Here's why:

* Ornithology is the branch of zoology that focuses specifically on the study of birds.

* Zoology is the study of animals, which falls under the broader category of life science.

* Earth science focuses on the Earth's physical features, processes, and history. While birds interact with the Earth's environment, their study primarily revolves around their biological characteristics, behavior, and evolution.

* Physical science deals with the fundamental laws of physics and chemistry. While physical principles apply to birds (e.g., flight mechanics), ornithology focuses on the biological aspects of birds.

Therefore, ornithology is a specialized area within the field of life science, specifically zoology.