One who studies birds (13)
I believe the answer is:
ornithologist
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">one who studies birds</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for ornithologist that I've seen before include "Studier of birds" , "Person who studies birds" , "Bird expert" , "One studying birds" , "Scientist" .)