Form of transport for bird (4)
I believe the answer is:
rail
I believe this is a double definition.
'form of transport' is the first definition.
(I've seen this before)
'bird' is the second definition.
(a wading bird)
'for' is the link.
(Other definitions for rail that I've seen before include "Form of transport" , "Complain bitterly - wading bird" , "transport system" , "Complain intemperately" , "Part of train track" .)