Gallinule and Water hen

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

Water hen is a synonym for gallinule. In some cases you can use "Water hen" instead a noun "Gallinule".

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.

Gallinule is a synonym for water hen. You can use "Gallinule" instead a noun phrase "Water hen".