Why advocate cayenne pepper to deter squirrels?
Read more at Gardening Know How: Foolproof Tips On How To Keep Squirrels Out Of Birdfeeders https://www.gardeningknowhow.com/plant-problems/pests/animals/squirrels-birdfeeders.htm In your article you state: "Or mix in some cayenne pepper into the food. The capsicum, the stuff that makes it hot, does not affect birds but will affect squirrels." Actually it does affect birds, especially if it gets in their eyes. Also, pretty cruel treatment for squirrels. SQUIRREL-SLIP is a much better alternative.
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Bird's, along with many other non-mammalian creatures lack capsaicin receptors. This means that they don't have a way to feel the "burn" associated with peppers, since this is caused by a chemical response when attached to a receptor.
Without this receptor, there is no ability to feel a pepper's burn. It remains one of the safest alternatives without being a synthetic chemical, since many chemicals, while being largely harmless can still accumulate in the soil causing imbalances from petroleum based substances breaking down.