All non mammalian life is "immune" to it. Capsaicin evolved specifically to affect mammals because their teeth can grind up the pepper seeds. So it's less that birds are resistant and more that the molecule never had any effect on them in the first place. This also makes pepper juices and seeds great to add to your bird feeders to keep squirrels out of them. Also if you want to keep rabbits and deer out of your garden dust with hot pepper powders or spray with diluted hot sauce. Most mammals tend to avoid it unlike our stupid psychotic monkey asses.