Recently, I received an e-mail from Kathryn McCatherin in Hermon. She recalled bluebirds had occupied nest boxes she had on her property some years back; but, year after year they seemed to be increasingly driven out by other birds.

“Bluebirds used to make their summer home in a birdhouse at my parents’ in Sangerville until a few years ago when my parents found a dead chick in it,” she wrote. “Although they have cleaned the house yearly, they never see a bluebird. Over the years at my home in Hermon, the bluebirds have nested in several of my birdhouses, moving from one to another as other birds chased them out. But now they merely use our property as a resting place in early spring and fall.

“Can you give us more information as to what is happening and how we might entice these beautiful birds back?”

The most likely reason for the Eastern bluebird nesting failure is the No. 1 reason for the birds’ decline in North America in the past: competition from the introduced European house sparrows and starlings. The answer is surprisingly simple and doesn’t necessarily involve an all-out war against the marauding invasive species. At the Fields Pond Audubon Center in Holden, each bluebird nest box is paired with another, and the two are placed back-to-back.

You may be thinking, “Oh, right, safety in numbers,” but that’s not quite the reason for extra boxes. Bluebirds are not known to be “colonial” nesters – that is, they won’t nest in close proximity to one another as do other birds, such as cliff swallows. In fact, they will not often nest closer than 300 feet from one another. However, they won’t mind tree swallows nesting nearby.

Tree swallows use the same type of nest box as do bluebirds and utilize the same habitat: open, partially overgrown fields and grassy areas with nearby perching sites. They forage for food in vastly different ways, however. Whereas the bluebird will perch-hunt and pounce on unsuspecting insect prey, sometimes sallying from their perch to catch a bug midair, tree swallows hunt entirely on the wing, much higher up in the air column, on the flying insects that swarm there. They will forage over ponds or lakes as well, even taking a drink of water on the wing.

The key to success here is the tree swallows’ stronger defense of their nest sites and territories against the very aggressive sparrows and starlings. In other words, they serve as “bodyguards.”

There are other measures you can take as well, according to the North American Bluebird Society. First, because species such as house sparrows nest near human habitation, remove any nest boxes that are located near houses, barns, or other outbuildings.

Second, if you do live in an area with a house sparrow problem, you may wish to plug the entrance holes to nest boxes over the winter. This prevents the sparrows from roosting there and getting first dibs on the boxes come breeding time.

Third, a certain type of box – called a Gilbertson PVC box – can be used, in addition to a traditional wooden box nearby. Sparrows are reluctant to use the PVC boxes for some reason. Boxes without perches should also be used as sparrows and house wrens – native cavity nesters who also compete with the bluebird for nest sites – are attracted to perches.

There are other, benign measures you can take to combat house sparrows and starlings. You may also wish to use more aggressive procedures if the house sparrow population is out of control. Starlings can be excluded from nest boxes simply by making sure the entrance hole is the correct diameter for a bluebird – which would then make it too small for a starling to enter.

For more information on control measures, nest box maintenance, and nest box plans, visit www.nabluebirdsociety.org. Or, you may contact the Fields Pond Audubon Center at 989-2591.

BDN bird columnist Chris Corio can be reached at bdnsports@bangordailynews.net