to build one of those (+ a martin house)…