Capsaicin

If the mention of David Julius doesn't ring any bells, it's safe to say you're new to the field of nociceptor or physiology.

If you haven't heard of David Julius, you should at least think about him next time you have wasabi or chilli pepper. He received the Nobel Prize after discovering the first capsaicin receptor – a protein called TRPV1. The compound capsaicin unlocks the secrets what makes chilli pepper "hot". When TRPV1 or capsaicin receptor is activated, a burning sensation results. 

We now realise TRPV1 receptors are abundant in the mouth, lips, throat and tongue; that explains the "burning" sensation of chilli pepper. In the meantime, capsaicinoid molecule is hydrophobic and not water-soluble. If you want to quench the burning by fetching a water jug or a beer, it is unlikely to work. So, a good solution is to reach for milk as it contains the lipophilic casein, which is an important key to remove the lipid-like capsaicins. 

David Julius then showed that mice lacking the gene for the TRPV1 ion channel were impervious to both hot peppers and heat. The same applies to birds, whose TRPV1 channel is not activated by capsaicin. The good news for birds (and chilli pepper, for that matter) is that they can consume chilli pepper seeds unharmed and disperse the viable seeds. 

What about other mammals like squirrels? They do respond to capsaicin in the same way as humans. Therefore, you can coat bird food like nuts with chilli powder dust (or Tabasco sauce) to keep squirrels away from the bird dining table.