Intriguing semantics

["martin butterfield" <>]

In looking through HANZAB to follow up some issues about the behaviour of the Kellys' Swamp spoonbills I was struck by the term "Agonistic Behaviour" since I have never come across the word 'agonistic' before.  As the prefix 'ant' could mean "opposite" as in antonym I assumed it meant the opposite of 'antagonistic' which would suggest something like cooperation.  However both the entries in HANZAB and the definitions in the Shorter and Concise Oxford Dictionaries the relevant definitions seemed to say both words meant 'competitive'.  

I have come across some defniitions from both pharmacology (one turns on a receptor, the other doesn't) and physiology (muscless which oppose each other can be referred to as agonist and antagonist) where the two words have different definitions implying some degree of oppositeness (if that is a word).  However I haven't been able to find such a pair of definitions relevant to behaviour and wonder if anyone on the list can explain why the competitive behaviour of birds is described as agonistic rather than antagonistic.

Martin