Why can birds eat hot chili peppers without feeling the burn?

Answer

Mammals feel the heat through the TRPV1 receptor, which birds' version ignores, so they can spread the seeds.

Context

In mammals, capsaicin binds to a protein channel called TRPV1 in sensory nerve endings. This channel normally opens in response to noxious heat above about 43 degrees Celsius, so capsaicin tricks the nervous system into signaling heat and pain. That is why chili peppers feel hot without any actual temperature change.

A study published in 2002 compared TRPV1 from rats and chickens and reported that the chicken version responds very weakly to capsaicin, even though it is still activated by heat. The authors traced the difference to particular amino acids in the channel. This fits the idea that birds are largely insensitive to the chemical, while mammals are sensitive. Later research on species differences identified individual amino acid changes that can alter how strongly a channel responds, which is consistent with this explanation.

The ecological reasoning is that chili plants benefit when birds eat the fruit and carry seeds away unharmed, while mammals tend to crush seeds with their teeth or digest them. Capsaicin may therefore deter animals that destroy seeds without bothering the animals that spread them. This is a widely discussed hypothesis, and it is supported by field and lab work, but the picture is not entirely uniform across all bird species and all mammals. The summary that birds feel no burn at all is also slightly simplified, since the findings concern the TRPV1 channel and not every possible pathway of irritation.