Behavioral Ecology Vol. 12 No. 3: 269-274
© 2001 International Society for Behavioral Ecology
Effects of begging on growth rates of nestling chicks
a Zoological Laboratory, Groningen University, The Netherlands b Servicio Interfacultativo de Animales de Laboratorio, Universidad de Granada, Spain c Estación Biológica de Doñana, CSIC, Spain
Address correspondence to M.A. Rodríguez-Gironés, Center for Limnology, Netherlands Institute of Ecology, PO Box 1299, NL-3600 BG Maarssen, The Netherlands. E-mail: rodriguez{at}cl.nioo.knaw.nl .
Received 2 November 1999; revised 1 May 2000; accepted 2 July 2000.
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ABSTRACT |
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We investigated whether an increase in begging levels delays growth of chicks. In experiment 1, we hand-reared nine pairs of ring dove squabs, divided into a control and a begging group. All squabs received similar amounts of food, but those in the begging group had to beg for a prolonged period in order to be fed, while squabs in the control group received food without begging. Squabs stopped responding to the treatment after 10 days and, at that time, there was no effect of induced begging on their body mass. In experiment 2, we hand-reared 27 pairs of magpie chicks for 3 days. The design of experiment 2 was similar to that of experiment 1. Daily food intake and begging affected growth rates. On average, chicks in the begging group grew 0.8 g/day less than control chicks, which represents a decrease of 8.15% in growth rate. Because growth is usually positively associated with expected fitness, this demonstrates that begging is a costly behavior, an assumption routinely made in models of begging behavior.
Key words: cost of signaling, handicap principle, magpies, Pica pica, ring doves, signaling of need, Streptopelia risoria.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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In species with parental care, young normally solicit food and other resources from their parents. These solicitations, often conspicuous, have attracted the attention of biologists for decades and have been interpreted in different ways. The research in food solicitation behavior has centered mainly on birds, where it has been suggested that begging is the result of parent—offspring conflict (Trivers, 1974
; see also Eshel and
Feldman, 1991; Mock and
Forbes, 1992), that begging influences the outcome of sibling
competition (Macnair and Parker,
1979;
Rodríguez-Gironés,
1999), and that it provides parents with reliable information
regarding the internal state of the chicks (Godfray,
1991,
1995;
Harper, 1986;
Hussell, 1988;
Rodríguez-Gironés,
1999). In these three contexts (which are not mutually exclusive),
begging is the visible outcome of an underlying tension, be it a conflict
between siblings or between parents and their offspring, which endows the
begging young with an advantage. If young are going to have stable
solicitation strategies, if we require that begging does not escalate ad
infinitum, it would seem that we must add a negative cost term to the equation
such that, at evolutionary equilibrium, the marginal cost of increasing
begging exactly balances its marginal benefit, thus stabilizing the begging
strategy. The presence of this cost term has been made explicit in most
discussions of begging, and several models seem to confirm the intuitive idea
that no evolutionarily stable begging strategy will exist unless begging is
costly (i.e., Godfray, 1991,
1995;
Harper, 1986;
Macnair and Parker, 1979).
In agreement with some of these models (Godfray,
1991,
1995;
Harper, 1986;
Rodríguez-Gironés,
1999), a number of experimental studies have shown that begging
intensity is correlated with levels of food deprivation and that parents
increase provisioning rates in response to increased levels of begging
(reviewed by Kilner and Johnstone,
1997). Furthermore, our understanding of biological signaling
systems in general is by and large conditional to signals being costly. Our
main theoretical framework for analyzing the evolution and stability of
signaling systems is the handicap principle, which states that reliable
signals must be costly (Grafen,
1990; Zahavi,
1977). Although, strictly speaking, signaling systems can be at
equilibrium in the absence of any cost (e.g.,
Enquist et al., 1998;
Lachmann and Bergstrom, 1998),
the amount of information that can be conveyed with cost-free signals is very
small if there is conflict of interests between sender and receiver. On the
other hand, recent findings suggest that the cost of begging required to
stabilize begging may be very small under some conditions
(Johnstone, 1999).
Considering that, according to existing models, the assumption that begging
is costly is absolutely critical for the stability of the begging strategies,
relatively little effort has been dedicated to carefully examining this
hypothesis. Discussions on the cost of begging have focused on two alternative
mechanisms that might make begging costly: by attracting predators and through
a direct energetic cost. The predation cost and other possible alternatives
are discussed below, and for the time being we will concentrate on the
energetic cost. McCarty (1996)
measured the energetic cost of begging in seven passerine species using
closed-chamber respirometry and found that the active metabolic rate while
begging was of the same order of magnitude as the resting metabolic rate. The
highest energetic cost he measured was in tree swallows, Tachycineta
bicolor, where the active metabolic rate while begging was 1.27 times the
resting metabolic rate. Leech and Leonard
(1996) and Bachman and
Chappell (1998) found similar
ratios (1.28 and 1.27) in tree swallows and house wrens, Troglodytes
aedon, respectively. This relatively low metabolic cost led McCarty
(1996) and Bachman and
Chappell (1998) to suggest that
the hypothesis that begging is energetically costly might have to be
rejected.
It is important to clarify, however, that when the models refer to begging
being costly, what is meant is that an increase in begging intensity is
associated with a decrease in expected fitness. In principle, a relatively low
increase in the resting metabolic rate can be associated with a high begging
cost. Indeed, most of the energy ingested by growing chicks is spent in
thermoregulation and maintenance (Weathers,
1992,
1996). Therefore, a modest
increase in energy expenditure can substantially reduce the amount of energy
available for growth, and this energy increase can be associated with a
meaningful decrease in growth (Leech and
Leonard, 1996; Verhulst and
Wiersma, 1997). For this reason, short-term measurements of
metabolic rate are insufficient to investigate the energetic costs of begging.
Bachman and Chappell (1998)
have measured metabolic rates in the field and in the laboratory for chicks of
different ages engaged in different activities. Their data suggest that the
energy spent in begging daily accounts for less than 1% of the daily energy
budget, well below 7.7% of the energy deposited in new tissues each day. Their
measurements probably incorporate short-term repayments of anaerobic
metabolism, but they cannot address the possibility of energy reallocations
during begging episodes.
The purpose of this study was to investigate the effect of begging on
growth. In the experiments reported, we handreared two groups of ring dove
(Streptopelia risoria) squabs and two groups of magpie (Pica
pica) chicks. For each species, we fed both groups the same amounts of
food, and we kept subjects in identical conditions, except that the
squabs/chicks in one group were required to beg for their food. The comparison
of the growth rates between both groups gives an indication of the cost of
begging in terms of body size, a parameter known to influence survival
probability and expected reproductive success (e.g., carrion crow, Corvus
corone corone: Richner,
1989,
1992; collared flycatcher,
Ficedula albicollis:
Lindén et
al., 1992; blue tit, Parus caeruleus:
Merilä and
Svensson, 1997; great tit, Parus major:
Haywood and Perrins, 1992;
Verhulst et al., 1997; zebra
finches, Taeniopygia guttata:
Haywood and Perrins, 1992; for
a recent review, see
Lindström,
1999).
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METHODS |
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This work was conducted after approval by the ethical committee of animal experimentation (CEEA) and following the Animal Behavior Society/Association for the Study of Animal Behavior (ABS/ASAB) guidelines for ethical treatment of animals.
Experiment 1
Experimental subjects
We kept six pairs of adult ring doves in breeding cages (80 x 90
x 100 cm) from January to May 1998. Cages had a nesting bowl and nest
material and birds were provided with grit, water, and food (mixed seed) ad
libitum. The nine two-squab broods that they produced were used for the
experiment. (All pairs produced second clutches. In three cases only one squab
hatched, and we left singletons with their parents until independence.) We
removed the squabs from their nests on the evening of day 3 (hatching = day 0)
and hand-reared them to independence.
Mortality
All four chicks in broods one and two died within 24 h. They were aged
between 5 and 9 days, shared an incubator, and showed signs of respiratory
distress, suggesting that they died from some contagious disease. The control
chick of the eighth pair died on day 15 (weight 88 g).
Housing of squabs
We raised the squabs following the procedure developed by Balsam (see
Balsam et al., 1992). We kept
young squabs in straw-lined bowls in an incubator at 32°C, one squab per
bowl. (The two squabs of a brood were always kept in the same incubator, and
therefore an incubator always had the same number of begging and control
squabs.) Bowls were 15 cm deep, and therefore squabs were in visual isolation.
After day 10, we placed the squabs, still in their bowls, in cardboard boxes
in a room at 26°C, one pair of siblings per box. All squabs older than 21
days and unable to feed on their own were kept together in a large aviary at
26°C. Independent young were kept in breeding cages with ad libitum food
and water.
Feeding of squabs
Since there are, to our knowledge, no published data concerning the daily
energy intake of growing ring dove squabs in the wild (nor in the laboratory),
we estimated their requirements from published values and allometric equations
derived from a number of species
(Weathers, 1996). From these
data, we can infer that the daily energy intake of squabs must be in the range
4.8·M0.845-6.24·M0.845
kJ, where M is the mass (g) of the squab. This is a rather wide
range, and the amounts provided in the experiment (see below) correspond to
the lower portion of the spectrum. This choice ensured that squabs were
slightly food deprived; with a very abundant food supply, both groups might be
growing at the maximum rates, the limit to growth being dictated by
physiological constraints other than energy intake.
Food composition as a function of squab age is given in
Table 1 (some drops of a liquid
multivitamin compound were added to this mixture). This diet mimics the
composition of crop milk in white Carneaux pigeons, Columbia livia,
and the diet of older squabs (Griminger,
1983; Leash et al.,
1971). Based on the energetic requirements of squabs and the
composition of food, we decided to feed squabs up to (and including) 8 days an
amount (cm3) that was equal to half their morning weight (g). We
fed older birds according to their age: squabs 9 and 15 days old were fed 24
cm3 of their corresponding diet, and 1 cm3 was added
each day to the last daily consumption, up to and including 30 cm3.
Squabs up to and including 12 days old were fed five times per day (0800,
1030, 1300, 1530 and 1800 h), while older squabs were fed only three times per
day (0800, 1300 and 1800 h). Squabs older than 21 days were weaned
progressively. We fed them decreasing amounts twice per day (0800 and 1800 h)
until they were able to feed independently.
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Treatments
Squabs in the begging group were stimulated by touching their bills and
crops for 2 min before feeding. Each meal was divided in seven small morsels,
so that squabs in the begging group were stimulated to beg for approximately
14 min during each meal, or some 70 min per day. Squabs in the control group
were fed as soon as possible after removal from the incubator. Squabs eagerly
accepted their meal in one or at most two portions, and feeding of control
chicks normally took less than 1 min.
Because feeding took place outside the incubator, and to control for the possibility that thermoregulation while out of the incubator was costly, control squabs remained outside the incubator while their siblings were being fed.
In the first brood produced by the breeding pairs, squabs were allocated randomly to the begging and control groups. Thereafter, we allocated the first-hatched squab of each new brood to the begging and control groups in alternation to make sure that both groups had the same proportion of first- and second-hatched squabs. With second clutches, the allocation of squabs (first- and second-hatched) to experimental groups (begging and control) was the reverse of the allocation in the first clutch of the same breeding pair. For statistical analysis, we looked at within-brood differences between begging and control chicks.
Experiment 2
Experimental subjects
We collected magpie chicks in Santa Fe (Granada, Spain) during April 1999.
Details of the study area can be found elsewhere
(Zúñiga
and Redondo, 1992). We collected 54 magpie chicks, with body mass
in the range of 20-100 g, from their nests on the evening before the
experiment (day 0). We handreared chicks in the laboratory for 3 days and
returned them to their original nests on the evening of day 3. The chicks were
used for an experiment investigating the plasticity of begging strategies, and
only the experimental details relevant for the study of begging costs are
reported here.
Housing and feeding of chicks
We kept artificial broods of two chicks in straw-lined bowls (hereafter
referred to as nests) at temperatures ranging between 27°C and 36°C,
according to the age of chicks. We fed them a mixture of 1 kg raw cow heart,
six boiled eggs, and 100 g of carrots, to which we added a multivitamin
complex. The morning weight of a chick determined the amount of food that it
would receive during the day. We calculated the relationship between body mass
and food intake of chicks from allometric relationships
(Weathers, 1996), calibrated
with the ad libitum food intake of 1-week-old chicks raised on the same diet
(Redondo, 1993). The daily
ratio of a chick was 0.98·M0.814 g, where
M is the mass (g) of the chick. Chicks received their daily food
ratio in 14 equal, hourly portions. Half of the artificial broods received a
"fasting" diet, in which daily intake was decreased by 5%.
Treatments
Each nest contained a chick from the begging and a chick from the control
(nonbegging) group. We fed chicks with one meal per hour. During a meal, we
visited each nest a random number of times, in 3- to 5-min intervals. The
number of visits per meal took values 1-4 and there were, each day, 4, 4, 3,
and 3 meals with 1, 2, 3, and 4 visits, respectively. We stimulated chicks to
beg (by saying "toma" close to their nest) in every visit. Chicks
in the control group received most of their portion in the first visit of each
meal (we held back a small amount to feed them in subsequent visits if they
begged again), while chicks in the begging group received their entire portion
in the last visit, after prolonged begging. We videotaped the begging
intensity of each chick every day, at three levels of food deprivation (30,
60, and 150 min). Regular meals were videotaped when time permitted.
Analysis
We scored from the videotapes the amounts of time that chicks spent begging
during a random subset of meals and all behavioral tests. The meals analyzed
corresponded to 13 broods, although several meals were analyzed for each
brood. For the analysis, we calculated the time that each chick spent begging
per meal.
In experiment 1, siblings had similar sizes at the beginning of the experiment, and therefore growth differences could be assessed by comparing the size of siblings at the end of the experiment. In experiment 2, initial size differences between chicks were pronounced. Due to the nonlinearities of the functions relating growth and food requirements of chicks to their weight, we cannot compare the growth rate of a 20-g chick with that of a 100-g chick. To study whether food intake and growth rates differed between control and experimental magpie chicks, we performed regression analysis of the variable of interest on the controlling variables and compared the residuals of control and experimental chicks. Thus, for the food intake, a second-order polynomial regression of food intake on initial weight was performed. The residuals of this regression give us an idea of whether the total food intake of a chick was greater or smaller than expected from its initial mass. These residuals were analyzed with a repeated-measures ANOVA with begging level as the within-subject comparison and diet as the between-subject factor. (In this analysis, brood was the independent unit.) For the analysis of growth, we included total food intake and its square as independent variables in a stepwise regression before calculating the residuals. All statistical tests are two-tailed. Results are reported as means ± SE unless otherwise noted.
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RESULTS |
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Experiment 1
Begging
Tactile stimulation of the bill and crop succeeded in inducing strong begging in young squabs of the begging group. Young squabs stretched their legs and body and flapped their wings vigorously for the duration of the meals. As squabs grew, tactile stimulation became less and less effective in inducing begging, and the squabs eventually stopped responding (median age 13, range 12-14 days).
Food intake
Daily food intake for 4- to 8-day-old squabs was weight dependent and hence
variable. Food intake during these days for the control (114.6 ± 3.7
cm3, n = 7), and experimental (113.6 ± 4.2
cm3, n = 7) squabs did not differ significantly (paired
t test: t6 = -0.31). From 9 to 15 days of age,
the only variability was due to some squabs occasionally refusing to eat their
entire share. Total food intake during these days did not differ between
control (195.7 ± 7.0 cm3, n = 6) and experimental
(195.5 ± 6.6 cm3, n = 6) squabs (paired t
test: t5 = 0.25). From 16 days onward, all squabs ate
their full share every day, and there were no differences between groups.
Growth
For the analysis of growth rates, only the pairs in which both chicks
survived to day 13 were used. Figure
1 shows the growth of the begging and control groups. At the end
of the experimental period (day 13, when most squabs stopped begging), the
body mass of control (87.20 ± 1.65 g, n = 7) and experimental
(87.86 ± 1.80 g, n = 7) squabs did not differ significantly
(paired t test: t6 = -0.14, p =.74).
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Experiment 2
Begging
There was a significant difference between the time that chicks in the
experimental and control groups spent begging per meal (paired t
test: t12 = 10.36, p <.0001). Chicks in the
begging group begged for longer periods (103.8 ± 11.2 s/meal,
n = 13) than chicks in the control group (29.6 ± 4.9 s/meal,
n = 13). The difference resulted from the combination of two factors.
For a given level of food deprivation, chicks in the begging group begged for
longer periods than chicks in the control group
(Rodríguez-Gironés
et al., in preparation). Furthermore, during most of the feeding events,
chicks in the begging group were hungrier than their foster siblings because
chicks in the control group were always fed in the first nest visit of a meal,
while their siblings were not. Hungrier chicks begged longer and at higher
intensities than satiated chicks
(Rodríguez-Gironés
et al., in preparation).
Food intake
Initial weight of chicks had a significant effect on their food consumption
during the experiment (F2, 51 = 157.71, p
<.0001; R2 =.866). The analysis of the residuals from
the regression of food intake on initial weight of chicks showed that diet did
not have a significant effect on food intake (F1, 25 =
3.928, p =.059), and both begging level (F1, 25 =
0.557, p =.462) and the interaction between diet and begging level
(F1, 25 = 0.380, p =.543) were clearly
nonsignificant. Diet had no significant effect on food intake because chicks
in the high intake rate groups left part of their food uneaten more often than
chicks in the low diet groups.
Growth
The average growth rate of chicks during the experiment was 9.8 ±
0.4 g/day (n = 54). The stepwise regression (overall:
F3, 50 = 691.98, p <.0001;
R2 =.977) showed that the final weight of chicks was a
function of initial weight (t50 = 8.105, p
<.0001), food intake (t50 = 5.516, p
<.0001) and the square of initial weight (t50 = -3.028,
p =.003). The regression equation was:
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The repeated-measures ANOVA on the residuals showed that diet had no significant effect on growth (F1, 25 = 0.040, p =.843), but begging significantly retarded growth of chicks (F1, 25 = 4.791, p =.038). The interaction was not significant (F1, 25 = 0.113, p =.739). On average, induced begging delayed growth by 0.8 g/day (Table 2).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Current explanations of the evolution of biological signaling systems in general, and of begging in particular, rely on the assumption that signaling (begging) is costly—costly in the sense that an increase in signaling intensity must be associated with a decrease in expected fitness (Godfray, 1991
,
1995;
Grafen, 1990;
Harper, 1986;
Macnair and Parker, 1979;
Zahavi, 1977). In the case of
begging, two potential sources of a signaling cost have been discussed:
begging may attract predators and hence increase the probability of predation,
or it may have a direct effect on expected survival and lifetime reproductive
success.
At least two experiments have tried to evaluate the predatory cost of
begging (Haskell, 1994;
Leech and Leonard, 1997).
Haskell (1994) played tapes of
begging calls from artificial nests baited with quail (Conturnix
coturnix) eggs and found that the begging calls increased predation rates
in ground, but not in tree nests. Leech and Leonard
(1997) used a similar approach
and found that begging tapes increased predation both on the ground and in
nest-boxes. These results suggest that begging is indeed costly, as
hypothesized by the models, and that the models can therefore be used with
confidence. There are, however, a number of reasons that the results of
Haskell (1994) and Leech and
Leonard (1997) must be
interpreted with care. For instance, the playback experiments may not provide
an accurate representation of the natural situation: in Haskell's
(1994) experiments, about 60%
(n = 24) of the control tree nests, and about 70% (n = 45)
of the ground nests with high rates of begging calls were predated within 5
days. Leech and Leonard (1997)
obtained predation rates of more than 30% (n = 88) within 2 days in
control nests. Clearly, predation rates in both experiments (even in the
control nests, and thus in the absence of begging calls) are much higher than
in natural nests: during a 2-week period, predation would be very close to
100% at this rate. We must conclude that the experimental setup has introduced
a factor that increases predation rate, and it is in principle possible that
the predatory cost of begging only exists in the presence of this experimental
factor. Leech and Leonard
(1997) acknowledged that
predators probably followed their scent to the vicinity of the nests, and once
there located the experimental nest before the control one because of the
begging calls.
Briskie et al. (1999) have
shown that, in a forest community, the begging calls of species with higher
predation risks are more difficult to detect by predators. The authors
interpret this finding as evidence that begging increases the probability of
predation. However, it seems clear that the higher predation risk cannot be
the consequence of having less conspicuous begging calls. It may, of course,
be that predation risk imposes a selection pressure leading to the evolution
of less conspicuous calls. But this does not necessarily imply that, in their
present form, begging calls increase predation risk.
Let us now assume that the predatory cost of begging has been shown beyond
doubt. Can this cost stabilize begging? The answer is probably not, other than
in species that rear one young per brood. In multichick broods, if begging is
going to be stabilized by its cost, this cost must be higher for the chick
that begs than for its siblings (Godfray,
1995). This premise is almost certainly false in the case of a
predatory cost, since any predator that follows the begging calls all the way
to the nest will most likely eat all the chicks. So begging may have a
predation cost, but this predation cost is not the one hypothesized by the
models. To apply the models to species with multichick broods, we must be able
to show that begging has a direct cost—that the chick that begs incurs a
higher cost than its nest mates do.
Begging can also reduce fitness if the energy allocated to begging
interferes with other activities or growth. In terms of total energy
expenditure, begging has a very modest cost
(Bachman and Chappell, 1998;
Leech and Leonard, 1996;
McCarty, 1996;
Soler et al., 1999). The
question remains whether this low cost had a large enough effect on growth and
development to have deleterious consequences for expected fitness. To answer
this question we need to know how big a difference in mass must be present to
affect expected fitness. In a population of great tits living in a patchy
environment, a mass increase of 3 g in the mass of a fledging chick (15% body
mass) more than doubled (0.3 to 0.75) the probability that the bird obtained a
breeding territory in the rich habitat the following year
(Verhulst et al., 1997). In
the zebra finch, a difference of 1 g in the mass of young birds (45 days) was
equivalent to a difference of 0.3 eggs in the expected clutch size
(Haywood and Perrins, 1992).
And in carrion crows, a 12% difference in the body mass of a fledging shifted
its probability of being large enough to breed from 25% to 64% (Richner,
1989,
1992). From these data, it
seems clear that mass differences of approximately 10% are sufficient to have
a significant effect on fitness. In experiment 1, begging had no effect on
growth and (in all likelihood) expected fitness of the ring doves. Begging
had, on the other hand, an effect on the growth rates of magpie chicks that
was both statistically significant and biologically meaningful.
It is difficult to interpret the results of the ring dove experiment for a number of reasons. Although it could be argued that a larger sample size might reveal a difference in growth rates between the begging and control groups, a power analysis suggests that this is not the case. A difference of 2.6 g (3%) on day 13 was associated with a power of 90%, and we may therefore assume that control squabs were not more than 2.6 g heavier than their sibs were. (A 10% weight difference was associated with a power of 0.9996.) On the other hand, it is difficult to compare the begging levels of the squabs during the experiment with those of squabs growing under natural circumstances. Although young squabs begged with high intensity, as they grew older it became more and more difficult to keep squabs begging, and begging intensity decreased with age. (Squabs do not open their beaks when begging. They had to be held by hand in order to insert the syringe used for feeding them in their crop. It is possible that older squabs associated begging with a being held in the hand of the experimenter and therefore refrained from begging.) Indeed, on day 9 size differences between begging (62.3 ± 2.1 g, n = 8) and control (63.8 ± 2.8 g, n = 8) squabs reached their largest difference, although this difference was still nonsignificant (two-tailed paired t test: t7 = 1.168, p =.28).
Experiment 2 produced clearer results. Despite large inter-individual variability, begging significantly retarded growth of magpie chicks. After 2 days of treatment, control chicks were, on average, 1.6 g heavier than chicks in the begging group with the same diet. Hence, intensive begging resulted in a growth rate 0.8 g/day (or 8.15%) lower than what chicks could achieve in the absence of begging. A note of caution is required here. It should be noted that the treatment had two effects: control chicks had to beg less than their nest mates, but they also had more time to rest. After being fed (in the first visit of a meal), control chicks could simply go to sleep. Chicks in the begging group might stay alert while waiting for future visits. Because we did not record the behavior of the chicks between visits, we cannot quantify the time spent sleeping by chicks of the two groups. In principle, it is possible that control chicks slept more and that this difference in time spent sleeping is responsible for the different growth patterns.
Although it is possible that the difference between the outcomes of
experiments 1 and 2 was due to methodological problems, it might also reflect
species differences. In an experiment almost identical to experiment 2, Kedar
et al. (2000) did not find an
effect of begging on the growth rates of house sparrow (Passer
domesticus) nestlings. Because this experiment induced clear and
systematic differences on begging intensities, and because chicks were
stimulated to beg more than 70 times per day, the lack of effect of begging on
growth cannot be ascribed to a methodological problem. It seems, therefore,
that begging may retard growth in some circumstances, but not always. In this
respect, the effect of begging may be species specific, but it may also be
related to growing conditions, such as ambient temperature and feeding regime.
At any rate, the results presented in this paper show that begging can have a
cost of the sort that is normally assumed by theoretical models of signaling
of need. With respect to the controversy concerning the importance of the low
metabolic rate measured in begging chicks, our results also confirm the
suggestion that short-term studies of oxygen consumption during begging are
insufficient to study the fitness consequences of begging
(Verhulst and Wiersma, 1997).
To be sure, energy consumption is an important component and deserves to be
studied. But the measurements obtained must be integrated in the general
framework of chick physiology and development if we are to understand the
pathways through which begging exerts its cost.
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ACKNOWLEDGEMENTS |
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This work was funded by a Marie Curie Fellowship of the European Commission to M.A.R.-G. and by the project PB95-0110 of Dirección General de Investigación en Ciencia y Tecnología (DGICYT) to Fernando Alvarez. We thank Amber Hersema, Pedro J. Rodríguez-Gironés, and Barbara Roggema for helping feed the chicks, Tosca Boeré, Sjoerd Veenstra, and Roeli Wiegman for taking care of the breeding pairs, Amber Hensema for helping find magpie nests, Ton Groothuis, Arnon Lotem, John P. McCarty, Simon Verhulst, and Popko Wiersma for comments and discussion, and Peter Balsam for invaluable information about hand-rearing squabs. We thank the University of Granada for the use of the equipment from the Unidad de Producción y Experimentación Animal (Centro de Instrumentación Científica).
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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