Birds do not have the same sensitivity to capsaicin, because it targets a specific pain receptor in mammals. Chili peppers are eaten by birds living in the chili peppers' natural range.
The seeds of the peppers are distributed by the birds that drop the seeds while eating the pods, and the seeds pass through the digestive tract unharmed. This relationship may have promoted the evolution of the protective capsaicin.
Products based on this substance have been sold to treat the seeds in bird feeders to deter squirrels and other mammalian vermin without also deterring birds. Capsaicin is also a defense mechanism against microbial fungi that invade through punctures made in the outer skin by various insects.