Is mountain dew bad for budgies?

No any type of sugary caffeine drink or food can be toxic to a bird. To much salt can also be. No chocolate, No avocados, No onions, garlic. No milk as all birds are lactose intolerant and it will cause many medical issues. I would get a good book on birds so you know what you can feed and what can and cannot be used around birds. Teflon pans can cause instant death for birds if overheated as they release toxic fumes we cannot smell. All areal sols are very bad as well. Scented candles are not good. Cleaning products and air fresheners. These are just a few things hope I helped some.