Cockatiel bites and hisses despite seemingly tame?

[liz]

I have a male cousin my height and my weight. He is muscular and I am not so I look fat next to him. If your bird is flying it will have more muscle and muscle weighs more than fat. Your birds weight could be from muscle and muscle requires more food to sustain it.

My vet said as long as I don't see and indentation in their breast then they are okay. Myrtles breast bone stuck out when I got her. She was severely underweight. She now eats everything that Rambo eats and gets hand outs from my food all day. She burns it off like my cousin.