Cockatiel Male or Female?

How can you tell if a cockatiel is a male or a female?

That is females will have grey or brown faces, spots under their flight feathers, and barring on their tails and generally have lighter cheek patches. Males will lack the spots and bars but have white or yellow faces and crests.

Which cockatiel is more aggressive male or female?

Rates of aggression were significantly higher for male cockatiels than female cockatiels. Results based on dyadic agonistic interactions showed males to rank significantly higher in the social hierarchy than females.