Latin sub-species names - why the change in endings?

[Carl Billingham <>]

I'm too young/common to have had the classical education so I flounder a bit 
with Latin;-)
 
I have noticed that the new IOC list has changed a couple of the endings on 
sub-species names. For example with the Shy Heathwren, cauta to cautus and 
halmaturina to halmaturinus.  Yet the same endings haven't changed for other 
birds such as Little Wattlebird on Kangaroo Island are still ssp halmaturina.  
Can somebody please explain what the difference is and what the rule is for 
whether they should end with an 'a' or 'us'.
Thanks in advance,
Carl
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