The Spotted Pardalote, a pretty Australian pipsqueak, uses bifocal vision to almost simultaneously see small insects underfoot and predators overhead.
By Rex Graham
That’s the theory of Canadian neuroscientists who completed the most exhaustive and detailed study to date of a tiny structure in the avian midbrain called the isthmo optic nucleus, or ION. When present in a bird, nerve fibers link the structure to the bird’s retinas, the light-sensitive inner lining of the eyes responsible for vision.
The neuroscientists studied the eyes of 81 bird species in 17 orders, and reported their surprising results in the journal PLOS ONE.
The ION was discovered over a century ago and has been found in many vertebrates, but its role has remained a mystery. Some scientists had theorized that it enables “pecking birds” to more accurately hit small food targets. Maybe it’s needed for gaze stabilization, dark adaption, shifting attention or detecting aerial predators.
The Canadian neuroscientists ascribe a more complex function to the ION that takes into account its size and complexity in the 81 species they studied, the birds’ visual habits, and the bifocal nature of their retinas.
Spotted Pardalote big ION
“The ION is more complex and enlarged in birds that have eyes that are emmetropic [far-sighted] in some parts of the visual field and myopic [near-sighted] in others,” said the researchers, led by Cristián Gutiérrez-Ibáñez, a neuroscientist at the University of Alberta in Edmonton, Canada. “We therefore posit that the ION is involved in switching attention between two parts of the retina i.e. from an emmetropic to a myopic part of the retina.”
They found that owls and other birds of prey have neither ION regions nor near-sighted vision. However, birds with myopic or near-sighted regions in their retinas also have relatively large IONs.
“Birds with large IONs feed close to the substrate, which can include the ground, flowers and tree trunks, and in species with lower-field myopia it is the part of the visual field containing the substrate that is myopic,” the researchers said.
Australia’s tiniest bird
The beautifully colored and patterned pardalote often hovers and hangs upside down high in Eucalyptus trees of southern and eastern Australia. The 10,000 cells in the pardalote’s ION, may give it a survival advantage as it forages for small invertebrates and lerps, the crystalized honeydew that coats the larvae of many sap-sucking bugs.
The Australian songbirds often feed singly or in pairs at the tops of trees where soaring predators might try to ambush them. The ability to multitask: monitor the air space around them while feeding at close range could be extremely valuable.
The researchers said the ION has evolved independently several times, for example in coots and allies, non-beak-probing shorebirds, songbirds, woodpeckers, hummingbirds, pigeons, pheasants and other birds. “Our results also suggest that ION has been ‘lost’ at least two times independently, in the nightjars and in the clade that includes the pelican and seabirds,” the researchers said.