Holly berries taste delicious to birds, but are slightly toxic to mammals. (enough to cause nausea, vomiting, and diarrhea, but not to kill you) Birds don't hav

Holly berries taste delicious to birds, but are slightly toxic to mammals. (enough to cause nausea, vomiting, and diarrhea, but not to kill you) Birds don't have receptors for capsaicin, the chemical that makes chilies spicy, which makes most mammals (but not humans) not eat them. Deadly nightshade isn't toxic to birds. Birds tend to travel farther during any given time period than (typically terrestrial) mammals do, making them better at spreading seeds over a wide area. So it's evolutionarily advantageous for plants to optimize their fruits to be eaten by birds instead of mammals.