While birds are known for their great vision, 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. The findings shed light on the nature of visual speed in birds and suggests that this adaptation may be a common feature in most birds.
The new study relied on 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. This ability can be compared to visual acuity, which is the number of details per degree in the field of vision.
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 level where the birds could no longer distinguish between the lamps- a level called the critical flicker fusion rate (CFF).
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, suggesting that this ability may have been naturally selected for by processes unrelated to size.
Supporting this idea is the necessity for airborne birds to detect and track objects that move with a high degree of speed across their retina. For example, flycatchers catch airborne insects, requiring fast vision and a high degree of forward planning to predict the insect's movements as each moment passes. This may be one of the selective pressures that led to the increase in visual speed.
"Fast vision may, in fact, be a more typical feature of birds in general than visual acuity," said Anders Ödeen, who headed the study. "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. 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 were published in the March 18 issue of the journal PLOS One.