Grass, twigs and moss and other materials suitable for a nest.
Bluebirds nest in cavities, both natural and man made. These include birdhouses and woodpecker holes for example. Their nests are made of fine grasses with a cup shape on top!
Some Bluebirds come from their Hushabye Mountain winter nesting grounds.
Generally house sparrows and house wrens. Both are cavity nesters and will compete with bluebirds for nesting spots.
Many died in communal roosts during winter nesting in areas where carbon monoxide was present. Clearing of dead trees removed nesting places. Now, bird nesting boxes have expanded by people, and bluebirds have returned to former numbers.
Sometimes bluebirds face competition with other birds over nesting sites. Another problem bluebirds have is a lack of natural habitat. In areas that are more developed there are less natural cavities for them to nest in.
By protected habitat areas, and by providing nesting boxes.
Probably because your home as everything they need to survive and make nests. If you want to get rid you them take down a birdhouse or two as bluebirds like nesting in birdhouse.
They may have moved to a different nesting site. Also they may have not had enough fresh food/water.
Bluebirds do sometimes just go in half way. They may be arranging nesting materials or doing something else. Entrance holes for bluebirds should be 1.5 inches.
their aggressive nature of nesting that will cause problems for your purple martins and bluebirds
Bluebird have adapted to use man made boxes for nesting. Their ready acceptance of the man made home has lead to a very successful conservation effort.
You can put nest boxes out with bird feeders and hay. The birds will feel safe within a nest box.
Not in one where the bluebirds already are nesting, but wrens are cavity nesters, and will nest in unoccupied bluebird houses.