As you drive through the Metroparks, look for the iridescent blue upper body and head and all white underside of Tree Swallows perching on roadside nest boxes. Although these nest boxes are intended to provide nesting habitat for the Eastern Bluebird, species like the Tree Swallow use nest boxes as well. The boxes are usually set in pairs, one for Bluebirds and one for the Tree Swallows. Neither bird species likes to nest too close to its own kind, but they are tolerant of each other.
Tree Swallows are more aggressive than Bluebirds and will chase them away from prime nesting spots, unless provided with their own box. Tree Swallows are also known to dive at the heads of people and any other animals checking out their nesting area. Swallow are very desirable to have on your property, as they are great insect eaters. If you place a nest box on your property, it is best to put up two, about ten feet apart. Nest box plans can be found here: https://www.audubon.org/news/how-build-bluebird-nest-box.