sentences:  While tree holes and cliffs may have comprised their historic nesting resource, the almost complete removal of ancient forest from their nesting range has resulted in adaptation to man-made sites.Swifts build their nests of air-borne material caught in flight, bonded with their saliva, in suitable buildings hollows, such as under tiles, in gaps beneath window sills, and most typically under eaves and within gables.Swifts form pairs that may couple for years, and often return to the same nesting site and partner year after year, repairing degradation suffered in their 40-week migratory absence.