Why is male cockatiel being so aggressive while nesting?

Male cockatiels can become aggressive while nesting due to protective instincts over their territory and potential mates. During the breeding season, they are particularly driven to defend their nest from perceived threats, which may include humans or other pets. This behavior is a natural response to ensure the safety of their eggs and chicks, reflecting their instinctual role as protectors in the breeding process. It's important to give them space and minimize disturbances during this time.