The Collaborative International Dictionary of English v.0.48:

Hagdon \Hag"don\ (h[a^]g"d[o^]n), n. (Zool.)
   One of several species of sea birds of the genus Puffinus;
   esp., Puffinus major, the greater shearwarter, and
   Puffinus Stricklandi, the black hagdon or sooty shearwater;
   -- called also hagdown, haglin, and hag. See
   Shearwater.
   [1913 Webster]