Butt problem

[Arash]

Hey guys
I bought 4 lovebirds 1 month ago and showed them to a vet they all were healthy 1 month ago.I don't know what is their gender I seperated them into 2 cages(cages are a little bit small 32w 45l and 60h centimeters). one ofthe lovebirds is realy stronger and bigger than other, he/she is at least between 10 to 20 percent bigger than his/her cage mate and eats normaly. one time I decide to put all lovebirds into their cage to play each other but the big one imediately attacked so badly and bit the new cage mate. and I understand I shouldn't put them all in one cage she/he just like her/his cage mate. also I can grab all other lovebirds but this one is realy wild she/he bites my fingers 10 times a second. recently ( 3 or 4 ) days I noticed that his/her butt doesn't have feathers and it's not normal but he/she acts totaly normal and healty and as I said he/she stronger than all others and is much better in acrobatic actions in comparing with my other lovebirds. in photos you can see him/her(I put my finger on top of the cage she/he scare from my finger and did this action in photos to see his/her problem) if someone guess what's the problem please help me.

 

[BIRDIGIRL]

Your Love Birds are lovely ... Is she a female could she possibly be preparing to lay an egg...Im not an expert on love birds by any means but I think perhaps it might be a good idea to have your AV check her as it could also be any number of other things and its always best to be safe than sorry.

 

[Arash]

Thank you very much
Yes you are right I think I must bring her to a vet also it's good to know exprienced people idea about this problem ( that's my first time having birds).