Oilbirds - An extreme example of a low light-level lifestyle among flying birds
In all eye types, visual performance is a compromise between the conflicting fundamental capacities of sensitivity and resolution. The balance of these capacities in any one eye is achieved through adaptations of both optical and retinal structures, and is assumed to reflect both the behavior and the ecology of the species.
In birds, flight is considered to be controlled primarily by vision and requires a high degree of spatial resolution. However, a small number of birds are active at naturally low light levels, where high sensitivity is required. The most extreme example of a low-light-level lifestyle among flying birds is provided by the cave-dwelling oilbirds, Steatornis caripensis.
The Oilbird, Steatornis caripensis, the only member of the Steatornithidae family, is a nocturnal frugivorous, endemic to South America. Oilbirds breed and roost in caves, often at sufficient depth that no daylight can penetrate, and this must result in the majority of individuals never experiencing throughout their lifetime (up to 12 years) natural light levels above those of maximum moonlight. Within the caves, oilbirds employ echolocation using audible click vocalizations, which provide low spatial resolution, to avoid in-flight collisions although, due to a low wing loading, flight speeds are low. However, their nocturnal foraging for fruit is thought to be guided primarily by vision, with olfaction playing a secondary role…
Photo credit: ©Robert Lewis | Oilbirds at Trinidad, Trinidad and Tobago.