Ultimately aggressive male pigeon, it's periodically becoming excited (9)
I believe the answer is:
ebullient
'excited' is the definition.
(thesaurus)
'ultimately aggressive male pigeon it's periodically' is the wordplay.
'ultimately' suggests the final letters.
'male' becomes 'bull' (bull is a kind of male).
'periodically' means one should take alternating letters (letters must be selected 'every so often').
The final letter of 'aggressive' is 'e'.
'pigeon'+'its'='pigeonits'
The alternating letters of 'pigeonits' are 'ient'.
'e'+'bull'+'ient'='EBULLIENT'
'becoming' acts as a link.
(Other definitions for ebullient that I've seen before include "Jolly" , "Full of cheerful enthusiasm" , "Libel tune (anag.)" , "bright and breezy?" , "Bubbling with enthusiasm" .)