Birds cannot feel the heat of chili peppers.
The burning sensation from chili comes from capsaicin, which triggers pain receptors in mammals. Birds have a version of the receptor that does not respond to it, so they eat the hottest peppers happily.
Why is this true?
From the plant's point of view this is the whole idea. Mammals chew and destroy the seeds, while birds swallow them whole and spread them far and wide in their droppings.
More facts in this category
More about food
Food facts sit right where biology, chemistry and history overlap. A fruit that is technically a berry, a spice worth more per gram than silver, a sweetener that never spoils in a sealed jar — the reasons usually come down to sugar, acid, water and how long humans have been moving plants around the planet. Each entry explains the mechanism in plain language, and links to a source if you want the longer version.