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Vision and foraging in cormorants: more like herons than hawks?

White CR, Day N, Butler PJ, Martin GR - PLoS ONE (2007)

Bottom Line: 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.We conclude that cormorants are not the aquatic equivalent of hawks.This technique appears to be driven proximately by the cormorant's limited visual capacities, and is analogous to the foraging techniques employed by herons.

View Article: PubMed Central - PubMed

Affiliation: Centre for Ornithology, School of Biosciences, University of Birmingham, Birmingham, United Kingdom.

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.

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Related in: MedlinePlus

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.
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pone-0000639-g002: 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.

Mentions: A total of 9673 discrimination trials were scored, and these were preceded by and interspersed with 11853 training trials which maintained 100% correct discrimination performance for high contrast, low spatial frequency stimuli. The number of trials per session varied significantly between birds (ANOVA, F4,252 = 30.4, p<0.0001) and ranged from 15.5±0.1 (SEM) to 26.3±0.2. Visual acuity was significantly effected by target illumination (Fig. 1, F5,18 = 39.0, p<0.0001), target contrast (Fig. 2, F4,15 = 10.2, p = 0.0003), and viewing distance (Fig. 3, F1,8 = 16.6, p = 0.003). Visual acuity was positively related to target illumination and contrast, and negatively related to viewing distances (Figs 1–3).


Vision and foraging in cormorants: more like herons than hawks?

White CR, Day N, Butler PJ, Martin GR - PLoS ONE (2007)

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.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC1919429&req=5

pone-0000639-g002: 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.
Mentions: A total of 9673 discrimination trials were scored, and these were preceded by and interspersed with 11853 training trials which maintained 100% correct discrimination performance for high contrast, low spatial frequency stimuli. The number of trials per session varied significantly between birds (ANOVA, F4,252 = 30.4, p<0.0001) and ranged from 15.5±0.1 (SEM) to 26.3±0.2. Visual acuity was significantly effected by target illumination (Fig. 1, F5,18 = 39.0, p<0.0001), target contrast (Fig. 2, F4,15 = 10.2, p = 0.0003), and viewing distance (Fig. 3, F1,8 = 16.6, p = 0.003). Visual acuity was positively related to target illumination and contrast, and negatively related to viewing distances (Figs 1–3).

Bottom Line: 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.We conclude that cormorants are not the aquatic equivalent of hawks.This technique appears to be driven proximately by the cormorant's limited visual capacities, and is analogous to the foraging techniques employed by herons.

View Article: PubMed Central - PubMed

Affiliation: Centre for Ornithology, School of Biosciences, University of Birmingham, Birmingham, United Kingdom.

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.

Show MeSH
Related in: MedlinePlus