She picks on another female - Skye. Lucy and Skye share a flight cage with another female (Priscilla) and 2 males, but Lucy is only aggressive towards Skye. Skye is not a pushover and tries to fight back, what results in intense beak fencing followed by Skye getting shoved off her perch (she is smaller than Lucy so she ultimately loses these fights). The two females only fight in the evening, after the cage has been closed for the day. When all my birds are out of the cage during the day, they all get along fine. Yesterday, the bullying got so bad, I had to put Lucy in a separate cage. Lucy knocked Sky off perches multiple times and wouldn't let her sit any higher than the second lowest perch. I put Lucy's cage next to the flight cage. Then one of my males, Mr. Lemon, came up to the side of the cage facing Lucy's and started chirping to her, while Lucy tried to figure out how to get back in. None of the other birds cared about keeping Lucy company. When I opened the flight cage today morning, the first thing Mr. Lemon did was to fly to Lucy's cage. I let Lucy out too and the bullying has stopped for now, but it will probably start again once I close the big cage for the night. Is it possible Lucy is jealous? Mr. Lemon is a ladies man and courts all three of my females as well as Rico, the other male. Skye's and Priscilla's ceres have recently started turning brown. Does Lucy view Skye as competition for Mr. Lemon's attention?
I'm planning on getting a bigger flight cage and adding another male to the flock to even out the gender ratio. Would it help if each female has a male to monopolize? If this behavior doe not stop, I will have to separate Lucy and Mr. Lemon, but I feel like this wouldn't be fair to him since he likes to interact with all his flock mates. These birds have lived together for several months without problems. This has only started in the past week or so.
I have two males that spend all day together but in the evening each has his own cage and separate space. Not only that, each single bird has his very own huge Prevue Hendryx F040 flight cage so that takes up a lot of room.
Personally, if the bullying problem only happens at night, I would simply have a separate cage for Lucy to sleep in.
It is a very easy solution and shouldn't cause any issues for her or any of the rest of your flock.
FaeryBee gives you a great solution - I was also thinking that if the problems only arise at night (and it seems that they're a problem of hierarchy since Lucy bullies about location of Sky), you could just separate her and Mr. Lemon at night to another cage. Since they're out during the day, Mr. Lemon will still get his time to mingle with his other birdies. Perhaps letting them hang out on neutral territory will help calm the bullying.
Thank you for your suggestions. Unfortunately separating Lucy every night with or without Mr. Lemon is not practical for me because none of my birds are hand-tame. Trying to catch Lucy is a traumatic experience for the entire flock. What did end up working for me was leaving a dim light on in the evening when the birds are back in the cage. This seems to signal them to calm down and get ready for bedtime rather than squabble over hierarchy and perches. I have not observed aggression between Lucy and Skye since I started doing this, so I hope this solution continues to work.