Thank you. A significant challenge with bluebirds is the starling population; the starlings are aggressive and will take over the bluebirds habitat. Starlings are unwitting colonists of North America, now quite common.

The European starling was purposefully introduced to North America in 1890–1891 by the American Acclimatization Society, an organization dedicated to introducing European flora and fauna into North America for cultural and economic reasons. Eugene Schieffelin, chairman at the time, allegedly decided all birds mentioned by William Shakespeare should be in North America (the bird had been mentioned in Henry IV, Part 1). A hundred of them were released from New York's Central Park.[6]

-- https://en.wikipedia.org/wiki/Starling#Distribution,_habitat_and_movements