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Success in competition for limiting parental resources depends on the interplay between parental decisions over allocation of care and offspring traits. Birth order, individual sex and sex of competing siblings are major candidates as determinants of success in sib-sib competition, but experimental studies focusing on the combined effect of these factors on parent-offspring communication and within-brood competitive dynamics are rare. Here, we assessed individual food intake and body mass gain during feeding trials in barn swallow chicks differing for seniority and sex, and compared the intensity of their acoustic and postural solicitation (begging) displays. Begging intensity and success in competition depended on seniority in combination with individual sex and sex of the opponent. Junior chicks begged more than seniors, independently of satiation level (which was also experimentally manipulated), and obtained greater access to food. Females were generally weaker competitors than males. Individual sex and sex of the opponent also affected duration of begging bouts. Present results thus show that competition with siblings can make the rearing environment variably harsh for developing chicks, depending on individual sex, sex of competing broodmates and age ranking within the nest. They also suggest that parental decisions on the allocation of care and response of kin to signalling siblings may further contribute to the outcome of sibling competition.
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doi: 10.1098/rspb.2010.1741
, 1273-1279 first published online 13 October 2010278 2011 Proc. R. Soc. B
Andrea Bonisoli-Alquati, Giuseppe Boncoraglio, Manuela Caprioli and Nicola Saino
the outcome of conflict among siblings over parental care
Birth order, individual sex and sex of competitors determine
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Birth order, individual sex and sex of
competitors determine the outcome of
conflict among siblings over parental care
Andrea Bonisoli-Alquati1,*, Giuseppe Boncoraglio1,2,
Manuela Caprioli1and Nicola Saino1
1
Dipartimento di Biologia, Universita
`degli Studi di Milano, via Celoria 26, I-20133 Milan, Italy
2
Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK
Success in competition for limiting parental resources depends on the interplay between parental
decisions over allocation of care and offspring traits. Birth order, individual sex and sex of competing sib-
lings are major candidates as determinants of success in sib sib competition, but experimental studies
focusing on the combined effect of these factors on parent offspring communication and within-brood
competitive dynamics are rare. Here, we assessed individual food intake and body mass gain during feed-
ing trials in barn swallow chicks differing for seniority and sex, and compared the intensity of their
acoustic and postural solicitation (begging) displays. Begging intensity and success in competition
depended on seniority in combination with individual sex and sex of the opponent. Junior chicks
begged more than seniors, independently of satiation level (which was also experimentally manipulated),
and obtained greater access to food. Females were generally weaker competitors than males. Individual
sex and sex of the opponent also affected duration of begging bouts. Present results thus show that com-
petition with siblings can make the rearing environment variably harsh for developing chicks, depending
on individual sex, sex of competing broodmates and age ranking within the nest. They also suggest that
parental decisions on the allocation of care and response of kin to signalling siblings may further contrib-
ute to the outcome of sibling competition.
Keywords: begging; competitive asymmetry; environmental sensitivity; hatching asynchrony;
sexual dimorphism; sibling competition
1. INTRODUCTION
Family life, rather than being a harmonious setting in
which genetically related individuals cooperate in view
of their overlapping evolutionary interests, is a stage for
intense conflicts both between parents and offspring and
among the offspring [15]. Understanding the
resolution of these conflicts is crucial not just to model
the dynamics of parental investment, but also to explain
the evolution of behaviour when the environment con-
tains genes that are shared among the interactors [68].
Despite recent progress towards a more dynamic
framework of parent–offspring coadaptation [7,8], con-
siderable uncertainty still exists concerning the role of
both parental and offspring traits in shaping the resol-
ution of these conflicts [9]. This is largely owing to the
fact that a wealth of factors is expected to affect perform-
ance in the intra-familial competitive arena [5,10,11] and
that the arena itself requires the consequences to be
assessed on inclusive fitness [5,1013].
Individual offspring of either sex can differ in their
ability to outcompete siblings and secure parental
resources, with the larger sex usually attaining a better
performance in scramble interactions [14]. The two
sexes can also differ in their sensitivity to rearing con-
ditions [15,16], the larger sex being more vulnerable to
food shortage because of higher energy requirements
[14] and therefore expected to evolve superior competi-
tive ability in order to prevent deterioration of body
condition [17,18]. Older siblings are generally expected
to benefit from size-related advantages in scramble
competition [14,19 21], and this gap can even be
emphasized when younger broodmates are of the weaker
sex and/or receive a poorer share of maternal resources
via the egg [3,21,22]. In fact, parents can influence the
outcome of scrambling among the offspring through
both pre- and post-natal strategies of differential resource
allocation [14,16,2325]. Parents can manipulate com-
petitive asymmetries among progeny members by
promoting variation in egg quality along the laying
sequence [25,26] and/or by modulating hatching
asynchrony [16], as is frequently observed in birds. Inter-
estingly, prenatal (maternal) effects have been suggested
as a mechanism for coupling offspring food soliciting
and parental provisioning [7,23].
Success in competition is therefore expected to depend
on the interplay between individual sex, sex of competing
siblings and birth order [14]. Surprisingly, studies focus-
ing on the combined effect of offspring sex and birth
order on sibling competition and access to parental provi-
sioning are rare [19,21,27]. This is of the utmost
importance for understanding the control of parental
investment, parent– offspring coadaptation and the
*Author and address for correspondence: Department of Biological
Sciences, University of South Carolina, Columbia, SC 29 208, USA
(andrea.bonisoli@unimi.it).
Electronic supplementary material is available at http://dx.doi.org/
10.1098/rspb.2010.1741 or via http://rspb.royalsocietypublishing.org.
Proc. R. Soc. B (2011) 278, 1273–1279
doi:10.1098/rspb.2010.1741
Published online 13 October 2010
Received 12 August 2010
Accepted 23 September 2010 1273 This journal is q2010 The Royal Society
on September 16, 2011rspb.royalsocietypublishing.orgDownloaded from
evolution of strategies for resource and sex allocation [7
9,23,28,29].
In birds, begging behaviour is a major mediator of
scramble competition among broodmates, with differ-
ences in individual begging strategies reflecting
asymmetry among the offspring in quality and/or com-
petitive ability [3,30]. Theoretical models dually depict
begging as a means of scrambling, with parents passively
following the outcome of sibling rivalry in their allocation
decisions, or as a reliable signal of offspring need or con-
dition (sensu [31]), with parents controlling food
allocation according to honest signalling of individual
quality by begging offspring [9,12,30,32].
Differential competitive ability according to age, as
determined by hatching order [10,19,20,22], can result
either from persistence of the effects of asynchronous
hatching or through differential maternal transfer of
resources over the laying sequence. Asymmetry in need
between competitors results in needier chicks probably
being more willing to compete for monopolizing food.
However, hunger levels being equal, high-quality, larger
offspring are expected to prevail over feeble siblings
[22]. Accordingly, younger chicks try to compensate for
their competitive disadvantage by begging more than
their siblings, although their individual strategies may
depend on the number, behaviour and size of competitors
[12,19,20,22,33], and therefore on their resource-holding
potential [33,34]. As individual sex, sex of the
opponent(s) and age rank within the brood are all
expected to affect individual relative resource-holding
potential within the nest, experimental tests where these
factors are jointly manipulated are extremely important
for understanding the resolution of sib sib and parent
offspring conflicts.
In the barn swallow, sib– sib competition can be severe
and is mediated by vocalizations, gaping and posturing
[17,35]. The intensity of begging increases with hunger
and hatching order, and parents provide more food to
the chicks that beg more intensely [35 37]. Offspring
of the two sexes, although similar in size, differ in their
sensitivity to the rearing conditions, as well as in their
short-term competitive ability. Males prevail over short
time periods but are more negatively affected than
females by harsh conditions [17,38]. In addition, male
and female barn swallow nestlings differ in their begging
behaviour [17,39].
Here, we provide a comprehensive experimental test of
the relative significance of sex and birth order for sibling
rivalry and begging behaviour. To this aim, we compared
the acoustic and postural begging of focal pairs of siblings
differing for seniority and optionally for sex. In our
framework, seniority reflects the effects of both hatching
order and variation in egg quality with laying order (see
electronic supplementary material, materials and
methods). Then, we assessed individual success in com-
petition by estimating food intake and body mass gain
during feeding trials. In the barn swallow, parents prefer-
entially follow a brood survival strategy [31] and hatching
order negatively affects individual condition [37]. Thus,
we predicted that needier, junior chicks should attain
higher begging levels and obtain more food than senior
siblings, particularly for males, owing to their superior
competitive ability in the short term [17]. In addition,
we tested whether the effects of sex and seniority on
begging behaviour and success in competition varied
according to the satiation level of chicks.
2. METHODS
(a)General field procedures
The study was performed during spring 2008 in three breed-
ing colonies near Milan (Italy). All nests were checked daily
to mark the eggs according to laying order. Around the esti-
mated hatching time, clutches were transferred to a
Covatutto 24 Eco incubator (Novital, Italy) and replaced
with dummy barn swallow eggs. The incubator was checked
every 3 h from 7.00 until 19.00. Hatching order in the
incubator closely paralleled laying order (r¼0.882, n¼151,
p,0.001; see also [40]). Hatchlings were individually
marked and immediately brought back to their nest; a
dummy egg was removed for each chick that was returned.
At day 7 (day 0 ¼day of hatching of the first chick of the
brood), we measured chick body mass and tarsus length
and collected a blood sample for molecular sexing according
to Saino et al. [39].
(b)Feeding trials and begging recordings
Based on sex information obtained at day 7, we identified
the pairs of siblings to be used in competition tests on days
13–16. According to laying order and brood size, chicks
were classified either as seniors (chicks from egg 1 or 2) or
juniors (chicks from the two last-laid eggs in clutches of
four to six eggs). In each nest, up to four of the following
comparisons were performed: senior male versus junior
male (n¼20); senior female versus junior female (n¼22);
senior male versus junior female (n¼23); senior female
versus junior male (n¼22). The tests confronting these
four different ‘seniority by sex’ classes of siblings were per-
formed one per day, starting on day 13, in random
sequence. We did not compare between male and female
chicks within seniority classes because we had already inves-
tigated the effect of sex per se on competition in a previous
study [17]. Pairs were tested both before and after a short
period of food deprivation [17,35]. On each test day, after
temporarily removing the non-focal broodmates, we
recorded begging vocalizations of each nestling while alone
at the nest during feeding visits of parents (see electronic sup-
plementary material). Recording sessions started in the
morning (7.00 8.00 h). After the second chick had been
recorded, we assessed the ability of each focal nestling to
obtain food while competing with its opponent under
normal satiation conditions (trial before food deprivation
BFD; hereafter). We weighed both nestlings, individually
marked them on the forehead with white markings, and put
them back together into their nest for a 1.5 h feeding trial,
while simultaneously video recording parental and offspring
behaviour. At the end of the trial, nestlings were weighed
again to record variation in body mass, reflecting individual
food intake. Then, focal nestlings were placed in a cloth
bag for a 2 h period of food deprivation and their non-focal
siblings were returned to the nest. Food deprivation simu-
lated a short period of starvation, similar to that naturally
occurring during spells of bad weather. A second session of
audio and video recording was performed after food depri-
vation (AFD), following the same procedure as in the first
trial. Body mass was also measured before and after the
second feeding trial (AFD trial). In subsequent analyses,
we used the number of feedings received by each chick
1274 A. Bonisoli-Alquati et al. Sex and seniority effects on competition
Proc. R. Soc. B (2011)
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during each trial to assess the inherent competitive ability of
the chicks in terms of number of interactions won. We also
used body mass at the end of trials as a proxy of the fit-
ness-related balance between costs and benefits of
scrambling, and because our estimates of feeding rates
could not account for variation in size of individual feedings.
A mean of 2.81 (1.01 s.d.) comparisons per nest was con-
ducted. Each chick was involved in up to two comparisons
(mean: 1.60 (0.49 s.d.)). The inclusion of chick identity in
the analyses (see below) statistically accounted for non-inde-
pendence of data from chicks used in different comparisons.
(c)Analysis of audio and video recordings
Audio recordings were analysed according to Boncoraglio
et al. [17] (see electronic supplementary material). Mean
bout and syllable duration (s), begging rate (number of sylla-
bles per second during begging bouts) and relative amplitude
(dB) of begging calls were measured following Boncoraglio
et al. [17].
Video recordings were analysed with movie editing soft-
ware (Vegas Pro 9, Sony Creative Software). We randomly
selected three feeding visits per trial (see electronic sup-
plementary material and Boncoraglio et al. [35] for further
details), and measured the maximum begging intensity
reached by each chick during each visit on a four-level scale
varying from zero (chick not begging) to three (chick stand-
ing on its tarsi and begging with fully stretched neck towards
the attending parent). Postural scores of each chick were
averaged within trial. The number of feedings obtained by
each nestling over the whole trial was also measured. All
measures were performed blindly with respect to treatments.
(d)Statistical analyses
Our main aim was to test for the independent and combined
effects of sex and seniority on begging behaviour and access
to food. Data from BFD were therefore first analysed separ-
ately from those from AFD trials, using linear mixed models.
Chick and focal pair identity, and nest of origin together with
its interactions with all fixed factors and covariates, were
entered as random factors [17,35]. We analysed the effect
of seniority, sex, sex of the opponent (fixed factors) and
their two-way interactions on feeding rates, final body mass
and begging features. For each dependent variable, we com-
pared the Akaike’s information cirterion for small samples
(AICc) values of all models that could be built by linear com-
bination of the main factors and their two-way interactions,
and selected the model with the lowest AICc value (‘best
model’ hereafter). All other models for which AICc values
did not differ for more than two units from the best model
were considered as equally explicative [41]. Except for one
case (see below), these alternative models never differed
from the best model for any significant effect.
To test for differential effects of satiation level on begging
behaviour and access to food depending on the concomitant
effects of chick sex and seniority, we ran for each variable
an additional linear mixed model on the whole data
sample. In these analyses, we included those terms that
were significant in BFD and/or AFD best models, a two-
level factor accounting for satiation level (BFD or AFD),
and all two- and three-way interactions involving food
deprivation that could be predicted based on the differences
in significance of the terms included in BFD and AFD best
models (see electronic supplementary material). All the
main effects and the two-way interactions that were needed
to properly test for three-way interactions were also included
in these models.
Throughout the manuscript, we report mean values of the
variables of interest, together with their associated standard
error (s.e.) in parentheses.
3. RESULTS
(a)Access to food
Junior chicks received more feedings than seniors in AFD
trials (seniority: F
1,26.2
¼7.72, p¼0.010; figure 1),
while in BFD trials the effect of seniority depended on
individual sex (seniority sex: F
1,120
¼6.39, p¼0.013).
Senior females received less food compared with
senior males (t
199
¼2.28, p¼0.025) and junior females
(t
83.9
¼22.92, p¼0.005; figure 1). Thus, juniors
obtained more feedings than seniors, and females were
weaker competitors than males. In the whole sample of
BFD and AFD trials, the three-way interaction between
2
3
4
5
6
7
8
9
number of feedings
BFD
a
bc
b,c,d
a,e
ed
AFD
food deprivation
Figure 1. Mean (+s.e.) number of feedings received by each nestling in AFD and BFD trials according to individual sex and
seniority. Significant differences in seniority sex feeding trial at post hoc tests are indicated by the same letter. White bars,
junior males; light grey bars, junior females; dark grey bars, senior males; black bars, senior females.
Sex and seniority effects on competition A. Bonisoli-Alquati et al. 1275
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food deprivation, seniority and sex was highly significant
(F
1,207
¼7.18, p¼0.008; figure 1).
The best models of final body mass for BFD or AFD
trials only included initial body mass (always p,0.001)
and seniority (BFD: F
1,22.9
¼4.45, p¼0.046; AFD:
F
1,85.1
¼4.89, p¼0.030). Juniors gained more mass
than seniors both BFD and AFD (BFD: mean body
mass gain: senior, 0.08 (0.05) g; junior, 0.20 (0.05) g;
AFD: senior, 0.27 (0.04) g; junior, 0.38 (0.04) g). How-
ever, two out of five equally informative models of BFD
trials (see electronic supplementary material, statistical
analyses) showed also that sex significantly predicted
final body mass depending on the sex of the opponent
(p,0.026 in both cases). In both models, males gained
more mass than females when competing with a male
(p,0.008) and males competing with a male gained
more than those competing with a female ( p,0.038),
irrespective of seniority (figure 2). While confirming the
importance of seniority and sex, the analyses on individ-
ual mass gain thus demonstrated that access to food
depended also on sex of the opponent. However, since
the effect of the interaction between individual sex and
sex of the opponent differed according to food depri-
vation, we also ran a model where we included the
effect of the three-way interaction between these factors.
This interaction was non-significant (F
1,103
¼0.83, p¼
0.36).
(b)Begging call features
During BFD trials junior chicks uttered longer begging
bouts than seniors (table 1), while in AFD trials the
effect of seniority depended on both individual sex and
sex of the opponent (table 1 and figure 3). Bout length
was greater in junior compared with senior males
(t
69.4
¼3.32, p¼0.001), while it did not differ between
junior and senior females (t
69.4
¼0.49, p¼0.623;
figure 3). In addition, junior chicks uttered longer bouts
when confronted with a female compared with a male sib-
ling (t
57.9
¼3.61, p¼0.001; female opponent: 2.31
(0.24) s; male opponent: 1.90 (0.24) s). The differential
effects of seniority according to individual sex did not
depend on satiation level (seniority sex food depri-
vation: F
1,183
¼0.07, p¼0.785). Thus, juniors begged
more, particularly among male chicks and when con-
fronted with a female sibling.
In AFD trials chicks uttered louder calls when they
had been exposed to a female (214.51 (0.35) dB)
compared with a male opponent (215.85 (0.36) dB;
F
1,161
¼7.90, p¼0.006) in the BFD trial, in a model
controlling for sex and seniority. This small difference,
however, might be biologically meaningless.
Begging rate was higher in female chicks, independent
of other factors (BFD: F
1,28.6
¼5.06, p¼0.032; AFD:
F
1,108
¼8.23, p¼0.005). This effect was due to males
uttering longer syllables than females for a given length
of begging bout (cumulative analysis on BFD and AFD
trials: F
1,125
¼8.02, p¼0.005; males: 0.143 (0.006) s;
females: 0.126 (0.006) s), consistent with previous
studies [17].
Finally, postural begging was affected by seniority in
BFD trials, with juniors begging more intensely than
seniors (F
1,40.9
¼4.52, p¼0.040; junior chicks: 1.86
(0.06); senior chicks: 1.62 (0.08)), while this was not
the case in AFD trials. However, the effect of seniority
did not depend on food deprivation in the overall
sample of tests (F
1,54.8
¼1.75, p¼0.192).
4. DISCUSSION
In this experiment, we found that junior chicks obtained
greater access to food than their senior siblings, as
reflected by both feeding rate and body mass at the end
of feeding trials. Access to food during the trial also
depended on individual sex and sex of the opponent,
with females generally being weaker competitors than
males. Consistently, junior chicks were found to generally
beg more intensely than seniors, independent of satiation
level.
From an evolutionary point of view, begging can be
interpreted either as a reliable indicator of inherent com-
petitiveness of the chicks or as a signal of need [9,12].
The ‘honesty’ of begging is supported by experimental
evidence that begging increases with hunger (e.g.
[42,43]). However, evidence that begging intensity posi-
tively predicts parental provisioning is consistent with
both interpretations of begging [9]. Similarly, both
models predict begging to be costly [42]. Rather, the
males females
body mass gain (g)
–0.1
0
0.1
0.2
0.3
0.4
0.5
Figure 2. Mean (+s.e.) body mass gain in BFD trials for
male and female chicks depending on sex of the opponent.
White bars, male opponent; grey bars, female opponent.
Table 1. Best linear mixed models of duration of begging
bouts in BFD and AFD trials with respect to seniority,
individual sex, sex of the opponent (AFD trials only) and
their interactions.
zFd.f. p
before food deprivation (BFD)
nest 0.81 0.208
seniority 8.62 1, 40.0 0.006
sex 2.17 1, 25.7 0.153
seniority sex 1.67 1, 55.2 0.202
after food deprivation (AFD)
nest 2.51 0.006
seniority 7.61 1, 42.9 0.009
sex 0.03 1, 72.3 0.868
opponent’s sex 0 1, 49.2 0.974
seniority sex 3.97 1, 74.5 0.050
seniority opponent’s sex 5.26 1, 45.9 0.026
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two models could be distinguished by the nature of the
signal, as begging behaviour is always assumed to be
honest under honest signalling, whereas this is not
necessarily the case under scramble competition [9].
Interestingly, however, the level of honesty and the accu-
racy of the begging signal are thought to strongly depend
on the context, as shaped by resource availability and the
age and sex composition of the brood [9].
If begging reflects individual capability in scramble
interactions, our finding that begging by juniors is more
intense implies that they outcompete senior siblings. A
possible explanation would then be maternal favouritism
in allocation of resources to the last eggs. In the barn swal-
low, egg size increases along the laying sequence because
of an allometric increase in the amount of protein-rich
albumen, this pattern being interpreted as evidence that
the parents privilege the last-hatched chicks by providing
resources that are fundamental for skeletal growth [40].
Although previous studies of this species have found no
variation in yolk testosterone content along the laying
sequence [44], maternal favouritism towards younger
chicks could also unfold via the uneven allocation of
other compounds (e.g. [45,46]).
On the other hand, if begging is a reliable signal of
need, last-hatched, needier chicks are expected to beg
more intensely because of the occurrence of carry-over
effects of hatching asynchrony. Higher begging levels
would be afforded by junior offspring because of the
higher potential gains [9]. Needier chicks could then be
favoured by parents because of the higher marginal
return from investing in disadvantaged offspring for a
given effort level, and/or by better-fed, larger siblings
adopting altruistic strategies in order to facilitate survival
of kin. Preliminary analyses on chick mass and condition
at day 12 indicated that junior chicks were in poorer con-
dition than seniors, with seniors being around 5 per cent
heavier when correcting for tarsus length at day 12 (A.
Bonisoli-Alquati, G. Boncoraglio & N. Saino 2008,
unpublished data). Adult barn swallows are known to
adopt a brood survival strategy (sensu [31,38,47]),
which implies that parents at least partly compensate for
the disadvantage of junior chicks arising from asynchro-
nous hatching. In addition, senior chicks in relatively
good physiological state may refrain from monopolizing
food items that would add a low marginal return for
them if these could prove valuable for needier siblings,
as predicted by kin selection theory [13,35]. Indeed,
senior chicks have repeatedly been found to invest less
in begging than junior chicks, when experiencing a com-
petitive advantage (e.g. [20]). The conditions for
interpreting begging as a signal of need are met, and
our findings thus suggest that parents and older siblings
might both favour juniors. On the other side, the finding
that junior chicks, even with a better access to food in the
short term, did not attain similar size to their senior
siblings might indicate that their begging signalling
comes at a greater cost than for siblings in better con-
dition, consistent with evolutionarily stable strategy
(ESS) models of animal communication [48,49].
Individual sex is a further layer of complexity in this
framework. Male and female barn swallow chicks differ
in their susceptibility to environmental conditions (e.g.
[36]). Although sexual size dimorphism is small, differ-
ences in need and sensitivity to rearing conditions might
arise from other factors (e.g. sex-specific androgen
levels; [16,50]). Interestingly, the effects of sex mainly
depended on seniority. Access to food, as indexed by
the number of feedings received during the trial, was
greater for males than for females, at least among seniors.
This effect could be due to senior males being stronger
competitors than senior females, but also to parents deli-
vering more food to the nest when attending sons rather
than daughters, because of generally higher begging
levels by males.
Present results show that exposure to competitors of a
particular sex makes the rearing environment variably
harsh to the chicks, depending on individual sex and
age ranking among broodmates. It has been shown that
seniority and the sex ratio of siblings both affect the con-
dition of barn swallow chicks around fledging [19,37,38].
More studies are required to validate fur ther predictions
about the effect of the covariation between sex and senior-
ity on chick phenotype. Although the isolation of two
competing chick is a common treatment in both theoreti-
cal studies (e.g. [12,51]; but see [10]) and empirical
studies (e.g. [13,35]), our results should be cautiously
interpreted when extrapolating to the overall dynamics
of sibling rivalry, which operate on a longer time scale
and with a larger number of individuals involved.
Confronting a strong competitor might have positive
consequences for individual fitness if parents respond to
escalating begging levels by increasing food provisioning.
Here, we found that, independent of seniority, chicks
from focal pairs including two males, which were likely
to attain the highest signalling level of any combination
of chicks, gained more mass than chicks in focal pairs
including a female. The effect of the opponent’s sex on
acoustic begging in AFD trials is also consistent with this
finding. Exposure to a female in BFD trials resulted in
longer bout duration (although in juniors only). We there-
fore speculate that the poorer feeding effort by parents to
focal pairs including females was owing to their lower sig-
nalling level compared with all-male pairs, leading to
reduced satiation level of the chicks at the end of BFD
trials. This may have prompted the chicks that were pre-
viously exposed to a female competitor to reach higher
begging intensity in AFD trials. Overall begging intensity
might be the signal to which parents are responding
[52,53]. This scenario is also consistent with previous
0
0.5
1.0
1.5
2.0
2.5
3.0
junior senior
seniorit
y
duration of begging bout (s)
Figure 3. Mean (+s.e.) duration of begging bouts in BFD
trials for junior and senior chicks of either sex. White bars,
males; grey bars, females.
Sex and seniority effects on competition A. Bonisoli-Alquati et al. 1277
Proc. R. Soc. B (2011)
on September 16, 2011rspb.royalsocietypublishing.orgDownloaded from
findings that a male-biased brood is beneficial for all off-
spring under harsh rearing conditions [21,38], and
suggests offspring control over food allocation. This
result has important implications for parent–offspring coa-
daptation dynamics [7,9,23,29]. Indeed, offspring control
of provisioning has recently been shown to predict parental
control on the evolution of prenatal effects, implying that
selection on parents drives the coadaptation of parental
and offspring traits [7,23]. The rise in parental provision-
ing rate can also be an indication of cooperative begging
within the brood, a subject of increasing interest for both
theoretical [51] and experimental studies [33,54].
Our results also confirmed that male and female nest-
lings have distinct begging features [17,39]. This discloses
the possibility of parental and sibling favouritism, as both
parents and siblings might be able to discriminate
between offspring of the two sexes. In this respect, we
cannot conclude whether parents and siblings respond
after actively assessing chicks’ need by means of their
signalling level and/or their sex-specific features, or
whether they are both passively accepting the outcome
of sibling competition within the nest, which is in turn
affected by sex-related features.
This study was performed in compliance with Italian laws on
animal research. We thankfully acknowledge the help of
E. Bonati, D. Cagnetti, A. Gerevini, A. Matteo and
V. Pignataro in conducting fieldwork. We thank
N. Bennett, A. Cockburn and three anonymous reviewers
for valuable comments on an earlier draft of the
manuscript. A.B.-A. was funded by a PhD grant from the
Italian Ministero dell’Istruzione, dell’Universita
`e della
Ricerca (MIUR). G.B. was funded by a grant from the
University of Milan and by a Marie Curie Intra-European
Fellowship (PIEF-GA-2009-252120).
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