The Mariana Island subspecies of the Common Moorhen resembles other moorhen subspecies found throughout the world. The Mariana Common Moorhen is a slate-black bird about 14 inches (35 centimeters) in length. It has sooty black on the head and neck; dark-bluish slate-gray on the upper back; brownish on the lower back & wing-coverts; blackish-brown on the tail; dark brown on the wings; white on the outer edge of the first wing primary; dark slate-gray on the breast & upper abdomen; dark underwings with white edges; white under-tail coverts; a red bill & frontal shield; yellowish on the tip of the bill; & olive-green legs & with large unwebbed feet. The adult female resembles the adult male, but has a smaller frontal shield on the base of the top of the beak. Immature birds resemble the adults, but the plumage is more mottled with white and brown, & the frontal shield is small. Males and females are nearly identical in appearance and are difficult to distinguish from each other.