On Feb. 26 the Lehigh Gap Nature Center held a bluebird housing project. Twenty houses were put together and taken home with instructions on the best method to install them.

Director Dan Kunkle said bluebird houses are apt to attract tree swallows also, and when placed too near trees a number of birds, especially wrens, are likely to move in. The only invasive bird is the house sparrow and when the boxes are checked, those nests can be removed.

Bluebirds like to be in an open area.

There are few natural nesting cavities because the old orchards and wooden fences that provided sites are no longer around. Bluebird houses can even be placed on a deck in an area that does not get heavy use because bluebirds are comfortable around people.

There should never be a perch on a bluebird house because it only provides a step from which cats can check the box more easily.

At the end of February or as spring nears, the boxes should be emptied because parasites may be there. Mice may also move in over winter. Kunkle said that when the weather is cold, birds may use the houses for shelter, so do not clean them until the weather begins to warm. Be sure to wear gloves when cleaning them out.

Bluebirds are territorial, so if boxes are placed too close together, the second one will more likely attract a tree swallow.

Facing a box to the east means they have more warmth in the early spring but it may be too hot in midsummer. Direction has to be determined by local conditions.

"Sometimes nests fail, but you've still helped a lot of birds," said Kunkle.