The Collaborative International Dictionary of English v.0.48:

bird-on-the-wing \bird-on-the-wing\ n.
   a common trailing perennial milkwort of eastern North America
   (Polygala paucifolia), having leaves like wintergreen and
   usually rosy-purple flowers with winged sepals.

   Syn: flowering wintergreen, gaywings, gay-wings, fringed
        polygala.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

bird-on-the-wing
    n 1: common trailing perennial milkwort of eastern North America
         having leaves like wintergreen and usually rosy-purple
         flowers with winged sepals [syn: flowering wintergreen,
         gaywings, bird-on-the-wing, fringed polygala,
         Polygala paucifolia]