The Eastern Bluebird
Even John James Audubon, most celebrated naturalist & artist of 18th century America, was a little gaga over the "bluebird of happiness" - the Eastern Bluebird.
John K. Terres, in his marvelous book, Songbirds In Your Garden (New York: Algonquin Books of Chapel Hill, 1994)(ISBN 1-56512-044-2), stated, practically, that it is usually necessary to live beyond the limits of large cities to attract bluebirds. Suburbs bordering on open country may get them with bird boxes. Rural areas or farms still have the best chance of attracting them.
These beautiful creatures began to vanish with the disappearance of old orchards and fence posts in the 20th century where they had historically nested. This, plus pesticides + "development"+ the introduction of foreign bird species such as the English Sparrow and the European Starling, caused a drastic drop in bluebird population across the Eastern half of the United States in the 20th century until farmers began putting up nesting boxes for them. (See How to Build a Bluebird House.)
An interesting fact is that young bluebirds leave the nest at about 3 weeks of age, but their parents continue to feed them for another 3-4 weeks, even after a new brood has been started. The fledgling you may find is not without protection.
Another nice thing -- as John K. Terres put it: "If you treat bluebirds with kindness and patience, these beautiful creatures may become very tame and friendly." (p.72) Even, he says, to the point of sitting on your shoulders or feeding from your hand. Is there anyone who wouldn't want the "bluebird of happiness" to come sit on their shoulder?
Tree Swallows and Purple Martins may also nest in your bird boxes.