Some birds, such as my pet Pionus chalcopterus, are not sexually dimorphic. People can't tell them apart, and even trained avian vets are not 100% certain with an inspection probing the vent.
I understand that when not "in season" there is no physical difference as internal sex organs are atrophied to save weight.
But how do they tell the difference?
Do they have dimorphic markings that bird eyes (but not humans eyes) can see?