Ultra-Rapid Vision in Birds

Abstract

Flying animals need to accurately detect, identify and track fast-moving objects and these behavioral requirements are likely to strongly select for abilities to resolve visual detail in time. However, evidence of highly elevated temporal acuity relative to non-flying animals has so far been confined to insects while it has been missing in birds. With behavioral experiments on three wild passerine species, blue tits, collared and pied flycatchers, we demonstrate temporal acuities of vision far exceeding predictions based on the sizes and metabolic rates of these birds. This implies a history of strong natural selection on temporal resolution. These birds can resolve alternating light-dark cycles at up to 145 Hz (average: 129, 127 and 137, respectively), which is ca. 50 Hz over the highest frequency shown in any other vertebrate. 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.

Fig 1. Flicker fusion frequencies for blue tits by three different light stimuli intensities.

Averages and ranges are shown with filled circles and brackets, respectively. Twelve individual 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.

Fig 2. Flicker fusion frequencies for collared (closed diamonds) and pied flycatchers (open squares).

Averages are shown together with ranges (brackets). Seven collared and eight pied flycatchers were repeatedly tested at up to five different light intensities each: 160 (n = 5 collared + 4 pied), 350 (n = 7 + 4), 750, 1500 (n = 7 + 8) and 3500 cdm^-2 (n = 6 + 7). The critical flicker fusion frequency (CFF), with a maximum of 141 Hz and 128.1 ± 9.8 Hz (±SD) on average for the collared flycatchers and up to 146 Hz and 138.2 ± 6.5 Hz on average for the pied flycatchers, was attained in both species at 1500 cdm^-2.

Fig 3

The flight paths of two blue bottle flies (Calliphora vomitoria) sampled from high-speed video (S1 Movie): A) at the rate of the visual system of a human (40 frames/s) and B) 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, resulting in a much more detailed view of the flight paths of the flies.