Excellent points would also like to add that sometimes passing through a digestive system is also part of the process. Hot peppers are actually a really good example of this. When a bird eats the pepper, it doesn't digest the seeds and it shits them out everywhere while it's flying. Mammals, by contrast, usually do digest / destroy the seeds. So capsaicin (the thing that makes the pepper hot) actually targets a receptor that mammals have and birds don't. You can actually buy squirrel deterring birdseed that's coated with capsaicin, but sometimes the squirrels just develop a taste for spiciness.