A bird vision may not be as sharp as we thought. A new study has found that in small passerines – also known as perching birds – their vision is not actually that sharp, but instead extremely fast, trumping other vertebrates and twice as fast as human vision. Yes, the vision of small perching birds is twice as quick as humans and faster than any vertebrates, according to a new study. The findings shed light on the nature of visual speed in birds, suggests that this adaptation may be a common feature in most birds.
Researchers from the Swedish University of Agricultural Sciences, Uppsala University and Stockholm University in Sweden conducted the behavioral experiments to examine the ability of birds to resolve visual detail in time, an ability that is the temporal resolution of eyesight, or the number of changes per second that any animal can perceive. The team studied three small wild passerine species: blue tit, collared flycatcher and pied flycatcher.
They trained wild-caught birds to distinguish between two pairs of lamps, one flickering and one constantly shining, by rewarding them with food. Afterwards, they measured temporal resolution by increasing the flicker rate to a threshold at which the birds could no longer distinguish between the lamps- a level called the critical flicker fusion rate (CFF).
This threshold, known as CFF averaged between 129 and 137 hertz (Hz). In the pied flycatchers it reached as high as 146 Hz, some 50 Hz above the highest rate encountered for any other vertebrate, researchers said. For humans, the CFF is usually approximately 60 Hz. For passerines, the world might to be said to be in slow motion compared with how it looks to us, they said.
The scientists found that blue tits and flycatchers had higher CFF rates and faster vision than what was predicted by their size and metabolic rates, indicating an evolutionary history of natural selection for fast vision in these species.
Supporting this idea is the airborne birds’ need to detect and track objects that move very swiftly across their retina. For example, for blue tits, to be able to see and avoid all branches when they take cover from predators by flying straight into bushes. Likewise, flycatchers catch airborne insects, requiring fast vision and a high temporal resolution to track the insect’s movements and predict its location the next instant. This may be one of the selective pressures that led to the increase in visual speed, researchers said.
Studies have shown that flickering light can cause stress, behavioural disturbances and various forms of discomfort in humans and birds alike, researchers said.
“Fast vision may, in fact, be a more typical feature of birds in general than visual acuity. Only birds of prey seem to have the ability to see in extremely sharp focus, while human visual acuity outshines that of all other bird species studied,” said Anders Odeen from Uppsala University, who headed the study, adding “On the other hand, there are lots of bird species similar to the blue tit, collared flycatcher and pied flycatcher, both ecologically and physiologically, so they probably also share the faculty of superfast vision.”
The findings raise concerns about the welfare of small caged birds, especially those kept in areas with modern low-energy or flickering lighting light, which can cause stress, behavioural disturbances and various forms of discomfort in humans and birds alike. Yet it appears perching birds may have traded their ultra-sharp vision at the expense of sharpness.
While the record for the sharpest vision still rests with eagles, which can detect finer details than any other animal; perching birds can only see in low resolution.
The study was published in the March 18 issue of the journal PLOS ONE.