Researchers from the Swedish University of Agricultural Sciences, Uppsala University and Stockholm University in Sweden studied the ability to resolve visual detail in time in three small wild passerine species - blue tit, collared flycatcher and pied flycatcher.

The ability is the temporal resolution of eyesight, that is the number of changes per second an animal is capable of perceiving. It may be compared to spatial resolution (visual acuity), a measure of the number of details per degree in the field of vision, researchers said.

They trained wild-caught birds to receive a food reward by distinguishing between a pair of lamps, one flickering and one shining a constant light. Temporal resolution was then determined by increasing the flicker rate to a threshold at which the birds could no longer tell the lamps apart.

This threshold, known as the CFF (critical flicker fusion rate), 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 blue tits and flycatchers proved to have higher CFF rates than were predicted from their size and metabolic rates. This indicates an evolutionary history of natural selection for fast vision in these species.

The explanation might lie in small airborne birds' need to detect and track objects whose image moves very swiftly across the retina - for blue tits, for example, to be able to see and avoid all branches when they take cover from predators by flying straight into bushes, researchers cited.

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