If you mean an Immune escape that is not a real problem for rabies because that virus has a slow evolution as you can tell from the fact that the same vaccines that were used in the 80s are still used today. Compare that for example to flu or Covid-19 vaccines where even two year old vaccine types are considered outdated and not used anymore.
What are birds? We just dont know! https://www.youtube.com/watch?v=Kh0Y2hVe_bw