The Collaborative International Dictionary of English v.0.48:

Redstart \Red"start`\ (-st?rt`), n. [Red + start tail.] (Zool.)
      (a) A small, handsome European singing bird (Phoenicurus
          phoenicurus, formerly Ruticilla phoenicurus),
          allied to the nightingale; -- called also redtail,
          brantail, fireflirt, firetail. The black
          redstart is Phoenicurus ochruros (formerly
          Phoenicurus tithys), and is now rare and protected
          by law in England. The name is also applied to several
          other species of Ruticilla and allied genera, native
          of India.
      (b) An American fly-catching warbler (Setophaga
          ruticilla). The male is black, with large patches of
          orange-red on the sides, wings, and tail. The female
          is olive, with yellow patches.
          [1913 Webster +PJC]

The Collaborative International Dictionary of English v.0.48:

Redtail \Red"tail`\ (-t?l`), n. (Zool.)
      (a) The red-tailed hawk.
      (b) The European redstart.
          [1913 Webster]

WordNet (r) 3.0 (2006):

redtail
    n 1: dark brown American hawk species having a reddish-brown
         tail [syn: redtail, red-tailed hawk, Buteo
         jamaicensis]
    2: European songbird with a reddish breast and tail; related to
       Old World robins [syn: redstart, redtail]