The Collaborative International Dictionary of English v.0.48:

Kingbird \King"bird\, n. (Zool.)
   1. A small American bird (Tyrannus tyrannus, or Tyrannus
      Carolinensis), noted for its courage in attacking larger
      birds, even hawks and eagles, especially when they
      approach its nest in the breeding season. It is a typical
      tyrant flycatcher, taking various insects upon the wing.
      It is dark ash above, and blackish on the bead and tail.
      The quills and wing coverts are whitish at the edges. It
      is white beneath, with a white terminal band on the tail.
      The feathers on the head of the adults show a bright
      orange basal spot when erected. Called also bee bird,
      and bee martin. Several Southern and Western species of
      Tyrannus are also called king birds.
      [1913 Webster]

   2. The king tody. See under King.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Tyrannus tyrannus
    n 1: large American flycatcher [syn: kingbird, Tyrannus
         tyrannus]