Methyl anthranilate [0], commonly used as artificial grape flavoring and naturally found in Concord grapes, is a bird repellent. They can certainly smell it, if not other odors.
If you want to keep birds out of your strawberry patch, for instance, you can sprinkle packets of grape Kool-Aid over it.
Comments
I don't know roadrunners specifically, but in general birds don't have a great sense of smell, and don't much use it day to day.
A visual warning would work better for birds. I wonder if there is something about the fence that makes it "invisible" to roadrunners.
Methyl anthranilate [0], commonly used as artificial grape flavoring and naturally found in Concord grapes, is a bird repellent. They can certainly smell it, if not other odors.
If you want to keep birds out of your strawberry patch, for instance, you can sprinkle packets of grape Kool-Aid over it.
[0] https://en.wikipedia.org/wiki/Methyl_anthranilate