Columbia subject headings:
Birds--Behavior.
Birds--Evolution.
Budgerigar--Behavior.
Budgerigar--Evolution.
Cockatiel--Behavior.
Cockatiel--Evolution.
Lovebirds--Behavior.
Lovebirds--Evolution.
Harvard subject headings:
Birds--Behavior.
Birds--Evolution.
Budgerigar--Behavior.
Budgerigar--Evolution.
Cockatiel--Behavior
Cockatiel--Evolution.
Lovebirds--Behavior
Lovebirds--Evolution.
Princeton subject headings: