Tracks of birds (5)
I believe the answer is:
rails
I believe this clue is a double definition.
'tracks' is the first definition.
(I know this)
'birds' is the second definition.
(rail is a kind of bird)
'of' is the link.
(Other definitions for rails that I've seen before include "Water birds" , "Metal bars of a barrier" , "Makes a protest" , "secretive birds" , "Barriers of horizontal bars and supports" .)