A raptor and a parrot remarkably commensurate (3,4)
I believe the answer is:
pro rata
'commensurate' is the definition.
(something done pro rata is fair or commensurate)
I don't understand the rest of the clue.
(Other definitions for pro rata that I've seen before include "maintaining correspondence" , "Proportionally" , "In proportion (Latin)" , "Proportional, according to the rate" , "fairly divided" .)