The social complexity hypothesis states that social complexity drives the evolution of complex communication systems; in this sense, the vocal repertoire can be studied to measure vocal complexity and, ultimately, social complexity. Oilbirds (Steatornis caripensis) are nocturnal, frugivorous birds that live in colonies inside caves. Their vocal repertoire was studied to test the social complexity hypothesis in this species. Recordings were made in three known colonies in Santander, Colombia. For categorization and a complete description of the vocal repertoire, a consensus between manual categorization and an unsupervised classification was reached. I found that the vocal repertoire is comprised of nine unique vocalization types. This vocal repertoire is larger than that of many other passerine and non-passerine species. There were both short-distance vocalizations that may serve as contact vocalizations, and long-distance vocalizations that may serve as alarm, aggressive, or group coordination vocalizations. The relatively large vocal repertoire and the structurally and functionally distinctive vocalization types support the social complexity hypothesis, suggesting a high social complexity in oilbirds supported by a highly complex communication system.
