Capsaicinoids

Chili heat: molecules that trip the body’s burn sensor while leaving birds — the seed couriers — cold.

Capsaicin activates TRPV1, the same receptor that detects real heat — the burn is neurological, not thermal.
The Scoville scale, from 1912, ranks pungency; pure capsaicin scores sixteen million units.
Birds lack the receptor’s sensitivity, so they eat chilies and spread seeds the plant deters mammals from chewing.