I'm having a bit of trouble determining how my budgies' weights are doing. Two of mine (housed together) look overweight to me. The other budgie, Sunny, is housed alone and looks maybe underweight? I'm not sure, though.
I looked up body condition scoring and learned about the keel bone, and I can see a furrow where the overweight budgies' keels would run, since they have too much fat on either side. I can't tell with Sunny, though. She's not tame, so when I tried to touch her chest, she bit at me. I don't want to traumatize her unnecessarily and undo my hand-taming efforts.
Sunny was 37 grams a month and a half ago. She's 2 years old. If I can get her in her carrier to weigh her again, is that a healthy weight? If I upload pictures, can someone determine if she's underweight?
She's not eating much that I can see, ever since I separated her due to her bullying. She throws food out of her cups onto the cage floor, though, so I can't weigh her cups to figure out how much she's eating.
Weight for a budgie is similar to weight for a person - a healthy range depends on many things. An English type is going to weigh more, for instance, than an American type.
If possible, see if you can get your Sunny to an avian vet so they can do a proper physical and health check. In the mean time, do you know what he favourite foods are? When I first got my boy, he was severely underweight and the avian vet recommended giving him millet on top of his regular food to help plump him up. But, again, that was vet approved. If your Sunny's favourite foods are things like shredded carrots or something like that though, start offering some with every meal. Does Sunny eat pellets?
As Moria has stated it varies per bird it could be fine for some and not for others. Since you have said her eating habits have changed it is important to begin weighing her to see if she is losing weight since the separation. When a budgie is overweight you will be able to feel fat pads on the chest and/or abdominal area.