Earthbound birds
I believe the answer is:
emus
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">earthbound birds</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for emus that I've seen before include "Aussie birds" , "Fast, but not high-fliers" , "Muse about flightless Australian birds" , "laboratory units" , "Large flightless birds of Australia" .)