Figure 2. Schematic depiction of the semicircular canals and polar histogram of LSC orientations in extant birds, measured in alert posture.

The semicircular canals are three interconnected tubes that define three nearly-orthogonal planes, and are part of the bony labyrinth of the inner ear. The measurements were obtained relative to the Earth’s horizon by Duijm (1951) when the birds were in a stereotyped alert posture at a zoological garden (n = 29). Notice that the orientation of the lagena and the cochlear duct in the vestibular apparatus normally varies among species; in our scheme they are steady for simplicity. Although the average orientation was close to zero, LSC angles when birds are in alert approximately ranges from −19° to 30° relative to the horizon.