This video clip is a great example of the complexity of loon vocalizations. We normally think of loons as having four basic calls and you can listen to them here. But anyone who has had the good fortune to study loons in the field over an extended period of time is aware that loons have a much broader vocabulary. By varying intensity and pitch, loons can also use inflection to vary the meaning of their calls. Jay Mager, of Ohio Northern University, has done some extensive studies on how loons vocally communicate.
In the case of this video, we can hear a range of “mews” and “coos” and moans mixed with various changes in pitch and volume. These calls are normally identified as courting behavior. As for exactly what they mean, we can only speculate. But we must admit that an extensive repertoire of vocalizations could allow a complex syntax.