Final answer:
The higher vocal control center and the nucleus robustus archistriatalis are the main structures involved in birdsong. The area postrema is not related to vocalizations in birds. Thus, the correct answer is D) both the higher vocal control center and the nucleus robustus archistriatalis.
Step-by-step explanation:
The major structures involved in birdsong are the higher vocal control center and the nucleus robustus archistriatalis. The higher vocal control center consists of interconnected brain regions that control the learning and production of birdsong, while the nucleus robustus archistriatalis is involved in the generation of the actual song patterns. The area postrema does not play a role in birdsong; instead, it's involved with the chemoreceptive trigger zone for vomiting, which is unrelated to vocalization. Therefore, the correct answer to the question is D) both the higher vocal control center and the nucleus robustus archistriatalis.