Icaros are medicine songs, used as part of the rituals of shamans and curanderos in the Peruvian Amazon Basin. The doctor spirits teach the shamans their icaros. They are expressed in the form of song and are a major system of delivery of the shaman's spiritual energy. They are used to bring on mareación  (the visionary effects of the ayahuasca), take mareación away, call in different plant spirits, call in the spirits of others or the deceased, take away dark spirits and dark energies, and manage the ceremony. Icaros are either whistled or sung, and can be expressed in any language. The shamans generally sing in a spirit dialect that is a mixture of their native language (i.e., Quechua, Shipibo-Conibo, Asháninka, etc.), Spanish, and different evocative sounds.

Icaros represent a system of communication between the shaman and the spirits, and the shaman and the participants in the ceremony. The shamans believe that every living thing has an icaro and that these icaros can be learned. The singing is sometimes accompanied by the chakapa, or shacapa, a leaf rattle that is used to carry the rhythm of the ceremony. The shaman will use his chakapa to direct energy and the icaros, as well as send away dark or unwanted energies. Each icaro is used to contact a different spirit, for use of healing. Source: Wikipedia.