Ibis (2013), doi: 10.1111/ibi.12040

Short communication

String-pulling behaviour in
a Harris’s Hawk Parabuteo
unicinctus

ERIN N. COLBERT-WHITE,1* E. MONTEENMCCORD,2 DESIREE I. SHARPE1 & DOROTHY
M. FRAGASZY1
1
Department of Psychology, University of Georgia, 125
Baldwin St., Athens, GA 30602, USA
2
HawkTalk, Inc., PO Box 130, Holly Springs, GA
30142, USA

Variations on the string-pull experiment have been presented to a variety of avian species. Here, we present theresults of a basic vertical string-pull task with a Harris’s
Hawk Parabuteo unicinctus. A 2-year-old subject
retrieved a shielded food reward within 8 min on each ofeight trials and spontaneously used solving techniques
similar to corvids and parrots. Our data contribute to thesmall body of literature on raptor cognition by showingthat it may be within the realm of at least one bird of
prey species to perform the string-pull task similarly toavian species renowned for their high cognitive abilities.

Keywords: avian cognition, bird of prey, problem
solving, raptor.
For more than half a century, the string-pull task has been
considered a reliable measure of avian problem-solving
ability (e.g. Great Tits Parus major Vince 1956, ﬁnches
Carduelis spp., Common Chafﬁnch Fringilla coelebs,
Canaries Serinus spp. Vince 1958, 1961, Budgerigar Melopsittacus undulatus, Common Myna Acridotheres tristis,
Western Jackdaw Coloeus monedula, D€ucker & Rensch
1977, Northern Raven Corvus corax, Heinrich 1995,
Heinrich & Bugnyar 2005, Grey Parrots Psittacus erithacus, Pepperberg 2004, Kea Nestor notabilis, Huber &Gajdon 2006, Werdenich & Huber 2006, Turquoisefronted Amazon Amazona aestiva, Hyacinth Macaw
Anodorhynchus hyacinthinus and Lear’s Macaws
Anodorhynchus leari, Schuck-Paim et al. 2009, New Caledonian Crows Corvus moneduloides, Taylor et al. 2010).
In the basic task, a food reward is suspended from a perch
by a string (Fig. 1) and the subject’s behaviour is*Corresponding author.
Email: colbere@uga.edu© 2013 British Ornithologists’ Union

recorded. Although many bird species have been presented with variations of the string-pull task, prior to thecurrent study, birds of prey had not been tested. As someraptor species rival parrot and raven species with respect
to the relative sizes of their telencephalon (the avian forebrain, the area of higher cognitive functioning; Burish
et al. 2004), we sought to provide preliminary string-pulling data for this group of birds.
The Harris’s Hawk Parabuteo unicinctus (also known
as the Bay-winged Hawk) was selected for this preliminary investigation because it shares an important feature
with parrots and some corvids (two high-performing
groups) that could be relevant to solving the task.
Harris’s Hawks are classiﬁed as social in that they huntcooperatively (Bednarz 1988, Ellis et al. 1993) andengage in shared nest attendance among adults (Mader
1975). Likewise, parrots (e.g. Seibert 2006) and somecorvids (e.g. American Crow Corvus brachyrhynchos,
Knopf & Knopf 1983) are also classed as social. According to the Social Intelligence Hypothesis (Byrne &Whiten 1988), human intelligence may have beenenhanced by the ability to learn from others, which
resulted as a consequence of social living. Empirical
work with non-human animals supports the relationshipbetween sociality and intelligence (e.g. Reader & Laland
2002, Burish et al. 2004, for counter-evidence see Beauchamp & Fernandez-Juricic 2004). Without a non-social
raptor species for comparison, it is impossible to draw
deﬁnitive conclusions about the relevance of sociality tosocial raptors’ cognitive abilities. Nevertheless, the common feature of sociality among Harris’s Hawks, parrots
and some corvids led us to predict that a Harris’s
Hawk’s performance on the string-pull task might besimilar to that observed in many parrots and corvids.
METHODS
subject
A 2-year-old male Harris’s Hawk was the subject of thestudy. The Hawk was captive-bred at Coulson Harris’

Figure 1. Diagram of baited string-pulling apparatus with clear
plastic shield surrounding food reward. Diagram not drawn toscale.

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E. N. Colbert-White et al.

motivation during testing. The apparatus and recording
equipment were removed from the enclosure after test
trials. All procedures were approved by the institution’s
animal use committee (Approval #A2010-10199).

Hawks, a federally licensed raptor propagation facility inLouisiana, USA. After 18 months of being ﬂown as a
falconry bird, the Hawk was injured and spent 2 months
at a raptor rehabilitation centre in northeast Georgia
before the beginning of testing in November 2010. At
the time of testing, the Hawk was housed in an outdoor
primary chamber (2.45 9 3.66 9 2.45 m). The Hawk
was fasted for 24 h prior to test days. Testing days werespaced at least 1 day apart.

Data analysis
Videotaped trials were coded in THE OBSERVER XT(v. 7.0) using a coding scheme adapted from Werdenich
and Huber’s (2006) ethogram that included exploratory,
effective and ineffective behaviours (see Table 1). An
outside observer coded a random 2-min segment ofvideo (11% of video footage). A Cohen’s kappa coefﬁcient of reliability was calculated using a behaviour
matrix and a ‘null’ category for when one rater coded a
behaviour when no behaviour was observed by the otherrater (A.B. Kaufman, E.N. Colbert-White & R. Rosenthal
unpubl. data). The coefﬁcient of reliability was
j = 0.73, where > 0.75 is considered ‘excellent’ reliability (Cicchetti & Sparrow 1981). Without the three
instances of null codes, reliability between raters
increased to j = 0.91.
Latency to solve was deﬁned as the time from when
the bait was lowered into the shield until it was grasped
by foot or beak. Time spent gazing at the baited string
was also recorded for each trial to provide a measure of
the bird’s attention to the task during each trial. Eyegaze was deﬁned as the Hawk’s head being oriented
downward toward the apparatus for at least 3 s. The
3-s criterion was included in each instance of eye gaze.
For example, if the Hawk continued to gaze at theapparatus for 4 s after the 3-s criterion was met, thegaze time would be recorded as 7 s. Eye gaze instanceswere tabulated and converted to a proportion of total
trial time.

Apparatus
All experiments were conducted in the Hawk’s primarychamber. Three strings (0.5 cm diameter braided nylon
cord, 60 cm in length, 23 cm apart) were suspended
vertically from the primary perch. The test string wasbaited with a food reward (a previous frozen mouse ofc. 25 g). One control string was baited with a pineconeof similar size (20 g), and the second control string was
unbaited (Fig. 1). To prevent access to the bait frombeneath, a clear plastic shield (45 cm; diameter 7.5 cm)
was attached to a steel base as shown in Fig. 1. A Sony
HandyCam mini-DV video camera mounted on a tripod
2 m from the apparatus recorded all trials.
EXPERIMENTAL PROCEDURE
Prior to ﬁrst testing, the subject spent 3 h habituating to
the recording equipment, shield and three unbaitedstrings suspended from the perch. Trials began by placing the Hawk on its perch. After starting the video camera, the experimenter baited the string, presented thebait to the Hawk, lowered it into the shield and thenexited the chamber. Trials lasted 30 min and the subject
was fed no less than 3 h following trials to maintain
Table 1. Ethogram of behaviours observed during string-pull trials.
Behaviour
Gaze (on or off)
Approach (1)Back-up (2)Circle (3)Fly away from apparatus (4)Grab string (5)Pull string (6)Release string (7)Walk string (8)Foot hold (9)Grab reward (10)Grab nothing (11)Grab shield (12)Tug string (13)Pull knot (14)Other (15)© 2013 British Ornithologists’ Union

Description of action

Classiﬁcation

Oriented toward baited string > 3 s
Moves towards test string (from any distance)
Moves away from test string (to any distance)
Moves body to opposite side of apparatus while gazing at baited string
Flies out of camera view to perch on other side of enclose
Grabs baited string with beak (5a) or foot (5b)Uses beak (6a) or foot (6b) to pull baited string in any direction
Lets go of baited string from beak (7a) or foot (7b)
Holds baited string in beak (8a) or foot (8b) while hopping laterally across perch
Secures baited string in foot and either holds or presses foot to perch
Successfully retrieves bait with either beak (10a) or foot (10b)Grabs for baited string, reward, or shield with beak (11a) or foot (11b) and misses
Grab shield with footShort pulls at baited string with beak near knot; does not affect position of bait
Bites and pulls up at baited string knot with beak
Other behaviour (e.g. loses balance on perch)

Exploratory
Exploratory
Exploratory
Exploratory
Exploratory
Effective
Effective
Effective
Effective
Effective
Solve
Ineffective
Ineffective
Ineffective
Ineffective
OtherString-pulling in a Harris’s Hawk

RESULTS
Eight trials were conducted. Upon ﬁrst presentation, theHawk retrieved the mouse in 380.1 s; subsequent solve
times ranged from 8.5 to 470.6 s (Fig. 2b). Interaction
with a control string occurred only once (a bite at theunbaited string’s knot during Trial 2), indicating a strong
preference for the baited string.
As shown in Tables 1 and 2, the Hawk engaged in avariety of behavioural sequences to reach the bait. Earlyon, exploratory (e.g. circling the apparatus while looking
at the bait) and ineffective (e.g. pulling the knot) behaviours were most common. For example, all 31 instancesof pulling the knot appeared during the ﬁrst two trials.
As sessions went on, however, ineffective behaviours
were replaced by effective behaviours such as pulling up
the string. Table 2 provides the ratios of effective behaviours to total behaviours for all eight trials. Over time,
inefﬁcient behaviours were phased out as the Hawk(a)3

learned that manipulating the string was integral toretrieving the bait.
During Trials 4, 6, 7 and 8 the Hawk spontaneously
engaged in sequences of pulling up the string with thebeak or foot, anchoring the string with the beak or foot,then pulling up again with the beak or foot (Supporting
Information Video Clip S1). The Hawk’s beak and both
feet were used interchangeably for all steps of the pullanchoring sequences, as well as the ﬁnal grasping of themouse. During Trials 2 and 8, the subject engaged in alateral side-step method of hopping across the perch withthe string loosely held in the beak or foot until the bait
was close enough to grasp. During Trials 1, 3 and 5, theHawk retrieved the reward by grabbing the string withthe beak or foot as close to the bait as possible, pulling up,
and then extending the leg to grasp the bait with the foot.
The proportion of time spent gazing at the apparatus(Fig. 2a) and time to solution (Fig. 2b) decreased acrosstrials. Although at least one ineffective behaviour (i.e.
pulling the knot, grabbing nothing, grabbing the shieldor tugging the string) occurred during all trials except
Trials 3 and 6, the number of ineffective behaviours
decreased over time (Fig. 2c). Overall, the trends shownin Fig. 2 demonstrated a clear learning curve, which wasestablished after only a few trials.
DISCUSSION

(b)

(c)

Figure 2. Increase in the Hawk’s solving efﬁciency across trials depicted in three panels. Both the proportion of total timespent gazing at the apparatus (a) and total solve time (b)decreased across trials. The number of ineffective behaviours(c) also decreased across trials.

This investigation provides the ﬁrst empirical evidence
that a member of a social raptor species can solve thestring-pull task in similar time and with similar solving
behaviours to some parrots and corvids. The test subject
retrieved the food reward within 8 min of ﬁrst viewing
the problem, a time within some species’ documented
time to ﬁrst solution (e.g. Northern Ravens, Heinrich
1995, Heinrich & Bugnyar 2005). Considerably fasterﬁrst-solution times have been documented in Lear’s
Macaws (12 s, Schuck-Paim et al. 2009), New
Caledonian Crows (6 s, Taylor et al. 2010) and Keas
(9 s, Werdenich & Huber 2006). The method of solving
was also similar, as the Hawk’s spontaneous use ofpull-anchoring and lateral side-stepping has been documented in other species (e.g. Heinrich 1995, Pepperberg
2004, Werdenich & Huber 2006).
Unlike some birds tested (e.g. Schuck-Paim et al.
2009), the Hawk immediately approached the apparatus
and displayed little neophobia. In addition, we observed
highly aggressive behaviour towards the apparatus, which
to our knowledge has not been documented in otherstudies of string-pulling in birds. We attribute the aggressiveness and low levels of neophobia to the subject’s
prior experience with baited lures during falconry training, its predatory nature or a combination of the two.
We also attribute to the Hawk’s predatory nature(which involves prey capture with the feet) its heavy
reliance upon the feet for manipulating (i.e. grabbing© 2013 British Ornithologists’ Union

4

E. N. Colbert-White et al.

Table 2. Behavioural data for each trial. Number and letter codes were used to construct behavioural sequences. Gaze was coded
as a state rather than event, and so was coded and timed as being ‘on’ or ‘off’.
Ratio of effective
to total behaviours

Trial

Behaviour sequence1

4, 1, 2, 1, 1, 4, 1, 1, 14, 14, 2, 14, 14, 14, 14, 15, 14, 2, 2, 2, 1, 2, 14, 1, 2, 2, 1, 2, 1, 5a, 6a, 7a,
14, 14, 14, 14, 14, 15, 15, 14, 14, 14, 14, 14, 14, 14, 14, 5a, 6a, 11b, 11b, 11b, 10b
1, 1, 14, 14, 4, 1, 3, 4, 1, 1, 14, 14, 14, 14, 14, 2, 3, 15, 14, 2, 1, 3, 4, 1, 15a, 2, 2, 3, 2, 1, 12, 14, 14,
2, 12, 5b, 6b, 8b, 6b, 8b, 10a1, 1, 2, 2, 2, 1, 2, 3, 3, 15, 1, 2, 5b, 15b, 10a1, 12, 5a, 6a, 7, 5a, 6a, 10b
1, 11a, 11a, 11a, 2, 1, 3, 12, 12, 5b, 6b, 11a, 6b, 10b
5b, 6b, 9, 5a, 6a, 7b 10b
12, 5b, 6b, 5a, 6a, 7b, 11b, 10b
5a, 6a, 7a, 5a, 6a, 7a, 5a, 6a, 7a, 5a, 6a, 7a, 5a, 6a, 7a, 5a, 6a, 7a, 2, 5a, 6a, 7a, 12, 5b, 6b, 9,
5a, 6a, 8a, 7a, 5a, 6a, 7, 8a, 9, 7, 10a

2
3
4
5
6
7
8

0.09
0.12
0.07
0.63
0.21
0.86
0.63
0.92

Because Gaze was coded as a state rather than an event, it is not included in the behaviour sequences. Instead, time spent gazingat the apparatus was recorded for each trial, which appears in Fig. 2(a). The ratio of effective behaviours to total behaviours wascalculated by dividing the number of effective behaviours (Fig. 1) in a trial by the total number of behaviours coded for that trial.
a
Indicates the one instance in which the subject interacted with (i.e. bit knot) a control string. bIndicates subject fell off the perch
momentarily after using the foot to grab the string very close to the location of the mouse.

and pulling) the string rather than simply stepping on it.
Although parrots and corvids can also grasp with theirfeet, as Heinrich (1995) pointed out, in a review ofstring-pull results only Werdenich and Huber’s (2006)
study with Keas described grasping and pulling up on
the string with the foot. However, this behaviour wasscored only once in the ﬁrst trial for two of their sevensubjects. The other studies with parrots (e.g. Pepperberg
2004, Schuck-Paim et al. 2009) and corvids (e.g. Heinrich
1995, Heinrich & Bugnyar 2005, Huber & Gajdon
2006, Taylor et al. 2010) described no foot use beyond
stepping.
We suggest that the Hawk’s behaviour when presented with the string-pulling task could be explainedas originating in species-typical behaviours of birds that
use their feet during foraging. Altevogt (1953)
described the tendency for species that use their feetduring foraging (e.g. holding or grasping food items) toengage in reﬂexive grabbing behaviours. One reﬂexive
grab could inadvertently lift the string and bait closer
to the bird. Taylor et al. (2010) tested this hypothesis
using a visually restrictive string-pulling apparatus with
New Caledonian Crows. When subjects could not see
the bait move as a result of their actions, performancedecreased. According to those authors, the ﬁndings indicated that in New Caledonian Crows, string-pulling
behaviour may indeed be better explained by operant
principles (if an action brings the bait closer, repeatthat action) than by positing the use of ‘insight’ orother reasoning processes. Nevertheless, we agreewith those who consider the string-pull task to be aninteresting measure of problem-solving ability in nonhumans, and we consider all species’ performance on
the task to be worthy of attention.

© 2013 British Ornithologists’ Union

Although Harris’s Hawks, parrots and some corvids
are social (in the sense speciﬁed above), the role sociality plays in their problem-solving ability cannot be
determined from the current study. In a relevant series
of investigations (Biondi et al. 2008), Chimango Caracaras Milvago chimango successfully solved a puzzle box
problem. These neotropical raptors also demonstrated
the ability to learn from conspeciﬁcs (Biondi et al.
2010), which the authors attributed to the species’‘gregarious’ and congregative natural behaviour. Building on these promising results, non-social raptor species
such as the Red-shouldered Hawk Buteo lineatus(Kerlinger 1989) should be tested to understand more
completely the inﬂuence of sociality on raptors’ cognitive abilities.
Cognitive research involving birds of prey is limited,despite ﬁndings that some species have forebrain homologues that rival those of parrots and corvids in terms of
relative size (Burish et al. 2004), and that at least one
species is capable of solving a puzzle box problem(Biondi et al. 2008). It is our hope that the resultspresented here will encourage others to explore further
the cognitive abilities of raptors.
We thank Justin Lukose and Tori Vratanina for their help withdata collection and analysis.

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Associate Editor: Eiven Roskaft.

SUPPORTING INFORMATION
Additional Supporting Information may be found in the
online version of this article:
Video S1. This video illustrates two solving methods
used by the Harris’s Hawk. The ﬁrst clip shows the pullstep-pull method; the second clip shows the lateral sidestep method.

© 2013 British Ornithologists’ Union