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Socially foraging animals can search for resources themselves (produce) or exploit the discoveries made by others (scrounge). The extensive literature on producer-scrounger dynamics has mainly focused on scramble competition over readily accessible resources, thereby largely neglecting the variety of scrounging techniques individuals may use as well as the role of investment in food handling. Furthermore, although individual differences in boldness and social factors such as dominance have been described to influence foraging tactics, their potential interplay and effect in foraging contexts beyond the conventional producer-scrounger game remains unclear. We investigated the relationship between social-foraging tactic use and dominance, pair bonds and boldness in a foraging experiment focused on food handling and alternative scrounging tactics. We conducted a producer-scrounger experiment in a captive group of rooks in which individuals could produce by pulling up baited strings, or scrounge by retrieving fallen food items or joining a producer. There were three key findings: (1) dominant rooks adopted the producer tactic more often and more successfully than subordinates; (2) producing and scrounging by tolerance led to mixed benefits to paired birds; (3) bold birds scrounged by retrieving more often than shy birds. Importantly, individuals were highly consistent in their tactic use across conditions differing in food availability. Our study highlights the importance of taking both social factors and boldness (heterogeneity) into account when studying social-foraging dynamics and offers empirical data on food handling and alternative scrounging tactics that can be used to extend current models and experiments on social foraging.
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Dominance, pair bonds and boldness determine social-foraging tactics
in rooks, Corvus frugilegus
Jolle W. Jolles
a
,
*
, Ljerka Ostoji
c
b
, Nicola S. Clayton
b
a
Department of Zoology, University of Cambridge, Cambridge, U.K.
b
Department of Psychology, University of Cambridge, Cambridge, U.K.
article info
Article history:
Received 15 October 2012
Initial acceptance 11 January 2013
Final acceptance 20 February 2013
Available online 12 April 2013
MS. number: 12-00790
Keywords:
boldness
Corvus frugilegus
dominance
individual differences
heterogeneity
pair bond
personality
producerescrounger
rook
social foraging
Socially foraging animals can search for resources themselves (produce) or exploit the discoveries made
by others (scrounge). The extensive literature on producerescrounger dynamics has mainly focused on
scramble competition over readily accessible resources, thereby largely neglecting the variety of
scrounging techniques individuals may use as well as the role of investment in food handling.
Furthermore, although individual differences in boldness and social factors such as dominance have been
described to inuence foraging tactics, their potential interplay and effect in foraging contexts beyond
the conventional producerescrounger game remains unclear. We investigated the relationship between
social-foraging tactic use and dominance, pair bonds and boldness in a foraging experiment focused on
food handling and alternative scrounging tactics. We conducted a producerescrounger experiment in a
captive group of rooks in which individuals could produce by pulling up baited strings, or scrounge by
retrieving fallen food items or joining a producer. There were three key ndings: (1) dominant rooks
adopted the producer tactic more often and more successfully than subordinates; (2) producing and
scrounging by tolerance led to mixed benets to paired birds; (3) bold birds scrounged by retrieving
more often than shy birds. Importantly, individuals were highly consistent in their tactic use across
conditions differing in food availability. Our study highlights the importance of taking both social factors
and boldness (heterogeneity) into account when studying social-foraging dynamics and offers empirical
data on food handling and alternative scrounging tactics that can be used to extend current models and
experiments on social foraging.
Ó2013 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Socially foraging animals can either search for resources them-
selves (produce) or exploit the discoveries made by others
(scrounge; Giraldeau & Beauchamp 1999;Giraldeau & Caraco 2000;
Giraldeau 2008). An extensive literature describes theoretical
models and experiments investigating the dynamics behind these
foraging behaviours and the conditions that inuence this pro-
ducerescrounger (PS) game (Barnard & Sibly 1981;Giraldeau et al.
1994;Giraldeau & Caraco 2000;Giraldeau 2008;Broom et al.
2008). Surprisingly, most PS studies have focused only on the
search for food and ignored the time needed to handle food. The
latter is ecologically relevant as many food items are not immedi-
ately available for consumption and require preparation (Giraldeau
& Caraco 2000;Bugnyar & Kotrschal 2002;Broom & Ruxton 2003;
Ha & Ha 2003;Morand-Ferron et al. 2007a;Broom et al. 2008), such
as the breaking of the protective shell of bivalves by many wading
bird species (Norton-Grifths 1967;Hockey et al. 1989;Stillman
et al. 1997). Furthermore, whereas most PS research has focused
on the exploitation of food patches through scramble competition
(Giraldeau & Beauchamp 1999), individuals may use a variety of
other techniques to obtain food from others, such as stealthy
sequestering and food sharing (Brockmann & Barnard 1979;
Giraldeau & Caraco 2000;Bugnyar & Kotrschal 2002). Investigating
the social-foraging dynamics related to food handling and alter-
native scrounging tactics may therefore provide valuable insight
into social foraging and the diversity of tactics that is frequently
observed (Giraldeau 2008).
Recent studies on the conventional PS game, which focuses on
the exploitation of patches through scramble competition, have
shown that phenotypic differences may play a major role in
determining the use of PS tactics (Barta & Giraldeau 1998;
Beauchamp 2000,2006;Liker & Barta 2002;Lendvai et al. 2006;
Giraldeau 2008;Kurvers et al. 2009). For example, the payoffs of
searching for food (producing) and exploiting the discoveries made
by others (scrounging) are often strongly related to an individuals
competitive ability as well as its explorative and risk-taking ten-
dencies, with bolder individuals mainly producing and dominants
mainly scrounging (Ens & Goss-Custard 1984;Barta & Giraldeau
*Correspondence: J. W. Jolles, Department of Zoology, University of Cambridge,
Downing Street, Cambridge CB2 3EJ, U.K.
E-mail address: jj352@cam.ac.uk (J. W. Jolles).
Contents lists available at SciVerse ScienceDirect
Animal Behaviour
journal homepage: www.elsevier.com/locate/anbehav
0003-3472/$38.00 Ó2013 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.anbehav.2013.03.013
Animal Behaviour 85 (2013) 1261e1269
1998;Stillman et al. 2000;Liker & Barta 2002;Lendvai et al. 2006;
Kurvers et al. 2010a). However, in a social-foraging context where
foraging requires extensive handling time and multiple scrounging
tactics are available, different phenotypic effects may be expected
(Brockmann & Barnard 1979;Bugnyar & Kotrschal 2002;Morand-
Ferron et al. 2007b;David et al. 2011). Extensive handling time
increases the investment of producing, making it especially costly
for lower-ranked individuals to produce because of their lower
competitive ability (e.g. Giraldeau 2008;Broom et al. 2008).
Furthermore, paired individuals may defend resources together
(Robichaud et al. 1996;Emery et al. 2007;Seed et al. 2008), tolerate
one another at a food patch (Beauchamp 2000;King et al. 2009)
and share food with each other (Emery et al. 2007;Seed et al.
2008). Therefore, paired and unpaired individuals are expected to
make different decisions as to whether to produce or scrounge. In
addition, consistent individual differences in boldness may be ex-
pected to affect tactic use differently in a context that involves a
large investment in food handling and in which alternative
scrounging tactics are available. Not only are bolder individuals
more likely to enter a feeding area rst (Dyer et al. 2009), they are
often the rst to discover food patches (Beauchamp 2001), and rely
less heavily on social information in making foraging decisions
(Kurvers et al. 2010a;Webster & Ward 2011; but see Marchetti &
Drent 2000). Bold individuals may therefore be expected to use
alternative and more risky scrounging tactics especially when
producing involves a long handling time. Thus, although in the
conventional PS context it has been shown that phenotypic dif-
ferences may play a major role on PS tactics, these and additional
phenotypic effects may be expected to play different roles in
foraging contexts in which food requires considerable handling
effort and/or in which alternative scrounging options are available.
Furthermore, investigating social factors in conjunction with
consistent individual differences in behaviour is very relevant as
they may be crucially linked to one another (Réale et al. 2007;
Scheid & Noë 2010;Webster & Ward 2011) and actually drive
consistent individual differences in the use of different foraging
strategies (Beauchamp 2001;Morand-Ferron et al. 2007a;
Bergmüller & Taborsky 2010).
We investigated the relationship between social-foraging tactic
use and three phenotypic factors, namely dominance, pair bonding
and boldness. Importantly, our foraging experiment extends the
conventional PS game by focusing on two often neglected factors:
time needed to handle food and alternative scrounging tactics,
namely joining a tolerant producer or retrieving food items drop-
ped on the ground by producers. To address these questions we
used rooks, Corvus frugilegus, as a model species as they are highly
social, form life-long pair bonds and establish a linear dominance
hierarchy (Clayton & Emery 2007;Emery et al. 2007). We presented
a captive group of 19 birds with bait attached to strings tied to
branches in their aviary, a set-up that has been successfully used in
avian cognitive research (Heinrich 1995;Heinrich & Bugnyar 2005).
Individuals could produceby successfully pulling up a baited
string, scroungeby joining or retrieving, or they could choose not
to participate at all. In this particular set-up, producing required a
considerable time investment and the ability to defend the
resource, while scrounging by displacing a producer was not viable
because of the high chance the string would drop. Rooks are rarely
observed foraging alone on the ground in their natural environ-
ment because it entails a higher risk of predation. Therefore,
scrounging by retrieving can be seen as naturally risky for rooks.
Furthermore, this tactic contains an element of uncertainty. A
scroungers success will depend not only on its own actions as there
is variability when food items fall after they have been successfully
pulled up by a producer. To determine the consistency of individual
tactic use we presented the foraging experiment in a low (one
baited string) and high (eight baited strings) food availability con-
dition. In addition, we measured individual feeding rates on a
maintenance diet to investigate the effect of hunger on foraging
motivation and behaviour (Lendvai et al. 2004). Prior to the social-
foraging experiment, we determined dominance ranks and aflia-
tion scores for each individual and acquired boldness scores using a
novel object test (Kurvers et al. 2010a).
We predicted that: (1) dominance would be positively related to
the use and success of the producing tactic; (2) paired individuals
would benet from the high tolerance with their partner as it may
increase their producing success and the ability to scrounge by
joining; and (3) boldness would be positively related to scrounging
by retrieving. We expected individuals to be consistent in their
tactic use independent of food availability.
METHODS
Subjects and Housing
A group of ve male and 14 female adult rooks served as sub-
jects and were housed in an outdoor aviary (8 20 m and 3 m
high) at the University of Cambridge Sub-Department of Animal
Behaviour, Madingley, U.K. The birds were members of a group that
was collected under English Nature Permit 20030108 from two
colonies in Cambridge on 16 and 17 April 2003 and that were
subsequently hand-raised. Aviaries were constructed of wood and
mesh, had gravel oors and contained several perches of varying
height, width and length throughout. All individuals could be
identied by coloured leg rings. Birds had ad libitum access to
water and a maintenance diet of soaked dog biscuits, egg, cheese,
bread, cooked vegetables, seeds, nuts and fruit, except during the
experimental procedures when the group was deprived of their
diet for 4 h. The behavioural observations and boldness tests were
conducted from May to August 2010 and the social-foraging
experiment was conducted from August to October 2010. Birds
were kept for future behavioural experiments after the conclusion
of the study. All experiments were conducted in accordance with
the universitys guidelines under the U.K. Home Ofce project
licence PPL 80/1975.
Dominance
Prior to the social-foraging experiment we scored agonistic and
afliative interactions to determine the dominance rank and pair
bonds in the group (Table A1 in the Appendix). Observations were
made from 10 m outside the aviary between 1300 and 1430 hours
for 16 days (MayeJuly 2010). Data were collected ad libitum
(Altmann 1974) and interactions were recorded as events with The
Observer 5.0 software (Noldus Information Technology, Wagenin-
gen, The Netherlands) and analysed with MatMan 1.1 (idem). We
scored 345 agonistic interactions, ranging from threats to active
chases with feather pulling.
Pair Bonds
Pairs were identied using dyadic afliation scores(DAS) using
the formula: DAS ¼(afliation directed from x to y þafliation
directed from y to x)/(afliation directed by x to all individuals in
the group þafliation directed from y towards all individuals in the
group), with the calculations based on the index of association
(Martin & Bateson 2007). We separated afliative behaviours into
active and passive afliations (Table A1 in the Appendix). Active
afliations are important for establishing and maintaining a pair
bond, while passive afliations are related to interindividual
tolerance (Emery et al. 2007). Individuals were only considered
J. W. Jolles et al. / Animal Behaviour 85 (2013) 1261e12691262
paired if they had the highest DAS with each other in the active,
passive and combined afliation categories.
Boldness Test
We conducted a novel object test to determine individual
boldness scores. We use the term boldnessfor approaching a
baited novel object (Wilson et al. 1993;Greenberg & Mettke-
Hofmann 2001;Frost et al. 2007), although some authors have
proposed using the term exploration(Réale et al. 2007). For 40
trials each of 5 min (maximum ve trials/day) we presented a novel
object next to a cup baited with eight waxmoth larvae, Achroia
grisella. We presented the novel object to the group rather than to
each individual separately because it has higher ecological rele-
vance (Webster et al. 2007), as in natural conditions it is likely that
rooks would be confronted with such a situation while in the
presence of conspecics. The novel object was unique in each trial
(varying in material, colour and size) and its position in the aviary
was randomized across trials. For each trial we recorded which of
the 19 individuals approachedthe novel object to within 1 m. Based
on the average number of trials in which an individual approached
(4.3 1.45 trials) we classied individuals with approach scores
higher than the average as bold (N¼6) and those with scores lower
than the average as shy (N¼13). To exclude the possibility that
feeding motivation inuenced approach scores in the novel object
test we ran an additional 40 control trials during which only the
baited food cup was presented. The conditions were presented
concurrently with randomized order for condition.
Social-foraging Experiment
In the social-foraging experiment individuals could scrounge,
either by retrieving food items dropped on the ground by pro-
ducing individuals or by joining a tolerant producing individual,
produce, by pulling up baited strings in the aviary (see Heinrich &
Bugnyar 2005; photo sequence Fig. A1 in the Appendix), or
decide not to participate at all. A week before the start of testing,
the experimenter habituated the birds to the strings and the pro-
cedure of baiting the strings (Heinrich & Bugnyar 2005) by daily
entering the aviary, clearly showing the bait (a peanut or piece of
cheese), and randomly attaching it to the end of one of 10 already
present 50 cm long strings (jute twine) hanging off branches in the
aviary. All strings were positioned at 2 m height and minimally
1 m apart to make each string approachable by multiple birds
simultaneously. We minimized the effect of possible interindi-
vidual differences in string-pulling prociency by starting the
experiment only after all individuals had been observed to pull up a
baited string successfully. In the test, we presented the rooks with
either a single baited string in 42 trials (one-string condition) or
eight baited strings in 35 trials (eight-string condition). The order of
conditions was pseudorandomized across trials such that the same
number of trials was run for each condition. Trials lasted for a
maximum of 5 min. Time between the experimenter entering and
leaving the aviary when baiting the strings was kept constant
across conditions. For each trial, we recorded which individuals
approached the string to within 1 m (producing attempt), suc-
cessfully pulled up the string and obtained the bait (producing
success), joined a producer and thereby obtained food (scrounge
by tolerance) or landed directly under the string to retrieve
dropped food (scrounge by retrieving). The measure of
approaching the string provides information about the extent to
which individuals would decide to bear the cost of approaching the
string. Scrounging by displacing a successful producer was not a
viable tactic because of the high probability that the string would
drop and thus food would again be out of reach. We used an
approach distance of 1 m from the string to indicate attempts to
produce as birds were always observed to land within this distance
when approaching the string to produce.
Feeding Rate
As a proxy of feeding motivation we measured individual
feeding rates at two adjacent feeding tables (50 50 cm and
140 cm high, approximately 150 cm apart) that are normally used
for the provisioning of the maintenance diet. After the string-
pulling experiment, for 10 consecutive days, we removed the
maintenance diet from the aviary for 4 h and recorded activity at
the tables for 20 min after providing the food again. We determined
the average percentage of time that an individual was on either
feeding table across the 10 trials (feeding rate). The size of the
tables and the distance between them allowed multiple birds to
feed simultaneously. Together with the relatively long time frame
used for recording feeding rates, this set-up ensured that variation
in dominance ranks would not inuence the measure of interest.
Statistics
Dominance
We organized all observed dominance interactions between
individuals in a sociometric matrix, which takes into account the
identity of each opponent and all the interactions and minimizes
inconsistencies (de Vries et al. 1993). To test for linearity we
calculated Kendalls coefcient of linearity K, Landaus index hand
the index of linearity h
0
using MatMan 1.1 (Noldus Information
Technology). Statistical signicance of Kwas calculated using a chi-
square test. Both indices vary from 0 (complete absence of linearity)
to 1 (complete linearity). The index h
0
is based on hand takes into
account the existence of unknown relationships. Statistical signif-
icance of h
0
is provided by a resampling procedure using 10 000
randomizations (de Vries 1995). If the dominance is linear, MatMan
calculates a rank order most consistent with a linear hierarchy by
minimizing the number of inconsistencies and then minimizing the
total strength of inconsistencies (de Vries 1998).
Boldness
Repeatability is a measure of the within-individual variance
compared with the between-individual variance and describes the
phenotypic variance explained by the individual (Kurvers et al.
2009). We determined the repeatability of individualsperfor-
mance across the novel object trials by calculating the mean
squares from a one-way analysis of variance (ANOVA) with indi-
vidual as the main effect following the method of Lessells & Boag
(1987). We compared the number of birds that approached the
cup between the novel object trials and the control trials using a
ManneWhitney Utest. Spearman correlation tests were run to
determine whether approach scores in the novel object and control
condition were correlated.
Social-foraging experiment
We used generalized linear models (GLMs) to test which vari-
ables explained producing attempts, absolute and relative pro-
ducing success, and scrounging by retrieving for each condition.
Data were tted to a binomial distribution with logit-link function
and separate models were run with the number of trials in which
an individual attempted to produce, had producing success and
scrounged by retrieving as the numerator and the total number of
trials as denominator. For the GLMs on the data of the relative
producing success the number of trials in which an individual
attempted producing was used as the denominator. As xed effects,
we tted dominance (rank), if an individual was paired or not (yes/
J. W. Jolles et al. / Animal Behaviour 85 (2013) 1261e1269 1263
no), boldness (bold/shy) and feeding rate (continuous). Models did
not include sex because dominance was strongly related to sex and
additional models including sex revealed it had no signicant in-
uence on any of the other numerators. We started with full models
with all explanatory variables. Interactions were not included in the
models as based on our sample size they may be driven by one or
two data points or, conversely, a real interaction may not be
apparent owing to lack of data. Minimal adequate models were
obtained by comparing models with dropped terms using Akaikes
information criterion (AIC), with the lowest AIC
c
value indicating
the best model t, which is considered the preferable method for
observational data (Burnham et al. 2011). The results of the GLMs
are presented in Table 1. Because of this model selection procedure,
discussion of the GLMsresults are based on the effect sizes and not
on signicance of Pvalues. For completeness, both effect sizes and P
values are presented in Table 1. Correlations of producing attempts
and scrounging by retrieving between the low food and high food
availability conditions were analysed using the Spearman rank
coefcient (r
S
). Results of all statistics other than the GLMs were
interpreted based on the signicance of Pvalues, with P<0.10
reported as a trend and P<0.05 as signicant; means are stated
SE throughout. All data were analysed in R 2.15.2 (R Development
Core Team 2012).
RESULTS
Dominance
Rooks had a linear dominance hierarchy (Kendalls coefcient
K¼0.53, P<0.001; Landaus linearity index h¼0.53, h
0
¼0.56,
P<0.001), such that all birds could be assigned a rank from 1 (most
subordinate) to 19 (most dominant; de Vries 1995). All males were
dominant over all females.
In both the one-string and eight-string conditions, dominance
was positively related to the total number of attempts to produce
and the total number of producing successes (Fig. 1)aswellasto
the proportion of successful trials out of all producing attempts
(Table 1). Dominance did not play a role in scrounging by retrieving.
Pair Bonds
Based on our observation of 340 active and 516 passive aflia-
tions six pairs were identied, one of which consisted of two fe-
males. Four of these pairs had built nests in the previous breeding
season. Paired individuals performed fewer producing attempts
than unpaired individuals (one string: paired: 24.0 6.0%; un-
paired: 31.9 5.5%; eight strings: paired: 38.9 5.7%; unpaired:
57.1 12%). Although in the one-string condition the pair bond did
not affect absolute producing success, paired birds had a higher
proportion of successful producing attempts (relative producing
success) than unpaired birds (paired: 33.3 7%; unpaired:
6.6 4%; Table 1). In 33 of the 42 trials, producing success was
achieved by a paired individual, 21 of which when together with its
partner. For 18 of these 21 trials the male was the rst to arrive and
pull the string; in 17 of these 21 cases the joining partner received
some of the food, and in seven of 15 trials this occurred after both
birds had pulled the string together. Furthermore, in 17 of 21 cases
the primary producer was successful only after its partner had
arrived at the string. In contrast to the one-string condition, in the
eight-string condition not only the absolute success but also the
relative success of paired birds was lower than that of unpaired
birds (Table 1). Scrounging by retrieving was negatively related to
the pair bond, in both the one-string and eight-string conditions
(Table 1,Fig. 2a).
Boldness
Individuals were highly consistent in their approaching behav-
iour in the novel object condition; the repeatability of individuals
approaches during the rst and second half of trials was high (0.93).
Although more birds approached in each trial during the control
than the novel object condition (U¼240.50, N
1
¼N
2
¼40,
P<0.001), there was no signicant correlation between the num-
ber of trials in which an individual approached in the novel object
test and a control condition (40 trials; r
S
¼0.038,N¼19,
P¼0.876), nor did the number of approaches of shy individuals
across the two conditions counterbalance those of bold individuals,
suggesting foraging motivation did not affect approach scores in
the novel object test.
Consistent individual differences in boldness were negatively
related to both producing attempts and producing successes.
Boldness did not affect relative producing success. These effects
Dominance High >
0
0.1
0.15
0.2
0.3
0
0.2
0.4
0.6
0.8
1
(a)
(b)
0.25
0.05
Ratio of trials produced successfully
< Low
Dominance High >< Low
Figure 1. Inuence of dominance rank on producing success in the (a) one-string
(N¼42 trials) and (b) eight-string (N¼35 trials) condition. Points are mean values
from raw data with ratio of trials based on the total number of trials per condition. Line
shows predicted means SE from the minimal model, thus controlling for other sig-
nicant terms in the model.
J. W. Jolles et al. / Animal Behaviour 85 (2013) 1261e1269126 4
were observed in both the one-string and eight-string conditions
(Table 1). In contrast, scrounging by retrieving food items fallen on
the ground was positively affected by boldness, with bold in-
dividuals scrounging considerably more than shy individuals, seen
in both food availability conditions (Table 1,Fig. 2b).
Feeding Rate
In the one-string condition, individuals with higher feeding
rates made more producing attempts. However, feeding rate did
not affect the number of successful pulls and was negatively related
to relative producing success (Table 1). In contrast, although in the
eight-string condition feeding rate was also positively related to
producing attempts, individuals with higher feeding rates had
more producing success, both in terms of absolute numbers and in
the proportion of pulls that were successful (Table 1). An in-
dividuals feeding rate did not inuence scrounging by retrieving in
the one-string condition and was negatively related to scrounging
by retrieving in the eight-string condition (Table 1).
Individual Consistency in Tactic Use
In each trial, individuals could either engage in one of the
foraging behaviours or not participate at all. On average, in the one-
string condition, individuals participated in 31.3 3.1% of the trials,
with producing attempts accounting for 27.0 3.9% and
scrounging by retrieving for 7.8 1.7% of trials. In less than 2% of
trials did individuals attempt both to produce and to scrounge,
which was not related to the frequency of producing attempts
(r
S
¼0.123, P¼0.615) or the relative producing success (r
S
¼0.09,
P¼0.714). The more individuals attempted to produce the less they
scrounged by retrieving (r
S
¼0.455, P<0.05). In the eight-string
condition, individuals participated on average in 69.0 24.9% of
the trials, with producing attempts accounting for 49.2 24.3% and
scrounging by retrieving for 19.9 18.1% of trials; they did not
attempt to produce in 50% of the trials in which they scrounged.
Individuals were highly consistent in their tactic use across the two
food availability conditions, both in terms of producing attempts
(r
S
¼0.879, N¼19, P<0.001; Fig. 3a) and in scrounging by
retrieving (r
S
¼0.755, N¼19, P<0.001; Fig. 3b).
DISCUSSION
In this study we investigated the use of various foraging tactics
by rooks in a PS game that focused on investment in food handling
and alternative scrounging tactics. In particular, we assessed how
two social factors (namely dominance rank and whether a bird was
paired or not) and individual differences in boldness inuenced the
use of producing and scrounging tactics. We tested a group of rooks
in a string-pulling experiment and determined individual consis-
tency in tactic use across conditions that differed in food avail-
ability. The experiment revealed three key ndings. First, dominant
rooks adopted the producer tactic more often, and more success-
fully, than subordinate rooks. Second, paired individuals often
attempted to produce together, which increased the scrounging
success of the joining partner, but resulted in mixed success for the
producing partner. Third, bold individuals scrounged more often by
retrieving than shy individuals. Individuals were highly consistent
in tactic use across conditions with low and high food availability.
Dominance has increasingly been taken into account in studies
on foraging dynamics (Barta & Giraldeau 1998;Stillman et al. 2000;
Liker & Barta 2002;Beauchamp 2006;Lendvai et al. 2006). The
majority of these studies have used the conventional PS game, in
which individuals can either search for food or exploit the discov-
eries made by others, and found that higher-ranked individuals
produce less and scrounge more than subordinates (but see
Beauchamp 2006). These ndings contrast with the result of our
study. Here, dominants made more producing attempts and were
Table 1
GLMs of approaching, absolute and relative success, and retrieving in the one-string and eight-string conditions of the social-foraging experiment
Fixed effect One-string condition zPEight-string condition zP
EstimateSE EstimateSE
Producing attempts
(Intercept) 1.410.29 4.80 <0.001 0.540.29 1.88 0.06
Dominance 0.040.02 2.06 0.04 0.050.02 2.63 <0.01
Paired 0.420.21 2.05 0.04 0.520.21 2.50 0.01
Bold 0.360.20 1.79 0.07 0.670.19 3.42 <0.001
Feeding rate 0.070.03 2.75 <0.01 0.110.03 4.12 <0.001
Producing success (absolute)
(Intercept) 4.000.45 8.96 <0.001 1.700.32 5.27 <0.001
Dominance 0.140.03 4.47 <0.001 0.100.02 5.03 <0.001
Paired 0.870.22 3.92 <0.001
Bold 0.680.40 1.72 0.09 0.420.21 1.95 0.05
Feeding rate 0.130.03 4.44 <0.001
Producing success (relative)
(Intercept) 3.290.68 4.87 <0.001 0.640.47 1.35 0.176
Dominance 0.230.05 4.65 <0.001 0.140.03 4.86 <0.001
Paired 0.750.41 1.81 0.07 0.780.33 2.35 0.02
Bold
Feeding rate 0.150.08 1.98 0.05 0.100.05 2.08 0.04
Scrounging by retrieving
(Intercept) 2.490.25 10.09 <0.001 1.130.30 3.75 <0.001
Dominance
Paired 0.770.28 2.89 <0.01 0.700.24 2.94 <0.01
Bold 0.990.27 3.62 <0.001 1.250.21 6.04 <0.001
Feeding rate 0.070.03 2.25 0.02
Minimal models were selected using an information theoretical model selection approach and results are discussed based on coefcient estimates. For completeness, Pvalues
are also presented. Coefcient estimates represent the change in the dependent variable relative to the baseline category and can therefore be interpreted as measures of effect
size. Data were tted to a binomial distribution with a logit-link function. The analyses were conducted on all 19 individuals.
J. W. Jolles et al. / Animal Behaviour 85 (2013) 1261e1269 1265
more successful at doing so than subordinates. This contrasting
effect is likely to be strongly inuenced by the extensive handling
time needed to pull up a string successfully. This may change the
payoffs and the probability of producing success in such a waythat,
for subordinates, attempting to produce might become too costly.
This effect might have been strengthened further by the ineffec-
tiveness of scrounging by displacing an individual after it had
successfully pulled up the string as this would result in it dropping
the string, thereby putting the bait out of immediate reach. In a
situation with a relatively high availability of food, the probability
of being displaced while attempting to produce is expected to be
low and producing success to be high. If this goes together with a
reduction in clumping of the food, this may minimize the benets
of being high in dominance status for food intake (see Theimer
1987;Giraldeau 2008). However, when we presented the birds
with eight baited strings (thus high food availability with a
reduction in food clumping), dominance was again positively
related to both the amount individuals attempted to produce and
their success in producing. Although this result might to a certain
extent have been inuenced by the feedback related to the prob-
ability of producing success between the two conditions, the in-
vestment in food handling is expected to play the major role.
Despite food being much more available and less clumped, the
probability of producing success may still be low because of the
probability of being displaced still being high owing to the exten-
sive handling time. Overall, these ndings thus seem to suggest
that a large investment in food handling may have an important
and consistent effect on the payoffs of producing related to
dominance.
The pair bond (paired/unpaired) had a strong effect on how
often birds attempted to produce and were successful as well as
0.2 0.3 0.4 0.5 0.6 0.70
0
0.2
0.4
0.6
1
0.1 0.15 0.2 0.25
Ratio of trials scrounging by retrieving [8]
0.2
0.4
0.6
0.8
1
rS = 0.879
P < 0.001
rS = 0.755
P < 0.001
0.8
Ratio of trials attempting producing [8]
(a)
(b)
0.1
Ratio of trials attempting producing [1]
0.05
Ratio of trials scroun
g
in
g
b
y
retrievin
g
[1]
Figure 3. Consistency in tactic use across conditions differing in food availability for
attempts of (a) producing and (b) scrounging by retrieving (one string [1]: N¼42;
eight strings [8]: N¼35). Shown are mean SE ratio of trials out of the total number
of trials per condition during which an individual (a) attempted to produce or (b)
scrounged by retrieving.
0
0.1
0.2
0.3
0.4
0.5
Bold
Shy
0
0.1
0.2
0.3
0.5
Paired
Unpaired
(b)
(a)
0.4
8 strings1 string
8 strin
g
s1 strin
g
Ratio of trials scrounging by retrieving
Figure 2. Inuence of (a) the pair bond and (b) boldness on scrounging by retrieving in
both the one-string and eight-string condition. Shown are mean SE ratio of trials out
of the total number of trials per condition during which an individual used the
scrounging by retrieving tactic. Paired individuals: N¼12; unpaired individuals:
N¼7; bold individuals: N¼6; shy individuals N¼13.
J. W. Jolles et al. / Animal Behaviour 85 (2013) 1261e1269126 6
how often they scrounged by tolerance and by retrieving fallen food
items. When food availability was low, paired individuals had a
higher chance of producing successfully than unpaired birds. On
average, in one out of two trials, paired individuals joined each
other at a producing attempt and, in more than 80% of these at-
tempts, success was achieved after the primary producer was
joined by its mate. This seems to suggest that paired birds benet
from their ability to defend resources together (Robichaud et al.
1996;Emery et al. 2007;Seed et al. 2008) and, furthermore, that
unpaired individuals may have to work harder to get their share of
the food. This is indeed supported by the nding that unpaired
individuals scrounged more by retrieving and performed more
producing attempts than paired individuals. However, looking at
the costs and benets of the pair bond in more detail, we see that
having a partner did not inuence absolute producing success in
the one-string condition. In addition, paired individuals might join
their producing partners preferentially only at the moment when
producing success seems likely, thus reducing the benetof
increased defensibility. Furthermore, in more than 80% of cases the
joining partner received (some of) the food (either by food sharing
or taking (some of) the bait); thus producing for paired birds may
actually be costly. This is supported by the nding that when food
availability was high, both absolute and relative producing success
was lower for paired compared to unpaired birds. Thus, although
for paired individuals scrounging by tolerance was a highly suc-
cessful strategy (see also Bugnyar & Kotrschal 2002), producing
when paired might actually be relatively costly in the short term,
especially when food availability is high. This is interesting as
scrounging by tolerance was performed by the female of a pair 85%
of the time. However, a producing male may still benet from
tolerating its partners scrounging, as this may strengthen their pair
bond (Emery et al. 2007;von Bayern et al. 2007) and ultimately
result in higher tness benets (e.g. increased mating opportu-
nities). To our knowledge such a nuance has never been addressed
in PS models. This might also explain the high levels of social
tolerance between the individuals in a pair (Beauchamp 2000;Seed
et al. 2008): during seven trials we observed paired birds to pro-
duce successfully by pulling the string together.
Consistent individual differences in boldness had a very strong
effect on use of the scrounging tactic. As predicted, bold individuals
scrounged more by retrieving fallen food items than shy individuals,
with both low and high food availability. We predicted this effect
because this particular scrounging technique entails both an
element of risk of predation and uncertainty because of dependence
on the producers action. In the wild, single rooks are rarely seen
foraging on the ground owing to a high risk of predation. Despite the
absence of predators in the aviary setting, the captive-housed rooks
spend most of their time away from the ground and in most cases
will land on the ground with multiple individuals simultaneously
(J. W. Jolles, unpublished data). Furthermore, retrieving fallen food
items contains an element of uncertainty that is beyond the action of
the scrounger, as food items will not fall during or after every suc-
cessful producing action. Although Kurvers et al. (2010a) reported
that boldness was positively related toproducing in barnacle geese,
Branta leucopsis, which seems to contradict our nding, both results
may be explained by the commonality of risk and uncertainty in
foraging tactics. In Kurvers et al.s study, individuals could produce
by discovering new food patches, which is a more risky and uncer-
tain tactic than scrounging on these new discoveries. This would
suggest that the relationship between boldness and the use of pro-
ducing and scrounging tactics might depend on the risk and un-
certainty involved in both tactics. This hypothesis is supported by
the nding that bold individuals are often the rst to enter a feeding
area and discover food patches (Beauchamp 2001;Dyer et al. 2009)
and are more exible in their responsiveness (Marchetti & Drent
2000;Frost et al. 2007). Owing to the small size of dropped food
items and the difculty of discerning them from the ground sub-
strate, we could not investigate the determinants of scrounging
propensity and scrounging success in more detail. Further studies
are needed to provide insights into the relative benets and costs of
this scrounging tactic and their effect on foraging choices.
In common with previous studies (Beauchamp 2001,2006;
Morand-Ferron et al. 2007a;Kurvers et al. 2010a), we found that in-
dividuals were highly consistent in their use of foraging tactics. In-
dividuals chose the same tactics in both food availability conditions,
even though high food availability could have enabled more in-
dividuals to choose successfully the more protable producing strat-
egy. Dominant individuals made more producing attempts and were
more successful than subordinates, while, in contrast, scrounging by
retrieving was positively related to boldness and the pair bond. This
effect was observed irrespective of food availability. These ndings
may indicate that dominance, boldness and the pair bond lower the
exibility of individualstactic choices. Still, the current state of in-
dividuals may playa role in their tactic use, as is substantiated by the
ndingthat individual differences in motivationto feed (feeding rates)
had opposite effects on producing success under low and high food
availability. Overall, these results suggest an interplay between social
factors and individual differences in boldness that has an important
effect on individual consistency in tactic use. This would support the
idea of social niche specialization which suggests that individuals
adopt different behavioural strategies to reduce competition with
other group members (Bergmüller & Taborsky 2010).
Our social-foraging experiment conforms to the rst and main
assumption of PS games, namely tactic incompatibility (Coolen
et al. 2001), and enabled the study of food handling and alterna-
tive scrounging tactics in a social-foraging context. According to the
second assumption of PS games, scrounger payoffs should be
negatively frequency dependent (Mottley & Giraldeau 2000). Our
results show that the investment in handling food may be an
important driving factor for individual foraging tactic choices and
that social factors and boldness may have strong and opposite ef-
fects relative to the conventional PS game. This may have impli-
cations for future studies on PS dynamics. Although foraging
efciency may play a large role in foraging tactic use under certain
conditions (see e.g. Stillman et al. 2000), we do not expect this to be
the case in our study. In our social-foraging experiment food-
handling time was long and competitor density high, factors un-
der which the inuence of foraging efciency is expected to be
minor (Stillman et al. 2000). Furthermore, only after all individuals
had already been observed to pull up a baited string successfully
did we start with the social-foraging task. Given that persistent
individual differences may often be related to dynamics that are
peculiar to the group in which they occur (David et al. 2011;
Morand-Ferron et al. 2011; but see Kurvers et al. 2010b), follow-up
research on our social-foraging experiment should also study howa
change in social composition may affect individual tactic use.
Finally, more research is needed to investigate the payoffs of the
various foraging tactics in relation to food-handling time, and how
these payoffs may differ between different phenotypes.
To summarize, our study shows that individuals were highly
consistent in the use of producing and scrounging tactics in a
social-foraging context that focused on food handling and was
strongly inuenced by dominance, the pair bond and boldness. Our
results add to a growing literature on heterogeneity in group
behaviour stressing the importance of not treating group members
as identical or interchangeable (Dyer et al. 2009;Nagy et al. 2010;
Webster & Ward 2011;Jolles et al. 2013). Moreover, our study
highlights the importance of considering social factors as well as
consistent individual differences (often referred to as animal per-
sonality) for investigating social-foraging dynamics. The current
J. W. Jolles et al. / Animal Behaviour 85 (2013) 1261e1269 1267
study offers empirical data on food-handling time and alternative
scrounging tactics that could be used to extend both current
models and experiments on social foraging.
Acknowledgments
We are grateful to the editor and two anonymous referees
whose comments have greatly improved the manuscript. This work
was supported by grants from the Erasmus Programme and the
University of Cambridge. We thank Alex Thornton and Neeltje
Boogert for valuable discussion and feedback, and Ivan Vakrilov and
Charmaine Donovan for avian husbandry.
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Appendix
Figure A1. Sequence of photos that shows a successful producing attempt as seen in the social-foraging experiment. After arriving above the string (a), an individual had to grab the
string with its beak (b), pull up the string (c) and stand on the string (d), and perform this sequence of behaviours up to three times (e) in order to get to the food item (f).
Table A1
Ethogram of agonistic and afliative interactions used to determine the dominance rank and pair bonds in the group
Behaviour Denition
Aggression
Displacement One individual retreats at the approach of another, which locates itself in the retreating individuals original position
Threaten One individual makes a movement directly at, pecks at, ies at or lunges at another without making contact
Pecking One individual uses its bill to peck at and make contact with another individual
Feather pulling One individual grabs the feather(s) of another individual and makes a pulling motion
Chasing A prolonged, continuous approach by one bird towards another bird that continuously moves away
Avoiding One individual moves out of the way as another individual approaches
Active afliation
Bill twining Two individuals interlock the mandibles of their bills, often accompanied by simultaneous displaying
Active food sharing One individual places a food item into the bill of another individual, often after the recipient makes begging calls
Sitting in contact Two individuals sit next to each other, maximum 5 cm apart
Allopreening One individual nibbles or strokes the feathers of another
Dual object manipulation Two birds manipulating the same object
Dual caching & cache recovery Two individuals caching the same object or recovering and eating a previously cached food item
Passive afliation
Co-feeding Two individuals foraging while in proximity to each other.
Sitting in proximity Two individuals sit within one body length of each other, but more than 5 cm apart
Passive food sharing One individual places a food item in front of another individual after which they eat from it together
Developed in collaboration with Corina Logan (Logan et al. 2012).
J. W. Jolles et al. / Animal Behaviour 85 (2013) 1261e1269 1269
... Among passerines, corvids made suitable subjects for cross-species comparisons of problem-solving abilities (Heinrich & Bugnyar, 2005;Taylor et al., 2010;Pendergraft et al., 2020) and social learning of extractive foraging tasks (Templeton et al., 1999;Holzhaider et al., 2010;Federspiel et al., 2019). While string-pulling studies have focused on the genus Corvus (e.g., Heinrich, 1995;Heinrich & Bugnyar, 2005;Jolles et al., 2013) in captivity, less is known about the cognitive abilities of adjacent family lines in the Corvidae (see Ericson et al., 2005), aside from recent assessments on captive California scrub-jays (Aphelocoma californica, Hofmann et al., 2016), captive green jays (Cyanocorax yncas, Manrique et al., 2017), captive azure-winged magpies (Cyanopica cyanus, Wang et al., 2019), and captive oriental magpies (Pica serica, Wang et al., 2020). It is noteworthy, however that McCune's et al. (2019) study of Mexican jays (Aphelocoma wollweberi) suggests captive birds may exhibit reduced problem-solving performance compared to wild conspecifics. ...
... Hypotheses attempting to explain variation in problem-solving performance have included individuals' willingness to approach novel objects (i.e., neophobia or boldness, Webster & Lefebvre, 2001;Day et al., 2003;Overington et al., 2011), explore new objects or environments (i.e., motor diversity, Overington et al., 2011;Sol et al., 2011; or persistence in problem-solving attempts (i.e., task-oriented motivation, Tebbich et al., 2010;Benson-Amram & Holekamp, 2012;Cauchard et al., 2013). Variation in problem-solving performance may also be influenced by age, sex, physical size, individual experience and social context experienced during experimental trials (Kummer & Goodall, 1985;Benson-Amram & Holekamp, 2012;Jolles et al., 2013). With additional experience, individuals are given more opportunities to improve and refine their problem-solving abilities after an initial attempt (Heinrich, 1995;Chow et al., 2016). ...
... Perhaps, due to their bold and explorative attributes discussed previously, some of the 19 birds that began pulling at the string would have eventually reached the peanuts if we had implemented any of the following modifications: (i) increased the duration of our study to allow for more trials overall, (ii) naturally excluded previously successful jays so others would have more opportunities to solve, or (iii) significantly decreased the amount of food reward to discourage scrounging. In wild great and blue tits (Cyanistes caeruleus), birds with a greater number of opportunities were more successful at eventually solving a novel task Cauchard et al., 2013), while in a study with captive rooks (Corvus frugilegus) dominant birds solved a string-pulling task more often while subordinate individuals regularly scrounged across varying levels of food availability (Jolles et al., 2013). ...
Article
Animals that exploit resources from human-modified environments may encounter unique problems when searching for food. Pulling a string tied to a food reward (string-pulling task) is one of the most widespread methods of testing a species’ problem-solving performance in non-human animals. Performance in problem-solving tasks may be influenced by an individual’s characteristics and social interactions, especially in its natural habitat. We examined problem solving by free-ranging Steller’s jays ( Cyanocitta stelleri ) when extracting food from a string-pulling task presented in their natural habitat. During the study, seven of 50 jays successfully solved the task on their first to eighteenth experimental opportunity; solvers differed from nonsolvers by showing higher levels of persistence by pulling the string in more trials. Of the successful jays, five birds solved without observing others, while two birds were present during successful trials and subsequently completed the task. All seven jays demonstrated improvement in the task by using less string pulls over additional successful trials. Nineteen other jays in the population interacted with the apparatus and pulled the string, but not enough to acquire the food. These 19 jays were significantly bolder (shorter latencies to approach), more explorative (contacted more parts of the apparatus), and had observed solvers more than the 24 individuals that did not pull the string. These results indicate a broad spectrum of individual differences in propensity for solving novel tasks in our population of Steller’s jays.
... It is possible skills and information facilitating burrow raiding behaviour is passed only amongst kin (e.g. Cornell et al. 2012;Schwab et al. 2008;Stöwe et al. 2006) or that certain individuals are bolder and more likely to explore (Jolles et al. 2013;Overington et al. 2011), leading to successful exploitation of prey in burrows. Exploratory behaviour or traits such as boldness in birds may at least partially be determined genetically (van Dongen et al. 2015). ...
... Culprit birds were primarily identified through images captured by remote-sensor cameras placed on penguin burrows over three breeding seasons (2013-2014-2016Fig. 2). ...
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Corvids are exceptional predators which can become problematic and difficult to manage due to their adaptability, intelligence, and abundance. On Phillip Island (Victoria, Australia) little ravens (Corvus mellori) prey on the eggs of burrow-nesting little penguins (Eudyptula minor) at an ecologically and economically significant colony. Raven depredation of penguin clutches is intense but appears to have emerged only in recent years. Understanding whether or how the behaviour is transferred provides an opportunity to disrupt this undesirable behaviour. We explore the role genetic relatedness may have on the occurrence of raven burrow-raiding behaviour. We test an hypothesis regarding one specific scenario of possible transmission, namely that ravens preying upon eggs/young in penguin nests share genetic traits that facilitate burrow raiding, and are therefore more genetically related than birds which do not raid burrows. Using 15 microsatellite loci, we found no significant difference in mean relatedness between culprits and other birds. Burrow-raiding behaviour does not appear to be restricted to kin, and targeting ravens based solely on their relationship to culprits is unlikely to reduce depredation rates. However, targeting known culprits may slow transmission of burrow-raiding behaviour.
... Many species show intraspecific differences in foraging ecology based on sex, either through habitat (Breed et al., 2006) or ecological segregation (Xavier et al., 2017). Social dominance (Dorning & Harris, 2017;Jolles et al., 2013) may also influence diet composition if socially dominant individuals have greater access to human foods (e.g., West & Peery, 2017). Finally, habitat use, encompassing both habitat type occupied and movement frequency, also influences ecological opportunity and may explain dietary differences between individuals or subpopulations (Marklund et al., 2017;Newsome, Garbe, et al., 2015;Powell et al., 2015;Scholz et al., 2020). ...
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Generalist species can exhibit individual specialization (IS), where individuals adopt specialized foraging behaviors not attributable to age, sex, or social dominance. While IS increases with the diversity of available foraging resources (ecological opportunity), the potential impact of human food subsidies on ecological opportunity is unknown. We quantified the isotopic niche width of Steller's Jays (Cyanocitta stelleri), a synanthropic predator and dietary generalist, across a gradient of human land-use categories ranging from subsidized (park campgrounds) to unsubsidized (forest interior) habitats in a protected area in California. We asked (1) if isotopic niche width was better predicted by individual foraging behavior than sex, social dominance, or habitat category (indicating IS) and (2) if degree of IS exhibited by jays increased with the availability of human food subsidies. We characterized the isotopic niche space occupied by individual jays relative to the population using carbon (δ 13 C) and nitrogen (δ 15 N) isotope values. Using linear models, we found that jay diet variation was best explained by individual behavior and habitat use rather than by sex or social dominance. While the degree of IS increased with the availability of human foods, individual jays exhibited a variety of foraging behaviors in all habitats that were segregated by foraging stratum and use of human foods. Individual diet also determined the degree of specialization, as jays that specialized on human foods had the narrowest niche width regardless of habitat. Management efforts targeted at generalist wildlife exhibiting large degrees of IS should therefore account for the impact of food subsidies on foraging behavior. Approaches that involve aversive conditioning may fail where highly specialized individuals are unlikely to encounter treatments; therefore, we recommend simultaneous efforts to limit human food subsidies (e.g., information campaigns and improved containment of human food and food waste), and deployment across the largest possible extent of foraging microhabitats and substrates in order to ensure exposure of whole populations to emetic eggs.
... Fundamentally, trait variation among individuals is the raw material natural selection acts upon [48]. Individual foraging behaviour can determine how other members of a foraging group behave, a group's choice of where to forage, and foraging methods and their successes [49][50][51][52], while individual temperament can affect predator-prey interactions [53]. An animal's temperament and the individual dietary choices it makes (including high degrees of individual dietary specialization) are often associated with ecological interactions that promote food web stability in diverse communities [53]. ...
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Article
Dietary variation within species has important ecological and evolutionary implications. While theoreticians have debated the consequences of trait variance (including dietary specialization), empirical studies have yet to examine intraspecific dietary variability across the globe and through time. Here, we use new and published serial sampled δ ¹³ C enamel values of herbivorous mammals from the Miocene to the present (318 individuals summarized, 4134 samples) to examine how dietary strategy (i.e. browser, mixed-feeder, grazer) affects individual isotopic variation. We find that almost all herbivores, regardless of dietary strategy, are composed of individual specialists. For example, Cormohipparion emsliei (Equidae) from the Pliocene of Florida (approx. 5 Ma) exhibits a δ ¹³ C enamel range of 13.4‰, but all individuals sampled have δ ¹³ C enamel ranges of less than or equal to 2‰ (mean = 1.1‰). Most notably, this pattern holds globally and through time, with almost all herbivorous mammal individuals exhibiting narrow δ ¹³ C enamel ranges (less than or equal to 3‰), demonstrating that individuals are specialized and less representative of their overall species' dietary breadth. Individual specialization probably reduces intraspecific competition, increases carrying capacities, and may have stabilizing effects on species and communities over time. Individual specialization among species with both narrow and broad dietary niches is common over space and time—a phenomenon not previously well recognized or documented empirically.
... accessible to all group members and cannot be monopolized by one or a few individuals), scramble competition should take precedence and resource access becomes less dependent on the dominance hierarchy. Although most studies have found that dominant individuals tend to be scroungers, some studies have found the opposite or no effect of social status on foraging strategy (common marmosets, Callithrix jacchus: De la Fuente et al., 2019; rooks, Corvus frugilegus: Jolles, Ostoji c, & Clayton, 2013;vervets, C. pygerythrus: Teichroeb, White, & Chapman, 2015; zebra finches, Taenopygia guttata: Beauchamp, 2006). ...
Article
Among social foragers, individuals can actively search for and find food (the producers) or join already discovered food patches (the scroungers). Compared to scroungers, producers often occupy more dangerous outer spatial positions in the group, but they benefit from the finder's advantage, which is the amount of food eaten before the arrival of others. Scroungers may occupy safer positions but they face feeding competition when joining a patch already occupied by others. Here, we report factors influencing intragroup spatial position, feeding strategies and feeding success for a group of wild vervet monkeys, C. pygerythrus, at Lake Nabugabo, Uganda. We collected data using behavioural observations and field experimentation (N = 132 trials) where we set up an artificial food patch with dispersed food rewards. We found that individuals who spent more time in the front-outer position of a moving group produced more. Producers that had a greater finder's time advantage (fed undetected by group members for longer periods) had a greater finder's advantage (consumed more food items before the arrival of other group members), and scroungers that arrived earlier had greater overall feeding success. We found that when the scrounger was higher ranking than other individuals at the patch, they used displacement scrounging (supplanted at least one individual from the patch to gain access to the food resource). However, feeding success did not differ between displacement scrounging and tolerated scrounging (when the scrounger fed at the patch with at least one other individual). Interestingly, we did not find higher-ranking individuals to have greater feeding success than lower-ranking individuals. Our findings corroborate previous studies showing that even among species with a linear dominance hierarchy and high occurrences of within-group contest competition, dominant individuals do not benefit from feeding advantages at large, dispersed food patches since they cannot monopolize the resource. Furthermore, our results emphasize the need to understand factors influencing feeding tolerance, particularly for subordinate individuals, who need to process ecological and social elements to maximize their food acquisition.
Thesis
La vie en groupe social apporte des avantages considérables mais augmente aussi la compétition, qui peut parfois être surmontée par la coopération. L’objectif de ma thèse a été de déterminer l’influence des liens sociaux sur l’expression de la coopération en situation de compétition chez les corbeaux freux, une espèce coloniale monogame. Deux formes de coopération ont été étudiées : l’affiliation suite à un conflit (réconciliation et affiliation avec un tiers) et la formation de coalitions. Pris dans leur ensemble, mes résultats montrent que les corbeaux freux recourent à la coopération surtout quand cela s’avère nécessaire et que, lorsqu’ils coopèrent, ils le font quasi-exclusivement avec leur apparié. Mes résultats suggèrent que les patterns de coopération des corbeaux freux sont largement conditionnés par leur organisation sociale : l’imprévisibilité de leur environnement social a pu conduire à la flexibilité dont ils font preuve dans le recourt à la coopération et, le lien de couple, fort et durable, détermine le choix du partenaire de coopération.
Article
Foragers within a group might increase individual foraging efficiencies by using public information to assess local resource availability. This information is often expressed as a change in behavior at resource encounter, which can be detected by nearby individuals. When the resource landscape displays sufficient degrees of clustering or fractility, joining nearby conspecifics in their successful foraging efforts becomes an attractive strategy. However, when group sizes increase, joining others might become less effective due to increased levels of intraspecific competition. In this work, we introduce a trimodal Lévy search in fractal resource landscapes, where highly diffusive, extensive searches are interchanged with localized, intensive searches and informed searches guided by attraction to successful conspecifics. Using an agent-based model, we are able to quantitatively determine what environmental characteristics facilitate joining to be beneficial on both the group and the individual level. We find that joining others is advantageous on the group level, but only if resources are sufficiently clustered and group sizes and joining ranges are not too large. In contrast, individual advantages, expressed as an increased survival rate mediated by a reduced variation in resource intake rate, are largest precisely in parameter regions where group benefits were smallest. These results highlight both the reach of a relatively simple agent-based model based on multimodal random searches, and the notion that groups might not necessarily optimize foraging efficiency when more strict conditions, such as survival, need to be met.
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The study of consistent individual differences has become an important focus in research on animal behaviour. These behavioural differences are typically measured through standardized testing procedures. One frequently used paradigm is the novel object test, in which animals are exposed to unfamiliar objects and their reaction is quantified. We used meta-analysis to evaluate how reliably novel object trials quantify individual differences. Overall, we found repeatability of responses to novel objects was strong and significant and was larger in short-term than in long-term studies. Average sample size and long-term estimates have both increased over the past three decades. Most short-term studies used different novel objects in repeated presentations, while long-term studies used either the same or different novel objects almost equally often. Novelty, the time interval between trials and their interaction together explained little of the total heterogeneity, while between-study heterogeneity remained large. Overall, novel object trials reliably estimate individual differences in behaviour, but results were very heterogeneous even within the same study species, suggesting susceptibility to unknown details in test conditions. Most studies that use novel object trials in a foraging context label the trait as neophobia, while novel object trials in a neutral context are labelled variously as shyness–boldness, exploration–avoidance or neophilia. To avoid ambiguity, we argue for the use of object–neophobia for trials near resources and object–neophilia for trials in a neutral context as the most specific labels for novel object responses.
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Preprint
The Great-tailed Grackle (Quiscalus mexicanus) seems to take advantage of inhospitable environments such as cities. However, it is not yet fully understood how these birds exploit hostile environments to their advantage. Casual observation suggests that this species can obtain resources of biological importance such as food or nesting material from the garbage. As a first approach to the problem, we located a patch outside a residential building in a high-density urban area, where the residents left their trash for pickup. A group of wild Great-tailed grackles was identified as regular visitors. In total, 25 days were recorded at the site (November 2017 - January 2018). Events such as foraging, the number of subjects present at the foraging area, aggressive behaviors between members of the group, and their relation with the presence or absence of the garbage collector truck were registered. The results show a higher number of grackles at the observation site and a higher frequency of foraging behaviors in the presence of garbage collection than in its absence. In its presence, the distribution of foraging during the day follows a normal distribution. In the absence, the distribution shows more variability towards the day. The highest frequency of interactions occurred between two grackles, yet there were records of up to eight subjects. The highest number of aggressions registered took place in the absence of garbage collection than in its presence. Moreover, the focal subject exhibits fewer agonistic behaviors than other group members, a result expected if the producer-scrounger game literature is considered. The outcome is explained in terms of deprivation and availability of resources. Finally, we conclude that grackles can exploit hazardous environments such as cities due to the highly social behaviors exhibited during foraging.
Chapter
In this chapter, I return to considering the traditional mapping sentence. The reason for my positioning this writing at this point and not in earlier chapters that took the traditional mapping sentence as their subject is because of the novelty of the application of the approach to investigate avian cognition and behaviour. My colleagues and I conducted the research I report (Hackett et al., International Journal of Comparative Psychology, 32, 2019) in which a re-analysis of a previously gathered set of data (Shaw et al., Animal Behaviour, 109, 101–111, 2016) was undertaken using a facet theory and mapping sentence approach. In the initial 2016 research conducted by Shaw and colleagues, a test-battery for avian intelligence was developed and administered to a group of New Zealand Robins (Petroica longipes). The data they gathered was analysed using principle component analysis (PCA). Hackett et al. (2019) used this data to facilitate analysis using traditional quantitative facet theory and it associated statistical techniques of Smallestspace Analysis (SSA) and Partial Order Scalogram Analysis (POSA). Based upon the results of these analyses, Hackett, et al., formulated a traditional mapping sentence for avian cognition and extended the understanding of the original analysis by Shaw et al. (2016) through identifying two facets with an interactive relationship specified in a mapping sentence. Hackett et al. (2019) suggested that this mapping sentence constituted a framework within which research into avian cognition may be designed, conducted, and interpreted and that the mapping sentence may be adapted and applied to analogous areas of animal behaviour.
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Flocks of birds in flight represent a striking example of collective behaviour. Models of self-organization suggest that repeated interactions among individuals following simple rules can generate the complex patterns and coordinated movements exhibited by flocks. However, such models often assume that individuals are identical and interchangeable, and fail to account for individual differences and social relationships among group members. Here, we show that heterogeneity resulting from species differences and social structure can affect flock spatial dynamics. Using high-resolution photographs of mixed flocks of jackdaws, Corvus monedula, and rooks, Corvus frugilegus, we show that birds preferentially associated with conspecifics and that, like high-ranking members of single-species groups, the larger and more socially dominant rooks positioned themselves near the leading edge of flocks. Neighbouring birds showed closer directional alignment if they were of the same species, and neighbouring jackdaws in particular flew very close to one another. Moreover, birds of both species often flew especially close to a single same-species neighbour, probably reflecting the monogamous pair bonds that characterize these corvid social systems. Together, our findings demonstrate that the characteristics of individuals and their social systems are likely to result in preferential associations that critically influence flock structure.
Article
Animals are known to affiliate after conflicts rather than avoid each other. Affiliation can occur between former opponents or between a former opponent and a third-party, and is more common between individuals with high-quality relationships. We investigate postconflict (PC) affiliation in 3 species of corvid (crows) to examine how both sociality and analysis method influence this behavior. We hypothesized that 1) there will be no former opponent affiliation because the highest-quality relationships in these species are between mates who never fight, therefore eliminating the need to repair this relationship; and 2) colonial rooks and jackdaws will show third-party affiliation with partners, whereas the territorial Eurasian jays will not show this behavior because they lack high-quality relationships outside of the breeding season when their data were collected. PC affiliation is generally analyzed using the latency to first affiliative contact, however this method has limitations. We explore 2 different measures: the frequency and duration of affiliation across each observation session. There was no evidence of former opponent affiliation in rooks or jays, but some in jackdaws according to affiliation durations. Rooks and jackdaws showed third-party affiliation with mates according to affiliation frequencies and durations, and jays showed third-party affiliation according to affiliation durations, but with any individual, not just mates. We suggest that PC affiliation is best investigated using more than first affiliation latencies, and that the frequency and duration of affiliation may indicate whether affiliation is used to address PC stress
Article
We briefly outline the information-theoretic (I-T) approaches to valid inference including a review of some simple methods for making formal inference from all the hypotheses in the model set (multimodel inference). The I-T approaches can replace the usual t tests and ANOVA tables that are so inferentially limited, but still commonly used. The I-T methods are easy to compute and understand and provide formal measures of the strength of evidence for both the null and alternative hypotheses, given the data. We give an example to highlight the importance of deriving alternative hypotheses and representing these as probability models. Fifteen technical issues are addressed to clarify various points that have appeared incorrectly in the recent literature. We offer several remarks regarding the future of empirical science and data analysis under an I-T framework.
Article
1. We develop an individuals-based model that predicts the strength of interference between foraging animals from basic elements of their behaviour. The model is based on the same principles as previous behaviour-based interference models, but extends and adds further realism to these models. One key difference is that in our model the responses of animals to competitors are not fixed, as is assumed in previous models. Instead, animals use optimal decision rules to determine responses which maximize their intake rate. 2. The general shape of interference function generated by the model is similar to that predicted by previous behaviour-based models. Interference is insignificant at low competitor densities, but steadily increases in intensity as density rises. However, comparison with the observed level of interference between oystercatchers, Haematopus ostralegus, feeding on mussels, Mytilus edulis, shows that the model's predictive power is substantially increased through the addition of optimal decision rules. When animals have a fixed response to encounters, too much interference occurs because dominant animals waste time avoiding subdominants and subdominants waste time attempting, but failing, to steal prey from dominants. When animals use optimal decision rules, only subdominants avoid, and only dominants initiate attacks. Interference is therefore reduced and is much closer to that observed. 3. The conditions under which optimal decision rules will lead to interference are described in terms of basic elements of foraging behaviour. Interference is predicted to occur when handling time and the probability of winning fights are high, and when prey encounter rate and the duration of fights are low. These parameters are used to predict successfully the presence or absence of interference in a range of shorebird-prey systems. 4. We suggest that behaviour-based interference models will need to incorporate optimal decision rules if they are to predict accurately the strength of interference observed in real predator-prey systems.
Article
I presented four groups of Common Ravens (Corvus corax) with a problem that they had never encountered before. Could they demonstrate the solution to this problem without first practicing or learning the correct sequence of intermediary steps? The problem posed was the reaching of food suspended by a string. The solution required perching above the string and the food, reaching down, pulling up a loop of string, setting the looped-up string onto the perch, stepping onto the string, releasing the string with the bill while simultaneously applying pressure with the foot onto it, then reaching down again to repeat the cycle six to eight times in that precise order before finally securing a piece of dried meat. The results varied enormously between individuals. However, typically a bird approached the string nervously, pecked or briefly yanked on the string, repeated the approach when given another opportunity, extinguished the approach behavior, or suddenly did the entire string-pulling sequence correctly. One of the wild birds performed the entire sequence correctly on his first approach to the string, even though no other bird of that group had shown the behavior. After a bird had acquired the behavior it thereafter performed the behavior correctly without fail. Other behaviors were associated with successful string pulling. From their first trial, the four hand-reared individuals dropped the meat attached to string (and perch) if they were shooed from the perch. In contrast, other birds that were handed the food attached to string attempted to fly off with it, and it required five to nine trials before they refused to do so, apparently learning the consequences of this behavior. Other problems related to food presented on the dangling string also were solved without first overtly trying out the alternatives. These problems involved: (1) crossing the string with food with another string that held a rock; (2) using a novel string with food, next to the previously-rewarded "old" string; and (3) having food on string next to a rock on string, but with insertion of the string on the perch now displaced laterally. In contrast, the birds performed very poorly at some tasks where simple trial-and-error learning quickly would have resulted in appropriate responses. For example, three birds never once (in 79 trials) pulled the correct string in the crossed-strings experiments that another mustered with no trials. The results are discussed in terms of possible insight and alternative mechanisms, including innate behavior and learning.
Article
(1) Oystercatchers Haematopus ostralegus eating mussels Mytilus edulis on the Exe Estuary were aggressive and often stole mussels from each other. A stable and linear dominance hierarchy existed amongst ten birds studied in detail. The intake rates (biomass consumed per 10 min foraging) of six out of eight birds on which adequate data were obtained, decreased with increasing densities of oystercatchers. However, this did not happen in the two top-dominant individuals, which therefore did not seem to suffer from interference. (2) Interference in the subdominants was due to increased intraspecific kleptoparasitism, to decreased capture rates probably caused by avoidance, and, perhaps, to increased time spent in aggression. Subdominants avoided high densities of conspecifics, but they did not stop feeding when high densities occurred. (3) It is possible to account for our data in terms of the increased opportunities for dominants to steal food from subdominants at high bird densities. Competition for small-scale feeding sites may have been important too. It is suggested that the dispersion of oystercatchers over the mussel bed can be described by a modification of the `ideal despotic distribution' proposed by Fretwell (1972).
Article
The producer–scrounger model assumes that producers are animals that search for food, and scroungers are animals that steal food found by producers. By following foraging decisions made by beach-foraging crows, we assessed individual variability in strategy use, variables influencing the finder's share (when defined as the probability of retaining a prey item in the presence of food theft behaviour) and the benefits and costs of producing and scrounging. Data were collected using focal and all-occurrences scan sampling. Results indicated that more individuals were likely to use both producing and scrounging tactics than to be 'pure' producers. No pure scroungers were documented. When producers obtained scroungeable items, scroungers attempted to steal the item 46% of the time. Forty-one per cent of attempted scrounges were successful. Thus, producers suffered a 19% loss rate of scroungeable items to scroungers, although producers had access to large numbers of small (nonscroungeable) items. Although there were significantly more scroungeable prey items available at lower tides, there was no significant relationship between prey density and scrounge attempts or actual thefts. No ecological factors (group size, temperature, time of day, season) or individual characteristics (age, sex) influenced the probability of retaining prey. Prey retention was determined only by characteristics of the prey items: larger prey items or those with longer handling times were most likely to be stolen.
Article
Animals are known to affiliate after conflicts rather than avoid each other. Affiliation can occur between former opponents or between a former opponent and a third-party, and is more common between individuals with high-quality relationships. We investigate postconflict (PC) affiliation in 3 species of corvid (crows) to examine how both sociality and analysis method influence this behavior. We hypothesized that 1) there will be no former opponent affiliation because the highest-quality relationships in these species are between mates who never fight, therefore eliminating the need to repair this relationship; and 2) colonial rooks and jackdaws will show third-party affiliation with partners, whereas the territorial Eurasian jays will not show this behavior because they lack high-quality relationships outside of the breeding season when their data were collected. PC affiliation is generally analyzed using the latency to first affiliative contact, however this method has limitations. We explore 2 different measures: the frequency and duration of affiliation across each observation session. There was no evidence of former opponent affiliation in rooks or jays, hut some in jackdaws according to affiliation durations. Rooks and jackdaws showed third-party affiliation with mates according to affiliation frequencies and durations, and jays showed third-party affiliation according to affiliation durations, but with any individual, not just mates. We suggest that PC affiliation is best investigated using more than first affiliation latencies, and that the frequency and duration of affiliation may indicate whether affiliation is used to address PC stress.