Vision and Foraging in Cormorants: More like Herons than Hawks?

Centre for Ornithology, School of Biosciences, University of Birmingham, Birmingham, United Kingdom.
PLoS ONE (Impact Factor: 3.23). 02/2007; 2(7):e639. DOI: 10.1371/journal.pone.0000639
Source: PubMed

ABSTRACT 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.
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).
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.

Download full-text


Available from: Graham R Martin, Sep 27, 2015
1 Follower
30 Reads
  • Source
    • ") and both terrestrial (Jetz et al. 2003) and marine birds (Wilson et al. 1993; White et al. 2007; Zimmer et al. 2008). In our study, murres made fewer, but longer and deeper, dives when light intensity was higher. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Studies of seabird–prey interactions often focus on biotic factors, such as prey abundance, seabird biomechanics and competition. In contrast, we examined the influence of abiotic factors, particularly weather, light and tide, on the diving behaviour of thick-billed murre (Uria lomvia) foraging in the Canadian Low Arctic. We found little evidence that tide and weather influenced dive behaviour. As visual predators, light availability limits foraging opportunities; however, prey often surface at night so there may be a trade-off between increased food availability and reduced foraging ability during low-light conditions. Our data lent support to both ideas, as dive depth increased with light availability and the proportion of vertically migrating schooling prey was highest during sunup and sundown. There was no difference in dive depth between sexes outside the period of sundown; males, which forage at night, dove shallower than females in the late afternoon, which we suggest is because they specialize on shallow prey often caught at night. Apparently, adaptation for higher oxygen stores or lower oxygen consumption in deeper-diving females overrode any adaptation for improved vision in night-specialist males. We concluded that light availability interacted with prey vertical migration to impact underwater foraging abilities of breath-hold divers.
    Marine Biology 08/2015; DOI:10.1007/s00227-015-2701-1 · 2.39 Impact Factor
  • Source
    • "Such prey would not seem to set a particularly exacting perceptual challenge no matter how poor the visual acuity of the birds might be underwater . For example, the binocular fields of Great Cormorants are similar to those of Guillemots and their acuity under water (White et al. 2007) has been shown to be 60 times lower than the acuity of aerial predatory birds (Reymond 1985), but this acuity and visual field configuration is clearly sufficient to allow evasive prey of various sizes to be taken by Great Cormorants, even in turbid waters (White et al. 2007). Therefore, on the basis of foraging tasks carried out during the breeding season even the reduced binocularity that occurs underwater can be seen to be sufficient for the foraging tasks at that time. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Significant differences in avian visual fields are found between closely related species that differ in their foraging technique. We report marked differences in the visual fields of two auk species. In air, Common Guillemots Uria aalge have relatively narrow binocular fields typical of those found in non-passerine predatory birds. Atlantic Puffins Fratercula arctica have much broader binocular fields similar to those that have hitherto been recorded in passerines and in a penguin. In water, visual fields narrow considerably and binocularity in the direction of the bill is probably abolished in both auk species. Although perceptual challenges associated with foraging are similar in both species during the breeding season when they are piscivorous, Puffins (but not Guillemots) face more exacting perceptual challenges when foraging at other times when they take a high proportion of small invertebrate prey. Capturing this prey probably requires more accurate, visually-guided bill-placement and we argue that this is met by the Puffin's broader binocular field, which is retained upon immersion; its upward orientation may enable prey to be seen in silhouette. These visual field configurations have potentially important consequences that render these birds vulnerable to collision with human artefacts underwater, but not in air. They also have consequences for vigilance behaviour.This article is protected by copyright. All rights reserved.
    Ibis 07/2015; DOI:10.1111/ibi.12297 · 1.92 Impact Factor
  • Source
    • "They were housed communally in a 130 m 2 outdoor aviary at the Edgbaston campus at Birmingham University. Details of bird care are given in White et al. (2007). The cormorants had been previously trained to swim through an underwater channel 2.8 m long fashioned from stainless steel grating and sunk in a 1 m deep, 8 × 4 m tank (White et al. 2007) that was continuously replenished with freshwater. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Externally attached remote-sensing devices used to study animals in their environment are a possible source of disturbance, notably in terms of drag, for diving species. The aim of the present study was to assess the possible effect of device-induced drag on the diving performance of great cormorants Phalacrocorax carbo. Based on wind-tunnel measurements, we assessed the effect of device size on drag and derived a formula to predict how drag changes as a function of both swim speed and device cross-sectional area. Tests on captive cormorants indicated that drag had an effect on the energy expenditure (using dynamic body acceleration as a proxy) during the dive. Wind tunnel-derived drag metrics were combined with data from the literature to construct a model predicting the power consumption of diving cormorants according to device size. Applying the model to dive data from 6 free-living great cormorants (recorded using implanted time-depth recorders) indicated that a device constituting only ca. 3% of the bird’s cross-sectional area could cause a 1.7% increase in power consumption when swimming. However, if a bird maintains constant power underwater, e.g. by decreasing foraging speed with increasing drag, this would result in a 7.1% reduction in the distance travelled during the bottom (active hunting) phase of the dive. Device-related increases in drag are also likely to reduce the maximum speeds achievable by these pursuit predators. The present study highlights the interaction between both drag coefficient and swim speed for diving animals with externally attached devices.
    Marine Ecology Progress Series 01/2015; 519:239-249. DOI:10.3354/meps11058 · 2.62 Impact Factor
Show more