Gallinule and Marsh 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.

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

Marsh 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 marsh hen. You can use "Gallinule" instead a noun phrase "Marsh hen".