Tail bobbing isn't really a symptom in itself, unless you are seeing other signs such as difficulty breathing, sleeping all the time, not eating/drinking. It could be you are just startling your birds, so they are hunching down when you are near, or they are just active and moving about if they otherwise seem healthy.
Offering a treat when you come by to visit can help them bond with you. If you do start seeing any signs of illness, or respiratory distress (heavy breathing, panting, etc) then scheduling a vet visit to rule out any more serious issue is best.