Because birds tend to hide illnesses and appear healthy until they are so ill that they are close to dying, your best action is to have the bird seen by the vet. Here are some articles with information on possible causes of the twitching. http://en.allexperts.com/q/Birds-General-2349/Possible-Health-Problems.htm Some tail bobbing is normal when breathing, but if the tail bobbing is excessive/constant and the bird also appears to be in distress otherwise, this could indicate a respiratory problem. http://www.tailfeathersnetwork.com/community/showthread.php?p=151996 Budgies tail bob for various reasons. Usually it means that they are having a hard time breathing either because of a respiratory illness or something in their abdomen is acutely wrong and paining them such as a tumor or fatty liver disease. Statistically, I've found that tumors cause almost 30% of budgie deaths since they are more common than one would think. We just lost one of our budgies not too long ago to an abdominal tumor. If you find it early enough, it can be possibly removed or treated. Normal tail bobbing in a budgie would be when they are nervous or scared. Also, if they have just been super active when they start to rest and come down from the "budgie crazyness" the tail bobbing can occur but it shouldn't last long at all, it should quit fairly quickly. A budgie who is consistently bobbing its tail, needs to be seen by an avian veterinarian. The sooner a bird with a problem is diagnosed and treated, the better the chances for a positive outcome.

maybe he's having a bad dream... kinda like a puppy dog