It can be an instinct; an innate behavior that is not learned, such as a bear preparing for hibernation, or certain birds migrating in fixed patterns. When animals display these specific behaviors even in the absence of instruction from their parents or other members of their species, then it must be genetic, even though it's probably not known how that would work specifically.

The accusation of a "just-so story" is made against a claim of how an adaptation came about. But that's not happening here, they just state that it is an evolutionary adaptation, without a theory of how it was acquired.