Vision and Foraging in Cormorants: More like Herons
than Hawks?
Craig R. White, Norman Day, Patrick J. Butler, Graham R. Martin
*
Centre for Ornithology, School of Biosciences, The University of Birmingham, Birmingham, United Kingdom
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.
Citation: White CR, Day N, Butler PJ, Martin GR (2007) Vision and Foraging in Cormorants: More like Herons than Hawks?. PLoS ONE 2(7): e639.
doi:10.1371/journal.pone.0000639
INTRODUCTION
Pursuit–dive foraging (taking prey from the water column or from
substrata at depth) is widespread among birds (c.150 species from
seven Orders). Although key aspects of the diet and foraging ecology
of many of these species are known, little information is available
regarding how these birds actually detect prey and what factors
constrain their diving behaviour. Amphibious behaviour presents
major sensory problems to birds, because of the markedly different
properties of air and water. The optical requirements for aquatic
vision are fundamentally different from those in air, because
underwater light environments differ from aerial environments in
spectral composition, luminance and turbidity [1,2]. Furthermore,
upon entering water, eyes of terrestrial vertebrates experience the
loss of corneal refractive power and to retain a sharp retinal image
this loss must be compensated for by changes in the lens [3]. This loss
of corneal refractive power also results in the reduction in the sizes of
visual fields, alteration of visual field topography and reduction in the
brightness of the retinal image [4,5].
Great cormorants (
Phalacrocorax carbo
: Phalacrocoracidae) are
generally regarded as visually-guided, pursuit-dive foragers, which
have the highest known foraging yield for a marine predator [6]
and very seldom injure fish without catching them [7]. It may
therefore be predicted that cormorants have excellent vision much
like aerial predators, such as hawks which detect and pursue prey
from a distance. Great Cormorants are widely distributed with
resident populations from temperate latitudes in the southern
hemisphere (e.g. New Zealand; 45
u
S) through the tropics to as far
north as Greenland (70
u
N) in the northern hemisphere [8,9].
Throughout this range they are often perceived as being in conflict
with human fisheries interests [10]. They exploit fish resources in
coastal waters, freshwater lakes and rivers. Cormorants exhibit
a range of solitary and social foraging behaviours and group
foraging appears to be particularly effective in highly turbid waters
[25]. Individuals are known to dive, presumably in pursuit of prey,
at night in the middle of winter at high latitudes [11]. They are
known to forage on both pelagic and benthic fish species [7,10].
Cormorant populations in Greenland and Iceland are known to
forage mainly on sculpins (
Myoxocephalus
) [6,12], which are a group
of cryptically coloured benthic fish with a disruptive outline
pattern that may have evolved in response to avian predation
pressure [13]. Given their ability to prey upon pelagic and cryptic
benthic prey, and a high capacity to accommodate their eye’s
optical system to compensate for the loss of corneal refractive
power upon immersion [3,14–16], it is reasonable to expect that
cormorants have a visual system well adapted to function in water
and that, as in aerial predatory birds, vision is the primary sense
that guides their foraging. Indeed, cormorant eyes appear to show
some specific adaptations to the amphibious life style. Thus, they
were reported to have a highly pliable lens whose curvature is
driven by powerful intraocular muscles [14–16] and this is thought
to accommodate for the loss of corneal refractive power that
accompanies immersion and ensures a well focussed image on the
retina [3].
Academic Editor:
Peter Bennett, Zoological Society of London, United Kingdom
Received
March 28, 2007;
Accepted
June 11, 2007;
Published
July 25, 2007
Copyright:
ß
2007 White et al. This is an open-access article distributed under
the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the
original author and source are credited.
Funding:
This research was supported by the UK Natural Environment Research
Council: Grant NER/A/2003/00542.
Competing Interests:
The authors have declared that no competing interests
exist.
* To whom correspondence should be addressed.
E-mail: g.r.martin@bham.ac.
uk
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July 2007 | Issue 7 | e639
However, demonstration of an anatomical capacity to accom-
modate visually for the changes occurring upon immersion
provides little indication of the visual information available to
a cormorant when foraging underwater. To develop an improved
picture of what a diving cormorant sees when foraging, we used
established psychophysical visual discrimination training tech-
niques to determine the upper limits of cormorant visual acuity
under water. We determined the visual acuity thresholds of free-
swimming cormorants under a range of viewing conditions that
mimicked those experienced in clear waters at different naturally
occurring light levels when viewing targets of different contrasts
and at different viewing distances. We then used these data to
model the appearance of a typical target fish viewed by
cormorants at a range of distances, target contrasts, and
illuminations representative of those encountered by naturally
foraging birds.
RESULTS
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, F
4,252
= 30.4,
p
,
0.0001) and ranged from 15.5
6
0.1 (SEM) to 26.3
6
0.2. Visual
acuity was significantly effected by target illumination (Fig. 1,
F
5,18
= 39.0, p
,
0.0001), target contrast (Fig. 2, F
4,15
= 10.2,
p = 0.0003), and viewing distance (Fig. 3, F
1,8
= 16.6, p = 0.003).
Visual acuity was positively related to target illumination and
contrast, and negatively related to viewing distances (Figs 1–3).
DISCUSSION
Our overall conclusion is that the ability of cormorants to resolve
visual detail in water is poor, and far below that predicted by
analogy with the vision of predatory birds that take prey in aerial
pursuit. Thus, the mean visual acuity of great cormorants for
targets with high (82%) contrast at an illumination equivalent to
that of twilight (1.4 lux) equalled 11.8
6
0.8 [SEM] minutes of arc.
Acuity improved at higher (day-time) levels of illumination [17],
but the difference was slight, and acuity was low at the levels of
illumination (ca 0.5 to 100 lux) that cormorants are known to
encounter during natural dives [18]. To provide a perspective on
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) =
2
0.00168 log(illumination)
2
+
0.0125 log(illumination)
+
0.0889. Symbols represent individual birds:
m
,
n
,
X
,
e
,
%
. Mean
values
6
SEM: 0.034
6
0.006, 0.055
6
0.004, 0.063
6
0.005, 0.064
6
0.007,
0.077
6
0.006, 0.087
6
0.006 for illuminations of 0.0012, 0.0058, 0.011,
0.028, 0.11, and 1.4 lux, respectively.
#
= mean data
6
SEM for five great
cormorants determined by Strod et al [17];
N
= mean aquatic visual
acuity threshold for unaided humans [19]. 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
[18], as are the illumination levels equivalent to
those received at the earth’s surface from natural sources between full
daylight and an overcast night.
doi:10.1371/journal.pone.0000639.g001
Figure 2. Effect of contrast on visual acuity of five great cormorants.
The relationship is significant: log(acuity) =
2
1.36
+
0.38 (contrast).
Symbols represent individual birds:
m
,
n
,
X
,
e
,
%
. Mean values
6
SEM:
0.054
6
0.005, 0.071
6
0.004, 0.096
6
0.009, 0.087
6
0.006, 0.095
6
0.007
(minutes of arc)
2
1
for contrast of 27, 54, 72, 82, and 93%, respectively.
doi:10.1371/journal.pone.0000639.g002
Figure 3. Effect of viewing distance on visual acuity of five great
cormorants.
The relationship is significant: log(acuity) =
2
0.751–0.151
(viewing distance). Symbols represent individual birds:
m
,
n
,
X
,
e
,
%
.
Mean values
6
SEM: 0.15
6
0.03, 0.12
6
0.03, and 0.087
6
0.006 (minutes of
arc)
2
1
for viewing distances of 0.62, 1.05, and 2.12 m, respectively
doi:10.1371/journal.pone.0000639.g003
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this relatively poor visual acuity in cormorants it should be noted
that the cormorants’ highest visual performance is only equal to
that of unaided humans in water [19], and approximately 60 times
lower than that of visually-guided terrestrial avian predators, such
as eagles, whose acuity threshold lies between 0.2–0.8 min of arc
[20–23]. This is a surprising result for a predator that exhibits high
foraging efficiency [6] and is assumed to be visually guided [8].
A model of prey detectability
To explore the consequences of the cormorants’ poor visual
resolution we have used our acuity data to model prey detectability
under a range of viewing conditions. We used curves fitted to our
acuity-illumination (Fig. 1), acuity-contrast (Fig. 2), and acuity-
viewing distance (Fig. 3) functions to describe a series of ‘‘threshold
acuity surfaces’’ which relate acuity to target contrast and
illumination, for each viewing distance (Fig. 4). This encapsulates
within a single figure the ways in which acuity is influenced by
a range of important parameters that describe the visual tasks
encountered by foraging cormorants. From this we have been able to
model visual prey detectability in cormorants under a range of
viewing conditions. Figs 5 and 6 show two examples from this
modelling using a prey item of a size (10 cm total length) commonly
taken by cormorants [24,25]. Even for a prey item of this size
detectability is low at all but the highest target contrasts, light levels
and short viewing distances. This raises a number of important
questions concerning the foraging techniques of cormorants and the
predator-prey interactions which underlie them.
We have modelled prey detectability in cormorants using
a relatively high contrast prey item stimulus based upon a pelagic
fish which would be taken from a water column with low turbidity.
This presents probably the simplest foraging situation for
a cormorant and therefore encapsulates what is likely to be the
maximum visual performance when foraging. Thus, acuity will
decline further with increasing turbidity [17], and high-contrast
pelagic prey are not typical items for cormorants. Potential prey
animals in benthic nearshore habitats have evolved to evade
detection through the use of both masquerade (i.e. resembling an
object that is not normally eaten) and eucrypsis (i.e. resembling the
background) strategies [26]. In the euphotic pelagic zone, prey
species have evolved transparency or reflectivity, with the latter
often accompanied by countershading [26]. The actual visual prey
detectability in cormorants is therefore likely to be far lower than
the upper limits modelled in Figs. 5 and 6 based upon simple
contrast parameters. The modelled prey detectability strongly
suggest that the foraging strategies of cormorants are likely to be
constrained by their poor aquatic visual acuity. We propose that
foraging cormorants must adopt a range of behavioural strategies
to overcome the limits of their vision.
Foraging strategies of cormorants
Under certain conditions cormorants are known to forage co-
operatively. Thus, in turbid conditions, where acuity will be
further reduced compared with the acuity thresholds reported here
[17], cormorants may use mass fishing techniques to drive fish to
relatively clear surface waters where they are more likely to be
detected when seen from below in silhouette against the down-
welling light [27]. However, cormorants more typically forage
alone, often in turbid conditions at depths greater than 10m where
light penetration is low, and sometimes at night [11], and it has
been suggested that cormorants might locate prey by touch using
the bill [28]. Tactile detection is thought to be successful only
when prey density is sufficiently high, when fish are relatively
immobile (as in the case in hibernating aggregations), or both [28].
We propose that these kinds of specialised behavioural strategies
play an important role in all cormorant foraging.
We propose that these observations on foraging behaviour,
together with our threshold acuity data (Fig. 4), suggest that
cormorants do not, and cannot, detect and pursue prey un-
derwater in a way that is analogous to that of predatory birds, such
as hawks, in air. Indeed, images from bird-borne cameras on the
congeneric European shag
Phalacrocorax aristotelis
show that
foraging typically occurs on the seabed rather than in the water
column [29], and high underwater swimming speeds indicative of
prey pursuits are very rare in great cormorants [30]. Cormorants
must either detect prey visually but only at very short distances, or
use a prey-flushing strategy [31] that forces prey to make an escape
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).
doi:10.1371/journal.pone.0000639.g004
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response. In either case it would seem inappropriate to describe
cormorants as pursuit foragers.
Cormorant foraging: more like herons than hawks?
The cormorants’ ability to strike rapidly at near prey employing
rapid extension of their long necks whilst virtually anchored by their
body mass and large webbed feet, might be a way to capture food
without an energetically expensive pursuit [30,32], This technique
may be key to this species’ ability to forage efficiently in a wide range
of aquatic environments and on different types of prey whose
combination would appear to pose a wide range of perceptual
challenges. Thus we propose that the foraging success of great
cormorants does not lay in particular adaptations of its vision to
resolve fine spatial detail within different aquatic environments, but
in the evolution of foraging techniques that operate within the
constraints of its vision. These foraging techniques, are analogous to
those employed by herons (Ardeidae) that use single-strike lunging to
take evasive prey [33]. We conclude that although cormorants are
highly efficient predators their aquatic foraging technique is more
like that of a lunging heron than an aerial pursuing hawk
MATERIALS AND METHODS
Five great cormorants (
Phalacrocorax carbo
) were trained using positive
reinforcement operant conditioning to conduct a simultaneous visual
discrimination [17,34] between pairs of horizontal and vertically
orientated gratings which were presented at the end of a stainless
steel swimway in a random sequence (Fig. 7). The gratings were
printed on acetate sheets and trans-illuminated by light from
a tungsten source. The level of trans-illumination was controlled by
neutral density filters and measured in situ at the gratings. The level
of grating contrast was controlled by the density of printing and
measured in situ with an Ocean Optics 80X Optometer. Stimulus
contrast was defined as (I
max
2
I
min
)/(I
max
+
I
min
)
6
100%. The whole
swimway and stimulus presentation apparatus was submerged in
a 1 m deep 8
6
4 m tank filled with continuously replenished
freshwater, which was housed in a light proof building. This ensured
high water clarity throughout the experiments. Turbidity was
monitored periodically with a portable Hach 2100P turbidimeter,
and remained below 1 NTU (nephlometric turbidity unit). The
building was illuminated by banks of fluorescent lights. Ambient
illumination was controlled by the number of these lights that were
illuminated, and was defined by the down welling illumination
received at the stimuli under different conditions.
At the start of a daily training or testing session each bird entered
the building from an adjacent aviary. After entering the water each
bird proceeded through a number of discrimination trials with each
trial signalled by the opening of a guillotine gate that controlled
access to the swimway (Fig. 7). When the gate was opened the bird
travelled along the swimway and performed the discrimination at
a known viewing distance from the gratings. Viewing distance was
established by vertically dividing the runway a known distance from
the stimuli, such that the bird chose to travel to either the left or right.
If the birds approached the horizontal stimuli (a ‘correct’ choice) they
were provided with a fish reward (a single sprat,
Sprattus sprattus
, ca
12 g). No reward was provided if the birds approached the vertical
stimuli (an ‘incorrect’ choice). Upon receiving the fish or making an
incorrect choice, the birds returned to the starting position. The
sequence was then was repeated until the birds were satiated.
The total number of correct trials, as well as the total number of
trials performed, was scored for each bird for each session. Minimum
separable acuity (i.e. the narrowest stripe width at which the birds
could distinguished horizontal and vertical stripes reliably) was
calculated as the interpolated 75% correct performance level. Trials
were conducted at six levels of ambient illumination (1.4, 0.11, 0.028,
0.011, 0.0058, 0.0012 lux; contrast = 82%, viewing distance =
2.12 m), five levels of contrast (93%, 82%, 72%, 54%, 27%; ambient
illumination = 1.4 lux, viewing distance = 2.12 m), and three viewing
distances (0.63, 1.05, 2.12 m, contrast = 86%, ambient illumina-
tion = 1.4 lux). Stripe width was 0.5 to 2.5 mm in 0.5 mm
increments at a viewing distance of 0.63 m; 1, 2, 3, 4 and 6 mm
at a viewing distance of 1.05 m; and 4 to 20 mm in 2 mm
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
u
.
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.
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increments at a viewing distance of 2.12 m. Stripe widths were
randomly ordered between successive trial days.
Data were analysed using a repeated measures ANOVA with
a single fixed factor: treatment (i.e. illumination, contrast, or viewing
distance), and a random factor: Bird ID.
a
was set at 0.05 for all tests.
All regulated procedures were performed by British Home
Office licensed personnel in possession of a Personal License, and
working under the auspices of a corresponding Project License, as
set out in the Animals (Scientific Procedures) Act 1986.
SUPPORTING INFORMATION
Movie S1
Video sequence of a cormorant performing the
simultaneous visual discrimination task. Initial sequence: shows the
gate (B) opening at the start of a trial. The bird comes in from
the left hand side of the starting area (A) and swims towards the
camera positioned at the choice point (C). Middle sequence: side
view of bird swimming along the middle section of the swimway.
Final sequence: the bird is viewed from the gate swimming
towards the pair of stimulus panel (D and E). In this instance the
bird makes an incorrect choice and exits through (G) to return to
the starting area for another trial.
Found at: doi:10.1371/journal.pone.0000639.s001 (1.13 MB AVI)
ACKNOWLEDGMENTS
We thank: Sarah Wanless and Yutaka Watanuki for discussions; Lesley
Alton and Steven Portugal for comments on the manuscript; Alex Kabat
for photography; Marie-Anne Martin for help in raising the birds from
nestlings to independence; staff of the Biosciences workshop for construct-
ing the swimway, training tank and housing; staff of the Biomedical
Services Unit for care and maintenance of the birds.
Author Contributions
Conceived and designed the experiments: CW PB GM. Performed the
experiments: CW GM ND. Analyzed the data: CW. Wrote the paper: CW
PB GM ND.
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