The blackened bits will likely ALWAYS drop off. The key is to try to prevent the frostrite before it happened. Have you tried bag balm BEFORE the freeze to see if it adds enough protection to keep the frost bite away? I'd give that a try next time. Keep areas that's dropped off clean and the birds should be fine. Are they getting frost bit inside their house or when they go out? I'd try the bag balm. For the rest of the season and see if it helps. Best wishes