A hard-shelled nut occurs in all habitats of a certain species of bird, but only some populations of that bird drop the nut on rocks to crack the hard shell. what is the most likely explanation for this difference in behavior?

It is a learned behavior taught by parents to offspring.

These populations are experiencing a learned behavior according to their environment, and this behavior is taught by parents to offspring.

Explanation:

The learned behavior is the one that the organism develope as a result of experience. Learned behavior contrasts with innate behavior, genetically programed that are performed with no previous experience. Learned behavior is not inherited from parents to progeny; instead, it is developed during life as a result of experience and the environmental influence, and taught to offspring.

Learned behavior allows the individual organism to adapt to changes in the environment, and they modify by the different experiences.

The fact that only some populations of birds perform this behavior and not the whole species is an indicator of learned behavior, which might be related to the habitat in which the population lives. If it was an innate behavior, the whole species would behave equally by letting nuts fall against something hard to break it.