There's actually an interesting reason for this! Capsaicin evolved primarily* to keep mammals from eating pepper seeds, since mammalian digestive systems destro

There's actually an interesting reason for this! Capsaicin evolved primarily* to keep mammals from eating pepper seeds, since mammalian digestive systems destroy the seeds, but birds' systems do not. Birds use a different protein complex for detecting heat than mammals, so capsaicin binds to the mammalian protein complex and activates it, but does not bind to the avian complex. * excuse the short hand anthropomorphizing of evolution here - I know.