Aquatic bird and Water hen

Aquatic bird noun - Wading and swimming and diving birds of either fresh or salt water.

Water hen and aquatic bird are semantically related. In some cases you can replace phrase "Water hen" with "Aquatic bird", this phrases are similar.

Water hen noun - Any of various small aquatic birds of the genus Gallinula distinguished from rails by a frontal shield and a resemblance to domestic hens.

Aquatic bird and water hen are semantically related. You can use "Aquatic bird" instead a noun phrase "Water hen".