Definition of blue heron in US English:

blue heron

noun

  • 1North American. Either of two North American herons, the large Ardea herodias (also more fully "great blue heron") and the small Egretta caerulea (also more fully "little blue heron").

  • 2NZ. The darker, slate-grey form of the eastern reef heron, Egretta sacra; = matuku.

Origin

Mid 16th century (in an earlier sense). From blue + heron.