The Common Gallinule is a bird in the Rallidae family which has been split in July 2011 from the Common Moorhen by the American Ornithologists' Union.
The most obvious physical difference between the two forms is the shape of the birds’ red forehead frontal shield.
The Old World species Common Moorhen show an elliptical, rounded frontal shield while the New World species Common Gallinule show a wedge-shaped frontal shield, wide and flat at the top, tapering toward the base.

1st & 2nd photo: Brazos Bend State Park, Texas, USA, 2 March 2009
3rd photo: Brazos Bend State Park, Texas, USA, 27 October 2006
4th & 5th photo: A juvenile. Brazos Bend State Park, Texas, USA, 27 October 2006