What breed is your bird?? In my experience, when my birds were in pain/sick they would get all fluffy. They would sit with their eyes closed and all the feathers would be puffed around the face. It may depend on the breed, but that is my experience with cockatiels and parakeets. Birds *usually* chirp only if they are happy/content/playful. I hope your birdie is OK!! What is his name??

Another thing that I just thought of is to watch their poop. (YUK) Any change in consistency or frequency is a big indicator. Hope this helps!