Oilbirds navigate, roost, and nest in dark caves. Unlike most birds, which navigate primarily by sight, oilbirds also have the unique ability to navigate using echolocation (also called sonar). While in the cave, echolocation enables the birds to avoid colliding with others in their colony. When they leave the caves to feed at night, they are able to avoid obstacles and obstructions.
While bats can detect objects the size of a gnat, oilbird echolocation skills are rudimentary. But they can detect objects greater than 20 centimeters (roughly the length of an adult human hand).
How does the echolocation work? Oilbirds emit short bursts of clicking noises, which bounce off of objects in the animals’ paths, creating echoes. The echoes return to the birds’ ears at different levels of loudness and intensity. The larger the object, the more sound waves that are deflected, making the echoes louder. This enables the birds to identify the size, shape, and location of the animal or object. What sounds like a single click to the human ear is in actuality an entire ‘burst’ of sonar signals. The oilbirds can thus receive constant spatial information from their surroundings, and can process this information much like the visual and audio sensations humans gather with our eyes and ears. This enables the birds to navigate at night without colliding into obstacles.
This summary was contributed by Victoria La Rocca.Edit Summary
"1. Oilbirds (Steatornis caripensis; Steatornithidae) have a bilaterally asymmetrical bronchial syrinx (Fig. 2) with which they produce echolocating clicks and a variety of social vocalizations. The sonar clicks typically have a duration of about 40 to 50 ms and can be classified as continuous, double or single. Agonistic squawks typically have a duration of 0.5 s and contain multiple harmonic components (Figs. 5, 6). 2. Both sonar clicks and agonistic squawks are initiated by contraction of the sternotrachealis muscles (Figs. 7 and 16) which stretch the trachea, reducing the tension across the syrinx and causing the cartilaginous bronchial semi-rings supporting the cranial and caudal edges of the external tympaniform membranes (ETM) to hinge inward, folding the ETM into the syringeal lumen (Fig. 17). Bernoulli forces created by expiratory air flowing through the restricted syringial aperture presumably initiate vibration of the internal and/or external tympaniform membranes." (Suthers and Hector 1985: 243)
"The physiology of vocalization by the oilbird (Steatornis caripensis) is of special interest for several reasons. The mechanism of phonation has never been studied in a bird possessing a bronchial syrinx such as the oilbird. Since the two halves of this syrinx are separated in different bronchi, the contribution of each half, or semi-syrinx, to vocalization can be more easily investigated than in birds having a tracheobronchial syrinx. Furthermore, despite the fact that the oilbird has a rather extensive vocal repertoire, its syrinx is relatively simple, having but one pair of intrinsic muscles. The mechanism of vocalization is more easily studied in this syrinx and the results can provide a step toward understanding the more complex syringes of song birds. Finally, oilbirds share with swiftlets (Collocalia spp.: Apodidae) the distinction of being the only birds capable of echolocation. These two species are among the very few vertebrates that echolocate with broadband clicks whose acoustic energy lies almost entirely within the human audible range, yet the structure of the oilbird's syrinx is very different from that of swiftlets. A knowledge of the mechanism by which echolocative signals are produced will contribute to an understanding of the capabilities, limitations and evolution of avian sonar systems... Although their eyes seem well adapted for nocturnal vision, they sometimes emit sonar clicks on dark nights when flying outside of their cave or when hovering at a feeding tree (Suthers, personal observation). " (Suthers and Hector 1985: 244)