Why is it so hard to sneak up on a bunch of sparrows on the sidewalk? According to a new PLOS ONE study, small birds boast ultra-rapid vision that’s faster than that of any other vertebrate – and more than twice as fast as ours.
The number of changes per second that an animal is capable of perceiving is called the temporal resolution of eyesight. Birds and other flying animals must accurately detect, identify, and track fast-moving objects (like tasty insects) all while maneuvering around branches and watching out for predators. For small, agile birds active in the daytime, high temporal resolution is a must, yet this hasn’t really been investigated.
Uppsala University’s Anders Ödeen and colleagues studied the ability of three perching birds to resolve visual detail in time: blue tits (Cyanistes caeruleus) from southeastern Sweden, and both collared flycatchers (Ficedula albicollis) and pied flycatchers (Ficedula hypoleuca) from the island of Öland in the Baltic Sea. Blue tits are insectivorous during breeding season, while flycatchers catch prey on the wing throughout the year.
Using food rewards, the team trained the birds to distinguish between a pair of LED-arrays: one that flickers and another that shines constantly. To determine the birds’ temporal resolution, the team gradually increased the flicker rate to the point at which the birds couldn’t tell the different lamps apart. This threshold is called the critical flicker fusion frequency (or CFF).
Flicker fusion frequencies for blue tits (left) and for collared flycatchers (right, closed diamonds) and pied flycatchers (right, open squares). J.E. Boström et al., PLOS ONE 2016
The flickering and constant lamps became indistinguishable at frequencies of up to 131 hertz (Hz) for blue tits and 141 Hz for collared flycatchers. The CFF of pied flycatchers reached as high as 146 Hz – or about 50 Hz above the highest rate for any other vertebrate. (Our CFF is about 60 Hz.)
Furthermore, their temporal acuity of vision is much higher than researchers predicted based on their size and metabolic rate – which suggests an evolutionary history of strong selection for temporal resolution. After all, at some point during the year, all three of these species live on the insects they catch at high speeds in dense forests.
However, temporal resolution is different than spatial resolution, which measures the number of details per degree in the field of vision. There are trade-offs between different aspects of vision, and the vision of these small birds isn’t as sharp as that of predatory birds. “Fast vision may, in fact, be a more typical feature of birds in general than visual acuity,” Ödeen says in a statement. “Only birds of prey seem to have the ability to see in extremely sharp focus.” Eagles have the sharpest vision known, and this excellent spatial resolution allows them to spot prey over long distances.
“Fast-eyed” species like tits and flycatchers have compromised spatial acuity, but their vision is as fast as eagle eyes are sharp.