Gill et al. (1974) provide the most detailed descriptions of the vocalizations of the Horned Screamer. Gill et al. distinguished three basic vocalizations: the Moo Co call, honking, and trumpeting. Vocalizations are given most frequently in the early morning, and only sporadically throughout the rest of the day. When vocalizations are given later in the day, these calls usually accompany the movement of groups or constitute what seems to be communication between groups of screamers.
The Moo Co call has two syllables with a large decrease in frequency from the first syllable to the second syllable. The overall pitch of the call may be related to the gender of the bird calling, as males probably have a deeper pitched voice than do females. The quality of the call is variable, as is the intensity. The calls usually are repeated at 3-10 second intervals, and there are instances where only the first syllable is called. This call can also form a duet: ha-moo-co. The two birds overlap only one bird's second syllable with the other's first. The Moo Co vocalization is used for alarm calls, distance calls, and greetings.
A typical Honking sequence consists of goose-like calls with two distinct patterns that last about 30 seconds. They are usually performed with head and neck-bobbing, especially if the vocalizing birds are in a group. One of the patterns has seven to eight strongly developed harmonics, spaced at intervals of about 750 Hz over a fundamental frequency of about 450 Hz. The dominant frequency is at 2600 Hz. The Honking sequence sometimes includes Moo Co calls and trumpeting calls. This sequence is used for distance calling and greetings.
The trumpet call is a long-distance call consisting of a low introductory note followed by a note with a fundamental frequency at 1000 Hz, four to five harmonics, and with a dominant frequency of 2600 Hz.