Although our nest box trail was established primarily for the benefit of fostering the dwindling bluebird population in the U. S., other bird species, primarily tree swallows and house wrens, are commonly found using the boxes. In recent years we have also had black-capped chickadees nesting in our boxes.
Since establishing the trail in 1993, we have found that many more tree swallows and house wrens nest in our boxes than bluebirds. In 2005 we fledged 69 young bluebirds, 94 tree swallows, and 126 house wrens from the 64 nest boxes on our trail there were also two black-capped chickadee nests, however neither were successful. While there were substantially fewer bluebirds in comparison to swallows and wrens, it was our most successful year since the trail was established, when only 18 bluebirds were fledged from 40 boxes. Thus, one can see that our 64 nest boxes get plenty of use.
Tree swallows are the primary reason we place our boxes in pairs. Since swallows are so common, if the boxes were put out in singles, we might have tree swallows nesting in every box. These birds are very territorial with their own kind and will aggressively defend their territories against other tree swallows. While tree swallows will defend their territories against other tree swallows, they are much more tolerant of other species, such as bluebirds. Putting nest boxes in pairs guarantees there wont be a tree swallow in the second box of a pair, thereby increasing the probability that bluebirds will have available nest sites.
House wrens also cause some problems. They are extremely aggressive birds especially for their size. While interspecies territoriality is not a problem for tree swallows, house wrens will go into the nest of other species and drive them out or even destroy the eggs, kill the young, or even kill the female sitting on the nest in order to obtain prime nesting territory.
While tree swallows and house wrens seem to cause some problems, both are native and are protected because they have an important role in the ecosystem. Competition for a home is just one of the struggles going on in the natural world.