A shaking or trembling bird can be alarming, but it is often completely normal. The trick is telling everyday trembles apart from the kind that signals a problem.
Normal reasons birds shake
Much shaking is harmless. Birds shiver to warm up when they are cold, tremble briefly from fear or excitement, and do a full-body feather shake to settle their feathers after preening or to relieve tension. A quick shake that passes, with a bird that then behaves normally, is rarely a worry.
When shaking means something's wrong
Be alert if the shaking is persistent or comes with other signs — staying fluffed up, sitting low, sleeping a lot, not eating, breathing oddly, or looking unsteady. Prolonged trembling can point to being genuinely cold, to stress, or to illness, and because birds hide sickness, it should not be ignored.
What to do
First, check the basics: make sure your bird is warm enough and not reacting to a sudden fright, and give it a calm, quiet space to settle. If the shaking passes and your bird is bright and eating, it was likely normal. If it continues, or comes with any signs of illness, keep your bird warm and contact an avian vet promptly.