Despite its showy red bill and bright orbital skin, Red-billed Ground-Cuckoo is elusive and poorly known. It is endemic to the lowlands of western Amazonia from northeastern Peru and western Brazil east to the Rio Negro; in addition, there are sight records from southeastern Colombia and eastern Ecuador. Although Red-billed Ground-Cuckoo occupies this large geographic area, it is considered rare, and likely occurs at very low density throughout its range. Red-billed Ground-Cuckoo may prove to be sympatric with the better-known Rufous-vented Ground-Cuckoo (Neomorphus geoffroyi) in areas of southern Colombia and northern Peru; this being the case, Red-billed Ground-Cuckoo cam be distinguished by its red bill, black crown and crest, and cinnamon-buff belly. Similar to other species of Neomorphus, Red-billed Ground-Cuckoos follow army ant swarms and herds of peccaries, running and jumping quickly to capture fleeing insects that are flushed from the vegetation. In addition, Red-billed Ground-Cuckoos have established a commensal relationship with Brown-mantled (Saguinus fuscicollis) and Moustached (S. myxtax) tamarins in northeastern Amazonian Peru, in which the ground-cuckoo lingers on the ground beneath monkeys feeding in the canopy, thus taking advantage of the used and partially-used fruits that are dropped from above. On one occasion, a Red-billed Ground-Cuckoo was observed resting, preening, and sunning at close proximity in the same tree as a troop of tamarins doing likewise. Red-billed Ground-Cuckoos make a nonvocal bill-snapping call while foraging and when disturbed, similar to other ground-cuckoos; the song is a deep moo note that is repeated steadily for a minute or longer. Unfortunately, much of the life history of this interesting terrestrial cuckoo remains a mystery.