I have a 15 yr. old B & G macaw. Since we received her, 7 years ago, she has trembled or shook, like shivering. She is a feather plucker and as she doesn't have feathers on her chest, her trembling is noticeable. One might think that she is nervous or cold, but she trembles when she is resting and when it is hot. She has a good diet, flies around the house and doesn't seem to have "sick" symptoms, other than the feather plucking of course (which, according to her vet is behavioural).