A medium-sized glossy blue-black monarch with a chestnut belly and undertail and a pale grayish bill. Distinctive crescent-shaped mark at base of bill is white in male and ruddy in female. Found in closed-canopy primary forest. Almost identical to Chestnut-bellied Monarch, but bill-base crescent diagnostic. Song is a series of rapid high-pitched whistles. Common calls are harsh, scolding chattering notes.