Topic: Echolocation in birds: oilbirds and swiftlets
The best known example of echolocating birds are the South American oilbirds (Steatornis caripensis), so called because their flesh yields abundant oil.
Echolocation, the sensory modality whereby sound is bounced off surrounding objects and received via the ears or other receivers, has evolved many times amongst the animals. Although best known in the bats and cetaceans (e.g. dolphins), echolocation has also evolved multiple times in the birds.
The best known example of echolocating birds are the South American oilbirds (Steatornis caripensis), so called because their flesh yields abundant oil. Although confined to South America today, where they are known as guacharos, in the geological past they had a much wider distribution with fossils being found in the Green River Formation of the western United States, and also in English deposits. In terms of relationships they appear to be fairly close to the trogons. Trogons inhabit caves and are effectively entirely nocturnal, foraging forth at night to feed on fruit. They have eyes, although they lack the capacity for binocular vision, and in their entire lives they never see the world in anything brighter than moonlight. The eyes, however, have some remarkable features including an extraordinarily large number of photoreceptors, achieved by having the rods banked up to three deep. This density exceeds that of any other bird, including the falcon, but it evidently provides extreme sensitivity and not acuity, and not surprisingly finds some interesting parallels to the eyes of deep-sea fish. Oilbirds can, therefore, see at extremely low levels of light. They can also navigate in total darkness and not surprisingly they employ echolocation, in the form of a series of auditory clicks that have a quite complex wave-form with some pulsatile intervals. The sounds themselves are produced in the syrinx and presumably received by the ears, although these do not have any obvious modifications.
The other group of birds that have evolved echolocation are the Asian swiftlets, a diverse assemblage that typically occupy the recesses of caves in areas of Asia such as Borneo and Ceylon. Swiftlets such as Collocalia and Aerodramas may not be familiar on account of their echolocation but are well known because of their ability to construct their nests with the aid of saliva, these nests being highly prized in Asia for the delicacy of “bird’s nest soup”. Like oilbirds they too produce a series of clicks (single and double) via the syrinx, and like them can navigate in complete darkness. Typically they employ the echolocation in the caves, where they roost in darkness, but at least one species (the Papuan swiftlet) may also be active at night. Whilst the clicks are certainly used for navigational purposes, they also appear to have a social role. In general despite the echolocatory capacity there are no obvious modifications to the relevant brain structure, or apparently the middle ear.
Importantly the ability of Asian swiftlets to echolocate has evidently evolved at least twice, and possibly more. In addition there is some circumstantial evidence for swiftlets in Brazil also employing echolocation which if correct is almost certainly independent of their relatives in Borneo and surrounding areas.
It is clear that whilst in general the ability to echolocate in olibirds and swiftlets does not rival that of the bats, nevertheless the birds are adept at flying in the pitch-dark, avoiding obstacles and finding the appropriate roost. In neither the case of the oil-bird or swiftlets, of course, are the birds blind and in non-crepuscular settings vision is also employed.
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Map of Life - "Echolocation in birds: oilbirds and swiftlets"
January 24, 2021