Competition: If your female budgie feels that the new male budgie is a threat to her territory or mate, she may become aggressive in an attempt to establish dominance.
Jealousy: Female budgies can become jealous of new budgies, especially if they feel that the new bird is receiving more attention. This jealousy can manifest as aggression towards the new bird.
Personality Differences: Some female budgies are simply more aggressive than others, regardless of the presence of new birds. This can be due to their individual personality traits or past experiences.
Unaccustomed to New Birds: If your female budgie is not accustomed to being around other budgies, she may be fearful or anxious in the presence of the new male budgie, which could lead to aggressive behavior.
Previous Negative Experiences: If your female budgie has had negative experiences with other male budgies in the past, she may associate all new male budgies with those experiences and become aggressive as a form of self-defense.
Size Difference: If the new male budgie is significantly larger than your female budgie, she may feel intimidated or threatened by his size, leading to aggression.