Ornithology is a part of zoology, and it's basically the science of studying birds. That includes how their bodies work, what they do, how we categorize them, and how we protect them. The word itself comes from ancient Greek words that mean bird and study.
Insects: The scientific study of insects is called entomology.
Sea Animals: The study of marine organisms is generally known as marine biology. Specialized branches include ichthyology (fish) and cetology (marine mammals).
Sea Plants: The study of aquatic or marine plants is a subfield of botany, often referred to as marine botany or phycology (specifically for algae).