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, 2 (7), e639

Vision and Foraging in Cormorants: More Like Herons Than Hawks?

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Vision and Foraging in Cormorants: More Like Herons Than Hawks?

Craig R White et al. PLoS One.

Abstract

Background: Great cormorants (Phalacrocorax carbo L.) show the highest known foraging yield for a marine predator and they are often perceived to be in conflict with human economic interests. They are generally regarded as visually-guided, pursuit-dive foragers, so it would be expected that cormorants have excellent vision much like aerial predators, such as hawks which detect and pursue prey from a distance. Indeed cormorant eyes appear to show some specific adaptations to the amphibious life style. They are reported to have a highly pliable lens and powerful intraocular muscles which are thought to accommodate for the loss of corneal refractive power that accompanies immersion and ensures a well focussed image on the retina. However, nothing is known of the visual performance of these birds and how this might influence their prey capture technique.

Methodology/principal findings: We measured the aquatic visual acuity of great cormorants under a range of viewing conditions (illuminance, target contrast, viewing distance) and found it to be unexpectedly poor. Cormorant visual acuity under a range of viewing conditions is in fact comparable to unaided humans under water, and very inferior to that of aerial predators. We present a prey detectability model based upon the known acuity of cormorants at different illuminances, target contrasts and viewing distances. This shows that cormorants are able to detect individual prey only at close range (less than 1 m).

Conclusions/significance: We conclude that cormorants are not the aquatic equivalent of hawks. Their efficient hunting involves the use of specialised foraging techniques which employ brief short-distance pursuit and/or rapid neck extension to capture prey that is visually detected or flushed only at short range. This technique appears to be driven proximately by the cormorant's limited visual capacities, and is analogous to the foraging techniques employed by herons.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Effect of ambient illumination (lux) on the visual acuity of five great cormorants Phalacrocorax carbo.
Visual acuity is expressed as the reciprocal of minutes of arc. The relationship is significant: log(acuity) = −0.00168 log(illumination)2 + 0.0125 log(illumination) + 0.0889. Symbols represent individual birds: ▴, ▵, ⧫, ◊, □. Mean values±SEM: 0.034±0.006, 0.055±0.004, 0.063±0.005, 0.064±0.007, 0.077±0.006, 0.087±0.006 for illuminations of 0.0012, 0.0058, 0.011, 0.028, 0.11, and 1.4 lux, respectively. ○ = mean data±SEM for five great cormorants determined by Strod et al ; • = mean aquatic visual acuity threshold for unaided humans . The range of mean illumination encountered during the bottom phase of dives is shown for European shags Phalacrocorax aristotelis and blue-eyed shags Phalacrocorax atriceps , as are the illumination levels equivalent to those received at the earth's surface from natural sources between full daylight and an overcast night.
Figure 2
Figure 2. Effect of contrast on visual acuity of five great cormorants.
The relationship is significant: log(acuity) = −1.36+0.38 (contrast). Symbols represent individual birds: ▴, ▵, ⧫, ◊, □. Mean values±SEM: 0.054±0.005, 0.071±0.004, 0.096±0.009, 0.087±0.006, 0.095±0.007 (minutes of arc)−1 for contrast of 27, 54, 72, 82, and 93%, respectively.
Figure 3
Figure 3. Effect of viewing distance on visual acuity of five great cormorants.
The relationship is significant: log(acuity) = −0.751–0.151 (viewing distance). Symbols represent individual birds: ▴, ▵, ⧫, ◊, □. Mean values±SEM: 0.15±0.03, 0.12±0.03, and 0.087±0.006 (minutes of arc)−1 for viewing distances of 0.62, 1.05, and 2.12 m, respectively
Figure 4
Figure 4. Visual acuity surfaces of great cormorants describing the effects of contrast, illumination and viewing distance.
Three surfaces are presented, corresponding with viewing distances of 2.12 m (upper surface), 1.05 m (middle surface) and 0.63 m (lower surface). Visual acuity is expressed as the minimum width of a detectable object (mm).
Figure 5
Figure 5. Prey detectability model for a great cormorant based upon the data of Fig. 4 demonstrating the effects of contrast and viewing distance.
The model is based upon a great cormorant foraging on a capelin (Mallotus villosus, 10 cm TL) type fish at an ambient illumination of 10 lux, which has a contrast of 90, 60 and 30% viewed from a distance of 0.63, 1.05 or 2.12 m. Each frame depicts a scene with an angular width of 10°. Scenes were generated by determining the angular resolution appropriate to each set of conditions from Fig. 4, and appropriately downsampling the high resolution images in the upper row.
Figure 6
Figure 6. Prey detectability model for a great cormorant demonstrating the effect of illumination.
The model is based upon a great cormorant foraging on a capelin (Mallotus villosus, 10 cm TL) of 60% contrast viewed at a distance of 1.05 m over a range of ambient illumination levels equivalent to those received at the earth's surface from natural sources between daylight to a moonless night. These span the range of target light levels used in this series of experiments and span the range of ambient light levels that are known to be encountered by cormorants during natural dives (ca 0.5 to 100 lux). Each frame depicts a scene with an angular width of 10°.
Figure 7
Figure 7. Stylised scale plan view of the tank (8 m×4 m) and swimway apparatus showing the typical paths followed by a cormorant during a discrimination trial (Top) and photograph of the tank and swimway apparatus (bottom).
Birds began a trial in the starting area (A). When the gate (B) was raised by the experimenter, signalling the start of a trial, the birds entered the swimway and performed the simultaneous discrimination at a point (C) a known distance from the stimuli (D and E). If the birds approached the correct stimulus they were provided with a fish reward (a single sprat). If they approached the incorrect stimulus, they received no reward. At the end of each trial, the birds exited the swimway at F or G, and returned to the starting position (A) to await the start of a new trial. Note that the stimuli are submerged beneath D and E in the photograph. Inset shows a single frame captured from a video of a bird swimming though the swimway, just prior to the choice point (C). See Movie S1 for the full video sequence.

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