If you can get close to the 'females' -- I assume you can -- there is absolutely no mistaking them. They have a 'vocal sac' in their chest, and they inflate that sac when they are threatening other birds. You can see it; and when I pat Felicity or Greedy when they have it inflated, I can feel it plainly.
(Don't mistake this for 'feather-flaring.' They usually both inflate the sac, and flare their chest feathers at the same time; but you can feel the sac under the feathers as you pat.)
[I am fortunate because there's always territorial drama happening here, but Felicity or Greedy will still trot up to me to get her sultanas. Thus, I get to pat them DURING their territorial calls/displays, when the sac is inflated.]
The second way to pick females is perhaps easier: females vocalise at night. In 'strings.' I can usually figure out which of my tame-wild birds I'm listening to. Felicity is usually about 9. Number One usually about 13.
Now, you shouldn't confuse females' night-time vocalisations with those of males. Its only in late winter here -- I think when less powerful males are vying for a last chance to get to sit on eggs in any season -- that you will hear males vocalise. Wild males and females have real and actual conversations about an hour before dawn. (And I'm not kidding you: they make appointments! If you hear a particular female talking to some male just before dawn, then you can be out with the binoculars, waiting for a male to hove into sight, heading to meet the female, about a half hour after first light!)
So, if -- much less likely -- you hear a boom-boom-booomer talking at night to a grunt-grunt-grunter, that's a pair talking. Much more likely, you'll hear, at different times during the night, a string of calls. The bird making those calls is female.
SE, unicup, Western Australia (Planet Rothschildi)
Edited by briefvisit - 12/21/15 at 2:16pm