Small birds have incredibly fast vision, faster than any other vertebrates and over twice the speed than that of humans, a team of Swedish scientists has found. Perching birds (small passerines) do not only have good visual acuity (excellent eyesight), they also see things at lightning speed. We live in a slow-motion world in comparison.
Scientists from Uppsala University, the Swedish University of Agricultural Sciences and Stockholm University, wrote in the academic journal PLOS ONE (citation below) that pied flycatchers, blue tits and collared flycatchers have incredibly fast vision.
In behavioural experiments, Professor Anders Ödeen, a lecturer at Uppsala University’s Department of Ecology and Genetics, and colleagues studied these perching birds’ ability to resolve visual detail against the clock.
Averages and ranges are shown with filled circles and brackets, respectively. Twelve blue tits were tested once at one of the light intensities 750, 1500 (n = 3) and 3000 cdm-2 (n = 6). The critical flicker fusion frequency (CFF), with a maximum of 131 Hz and 130.3 ± 0.94 Hz (±SD) on average, was reached at 1500 cdm-2. (Image: journals.plos.org)
Visual acuity vs. speed of vision
This ability is referred to as the temporal resolution of eyesight – the number of changes per second a bird, human or any other animals is capable of perceiving. It may be compared to visual acuity (spatial resolution), which measures how many details per degree are detected in the field of vision.
The wild-caught birds were taught to receive a food reward whenever they were able to distinguish between a pair of lamps: one shining in a constant light and the other flickering.
Temporal resolution was determined by raising the flicker rate to a point at which the birds could no longer tell the two lamps apart.
Critical Flicker Fusion rate
This threshold is called the Critical Flicker Fusion (CFF) rate, which in the case of the three small bird species averaged between 129 and 137 Hz (hertz). A 146 Hz was recorded with one of the pied flycatchers, which is about 50 Hz higher than anything encountered for any other vertebrate.
Averages are shown together with ranges (brackets). 7 collared and 8 pied flycatchers were repeatedly tested at up to 5 different light intensities each. (Image: journals.plos.org)
Humans’ CFF averages at about 60 Hz. The authors say that for passerines, we live in a world where everything moves slowly.
Scientists have long thought that small and agile wild birds had extremely fast vision – however, this was never investigated, until now.
The researchers were surprised to find that flycatchers and blue tits had faster CFF rates than they would have predicted from their size and metabolic rates.
This suggests that an evolutionary history of natural selection for fast vision occurred in these species. Small airborne birds have needed to detect and track tiny objects whose images move ultra-fast across the retina.
Rapid vision needed to hunt and avoid predators
For example, blue tits need to be able to see things quickly if they want to avoid crashing into branches and twigs when taking cover from predators by darting straight into bushes.
These three bird species subsist to varying degrees on tiny insects that zig-zag and change direction rapidly in the air. Flycatchers, as their name suggests, are expert airborne insect hunters.
The flight paths of two blue bottle flies sampled from high-speed video at the rate of the visual system of a human (40 frames/s) and at the rate of a pied flycatcher (120 frames/s) at a light intensity of approximately 500 cdm-2. The flycatcher refreshes visual input almost three times faster than a human, resulting in a much more detailed view of the flies’ flight paths. (Image: journals.plos.org)
When a flycatcher hunts insects it does not simply fly straight at its prey. Some forward planning is required – the bird needs high temporal resolution to see every tiny movement of its target insect, as well as predicting its next move and location. In other words, it flies straight at where it thinks the prey will be next.
Imagine how much easier it would be for us to grab a fly in mid flight if it flew at one third of its speed – that is what the world is like for the pied flycatcher.
Captive birds and modern indoor lighting
The authors commented that captive birds’ welfare may be affected by the phasing-out of incandescent light bulbs and the introduction of low-energy bulbs, LED lighting and floursercent lamps.
Many of these flicker at 100 Hz, which is fine for humans but a dazzling nuisance for small perching captive birds. Previous studies have demonstrated that flickering causes stress, behavioural disturbances and several forms of discomfort to both humans and birds.
An Australasian Figbird catching an airborne beetle. The authors believe that most birds have ultra-fast vision, especially those that subsist on catching their prey on the wing. (Image: Wikipedia)
The eagle has the sharpest vision in the animal kingdom – it can discern 143 lines within one degree of field vision, compared to humans’ (with 20×20 vision) 60 lines.
The difference between a human and an eagle regarding visual acuity is about the same as a human’s versus the pied flycatcher’s vision speeds – 60 and 146 Hz respectively. In other words, the flycatcher’s vision is faster than ours roughly to the same extent as an eagle’s vision is sharper (than ours).
Small passerines’ rapid vision is an evolutionary adaptation just as impressive as the visual acuity of birds of prey.
Eagles, hawks and other birds of prey have the best visual acuity in the animal kingdom – they can see the most detail per square inch. Small perching birds have the fastest vision – they can see the most movement per second. (Image: Wikipedia)
Most birds likely have ultra-fast vision
Prof. Ödeen puts the research findings in perspective:
“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.”
“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.”
In an Abstract in the journal, the authors wrote:
” We argue that rapid vision should confer a selective advantage in many bird species that are ecologically similar to the three species examined in our study. Thus, rapid vision may be a more typical avian trait than the famously sharp vision found in birds of prey.”
Citation: “Ultra-Rapid Vision in Birds,” Anders Ödeen, Jannika E. Boström, Olle Håstad, Marina Dimitrova, Cindy Canton & Anna Qvarnström. PLOS ONE. 18 March 2016. DOI: 10.1371/journal.pone.015109.
Video – How humans and small birds see movement
This video shows how much faster a small bird’s vision is compared to ours. It is much easier for them to follow and anticipate the movements of two flies, with nearly three times the update rate than that of a human. This ability is crucial for the survival of a bird that survives on catching darting insects.