Some birds do this at the speed of a fighter plane? (7)
I believe the answer is:
migrate
I believe this clue is a double definition.
'some birds do this' is the first definition.
'the speed of a fighter plane?' is the second definition.
(the rate of a MiG Russian fighter)
'at' is the link.
(Other definitions for migrate that I've seen before include "move base?" , "Move from one country to another to settle" , "Ragtime (anag)" , "Move with the seasons" , "Change habitat (seasonally, perhaps)" .)