Plants like to have capsaicin in them is because to prevent animals, insects, fungi, and bacteria from eating them, hence, having a higher chance of maturity; however, the nervous system in birds prevents them from feeling the pain of capsaicin, which means birds can eat peppers without knowing that they are hot. A big chance capsaicin works this way is because birds are excellent seed spreaders, so birds, having them immune to capsaicin, could then spread capsaicin plant seeds.