Answer : 4. "Study of birds"
Explanation :

A. Ornithology is a branch of biology that deals with the study of birds. It includes topics related to the description of the external and internal structure of birds, their classification, expansion and development, their daily routine and direct or indirect economic utility for humans etc.

B. Ornithology deals with the following subjects.

1. Anatomy of birds: External structure of birds, including their feathers, bills, claws, beaks, etc.

2. Internal structure of birds: Internal structure of birds, including their muscles, bones, nervous system, etc.

3. Classification of birds: Birds are classified on the basis of their genetic relationships.

4. Development of birds: The development of birds occurs from egg to adulthood.

5. Bird Behavior: Behavior of birds, including their feeding, breeding, nesting, etc.

6. Ecology of Birds: Ecology of birds, including their habitats, migration, etc.