Why is male cockatiel being so aggressive while female is nesting?

Male cockatiels can become aggressive while a female is nesting due to protective behavior. Males often feel the need to guard their mate and territory from perceived threats, which can include humans or other pets. This instinct is heightened during the nesting period as they want to ensure the safety of their eggs or chicks. Additionally, hormonal changes during breeding can also contribute to increased aggression in males.