The correct answer is Ornithologist.

Explanation:
Let us analyze the definitions of the options provided:

1. Ornithologist: A scientist who specializes in ornithology, which is the branch of zoology concerned with the study of birds.
2. Paleontologist: A scientist who studies fossils of plants and animals to understand ancient life forms and Earth's geological history.
3. Ichthyologist: A scientist who specializes in ichthyology, the branch of zoology devoted to the study of fish.
4. Herpetologist: A scientist who specializes in herpetology, the branch of zoology that deals with reptiles and amphibians.

Thus, the correct one-word substitute for "A scientist who specializes in the study of birds" is Ornithologist.