Behavioral Ecology Vol. 11 No. 3: 274-281
© 2000 International Society for Behavioral Ecology
Responses of breeding Cory's shearwater Calonectris diomedea to experimental manipulation of chick condition
a Instituto da Conservação da Natureza, Rua Ferreira Lapa 38, 6[UNK], 1150 Lisboa, Portugal b Ornithology Group, Institute of Biomedical and Life Sciences, Graham Kerr Building, University of Glasgow, Glasgow G12 8QQ, UK c A Rocha Field Study Centre, Cruzinha, Apt 41, 8500 Mexilhoeira Grande, Portugal d Department of Zoology, Box 351800, University of Washington, Seattle, WA 98195, USA
Address correspondence to J. P. Granadeiro, Instituto da Conservação da Natureza, Rua Ferreira Lapa, 38, 6[UNK], 1150 Lisboa, Portugal. E-mail: granadeiroj{at}icn.pt .
Received 18 June 1999; revised 4 August 1999; accepted 31 August 1999.
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ABSTRACT |
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We studied the regulation of provisioning in Cory's shearwater at Selvagem Grande during the chick rearing period. Provisioning was examined in terms of feeding frequency and amount of food delivered to chicks. Two groups of chicks were subjected to short-term contrasting manipulations of their nutritional status: one group of chicks was given a food supplement of about 30 g, and another group was deprived of up to 30 g of food. Adults tending deprived chicks increased the frequency of feeding visits (but not the size of feeds), which resulted in an increase in the net rate of food delivery. At the end of this study, deprived chicks were growing at the same rate as fed chicks. Parents attending fed chick did not change their provisioning rates in response to the treatment. Our results indicate that Cory's shearwaters are able to adjust their provisioning rate in response to short-term variation in the nutritional status of their chicks. We also examined the change in the begging rate of fed and deprived chicks in response to the treatment. There was no relationship between the begging rate and the condition of chicks, which is taken to be a measure of the chick's physiological condition, related to its ability to withstand imposed periods of fasting. However, fed chicks decreased their begging rate after the increase in their condition due to supplementary food. Conversely, deprived chicks, which were only able to sustain their condition before the onset of the treatment, maintained high levels of begging. To some extent, these results suggest that parental provisioning can be influenced by the begging behavior of chicks.
Key words: Calonectris diomedea, chick provisioning, Cory's shearwater, experimental manipulation, parental behavior.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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The spatial and temporal variability of food resources in the marine environment is considered to be a major factor determining the life-history traits of seabirds (Lack, 1968
). During breeding episodes, seabirds have to resolve the
conflicting needs of visiting their nests regularly (to carry out incubation
or feed their brood) and maintaining their body condition above the level that
would compromise their future breeding attempts
(Stearns, 1992). Among
seabirds, Procellariiformes (albatrosses, shearwaters, and storm petrels) are
distinctive in that they visit the colony infrequently, presumably because
they forage over vast oceanic areas where the resources are believed to be
scarce and unpredictable in location
(Ashmole, 1971). This group of
seabirds exhibits extreme life-history traits, such as deferred maturity, low
fecundity, and high adult survival
(Warham, 1990). Additionally,
the slow-growing, semiprecocial chick accumulates large amounts of fat while
at the nest, which has been viewed as insurance against prolonged periods
without feeding (Lack, 1968).
These traits do suggest that for this group of pelagic seabirds, foraging
success at sea is often poor (Lack,
1968). However, most recent studies have acknowledged that
persistent unfavorable oceanographic conditions, resulting in prolonged
periods without any parental visits to the nest, are relatively infrequent
(Hamer and Hill, 1993;
Hamer and Thompson, 1997;
Ricklefs et al., 1985).
Additionally, the magnitude of fat accumulation by chicks is apparently in
excess of that needed to withstand normal fasting periods
(Ricklefs et al., 1980). These
observations have cast doubts on the view that such developmental traits are
directly connected to the scarcity and unpredictability of food resources at
sea (e.g., Bolton,
1995a,b;
Hamer, 1994;
Mauck and Grubb, 1995;
Ricklefs et al., 1985).
In this context, considerable attention has been paid to examining natural
patterns of food delivery to nestling petrels to determine the proximate
factors controlling their rates of food delivery
(Bolton, 1995b;
Granadeiro et al., 1998a;
Hamer and Hill, 1993;
Hamer and Thompson, 1997;
Ricklefs et al., 1985;
Weimerskirch et al.,
1997a,b).
Additionally, several studies have examined the parental provisioning response
after experimentally decreasing their foraging ability
(Mauck and Grubb, 1995;
Sæther et al., 1993;
Weimerskirch et al., 1995) or
after manipulating chick demands at the nest (e.g.,
Andersen et al., 1995;
Bolton, 1995a;
Hamer and Hill, 1994;
Ricklefs, 1987,
1992;
Weimerskirch et al., 1997a;
Hamer et al., 1998). The
results of these experimental studies revealed marked differences in the
capacity of different species to respond to changes in reproductive demands.
Indeed, while some studies indicated that adults are able to adjust their
effort in response to changes in chick demand
(Bolton, 1995b;
Granadeiro et al., 1999;
Hamer and Thompson, 1997,
Hamer et al., 1998;
Weimerskirch et al., 1995,
1997a), other studies
supported the idea of a relatively inflexible adult provisioning rate (Hamer
and Hill, 1993,
1994; Ricklefs,
1987,
1992). Similarly, brood
enlargement experiments have provided ambiguous evidence in relation to the
parental adjustment to offspring needs (review in
Ydenberg and Bertram, 1989).
This diversity in parental response casts doubt on the existence of a single
mechanism through which Procellariiformes may regulate food provisioning
(Weimerskirch et al.,
1997b).
The decisional processes by which petrels determine their levels of nest
visiting are still poorly understood, but there is increasing evidence that
adult condition plays a central role in these regulation processes (e.g.,
Chaurand and Weimerskirch,
1994; Lorentsen,
1996; Tveraa et al.,
1997; Weimerskirch,
1998; Weimerskirch et al.,
1997b). In some species, the duration of a given foraging trip
depends on the condition of the parent at the end of the previous one
(Chaurand and Weimerskirch,
1994; Weimerskirch et al.,
1994,
1997a,b;
see Ricklefs and Schew, 1994,
for a model). This emphasizes the need to examine the behavior of individual
adults and not simply the delivery rate experienced by chicks at the nest
(Granadeiro et al., 1998b;
Weimerskirch et al.,
1997b).
On the other hand, the role of offspring in determining the patterns of
food delivery at the nest is still unclear. In many avian species offspring
solicitation behavior conveys information on their current nutritional needs
(Cotton et al., 1996;
Kilner, 1995;
Kilner and Johnstone, 1997;
Ottoson et al., 1997). Parents
are apparently able to perceive the nutritional status of their chicks and can
adjust their provisioning rates accordingly (e.g.,
Ottoson et al., 1997). The
importance of begging behavior as a means of providing information on current
nutritional status of chicks has also been recognized in some seabird species
(e.g., Harris, 1983;
Henderson, 1975;
Iacovides and Evans, 1998;
Impekoven, 1971). Among
seabirds, Procellariiformes are particularly suitable to address hypotheses
relating levels of chick signaling to parental response. All species lay a
single-egg clutch, and they nest on islands where predation levels are
generally low. Hence, models of solicitation-mediated provisioning can be
analyzed without the complexities arising from sibling competition (e.g.,
Godfray, 1995;
Kacelnik et al., 1995) and
costs due to increased risks of predation (e.g.,
Haskel, 1994;
Leech and Leonard, 1997).
Surprisingly, the evidence for effective use of this type of signaling
among Procellariiformes is still scant. Ricklefs
(1992) did not record any
relationship between chick condition and vocal response to handling (which was
assumed to reasonably match begging behavior) in Leach's storm petrel
Oceanodroma leucorhoa chicks. If Procellariiformes do not respond to
chick begging calls, either this behavior does not convey any meaningful
information on the nutritional status of chicks, or parents are unable to
respond to it (Hussel, 1991;
Ricklefs, 1992).
Cory's shearwater Calonectris diomedea is a pelagic
Procellariiform breeding on subtropical northeast Atlantic islands and in the
Mediterranean (Cramp and Simmons,
1977). The aims of this study were to (1) examine the extent to
which breeding shearwaters were able to adjust the rates of food provisioning
in response to manipulated chick demand at the nest and (2) test whether
begging behavior was related to current chick needs. We adopted an
experimental protocol where two groups of chicks experienced opposing
manipulation of their body condition. One group was given a food supplement,
while the other group was deprived of part of the food provisioned by their
parents. We examined the response of parents to these manipulations in terms
of changes in amount of food delivered during the night and by variation in
feeding rate by each adult. Furthermore, we analyzed the changes in the
begging behavior of chicks under these contrasting treatments.
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METHODS |
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TOPABSTRACTINTRODUCTIONMETHODSRESULTSDISCUSSIONREFERENCES |
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Fieldwork was carried out on the island of Selvagem Grande (30°09' N, 15°52' W, approximately 300 km south of Madeira), between 12 August and 4 September 1997. The breeding population of Cory's shearwaters on Selvagem Grande (hereafter referred to as "Selvagem") is currently estimated at about 13,000 pairs (Mougin et al., 1996a
).
Hatching is relatively synchronous, and most chicks hatch between 18 and 31
July (Granadeiro, 1991). On 12
August, we randomly selected 32 burrows of Cory's shearwaters and equipped
them with an automatic detector. This monitoring system continuously logged
(in real time) all movements of adults in and out of the burrows (see
Granadeiro et al., 1998a, for
details on system design and reliability). The detection units, placed at the
entrance of the burrows, are only sensitive to the movements of a strong
magnetic field. Birds at Selvagem mainly return to the colony to feed their
chick early at night, but occasionally they arrive in late afternoon. Both
parents were captured by hand (under license) as they entered the burrows. We
equipped the adults of study nests with one small magnet (weighing < 1 g),
attached to a tail feather with cloth-backed adhesive tape. Animals without a
magnet, such as prospecting birds occurring in the area, did not trigger the
system. All 64 birds were ringed, and their sex was determined from calls
(Bretagnolle and Lequette,
1990) or bill measurements
(Granadeiro, 1993). The
polarity of the magnets deployed in males was opposite to that of females
(Granadeiro et al., 1998a),
which enabled identification of each member of a pair as they entered or left
the nest. Males and females received a distinctive paint mark on the forehead.
We used occasional observations of marked animals during the night to validate
records obtained by the automatic logger.
On 19 August, we weighed (to the nearest 1 g) and measured (to the nearest
mm) the wing length of study chicks, from which we estimated their ages to the
nearest 2 days (Granadeiro,
1991). On this date, a randomly selected group of 30 chicks from
an adjacent area was also weighed and acted as a control group. Chicks in the
control group were subsequently weighed on 24 and 28 August and 1 September.
On 22 August (hereafter designated as day 1), we began weighing study chicks
at 6-h intervals, starting at 1900 h, following the protocol proposed by
Ricklefs et al. (1985). This
routine was maintained each day until the morning of 4 September (day 14).
Chicks were weighed in the same order, and none of them regurgitated food
during handling. The weighing procedure took about 1 h to complete. We
computed growth rates of each chick as the slope of the equation obtained by
regressing weight at 1900 h each day upon date.
We assessed food delivery in terms of both chick feeding frequency (proportion of chicks receiving at least one feeding visit) and meal size (total amount of food received overnight by the chick on nights when food was delivered). The terms "single feed" and "double feed" are used to refer to the amount of food delivered by a single or by both parents respectively, and "nightly food delivery" is defined as the average amount of food received overnight, irrespective of chicks being visited or not (zero meals included).
We estimated meal size from the sum of positive mass increments recorded
between weighings (SUM; according to
Ricklefs et al., 1985). This
measure underestimates meal size because it does not account for losses due to
respiration and excretion. The rates of weight loss between weighings after
and before a meal was delivered to chicks were linearly related to chick
initial body mass and size of meal (but not age) by the following
equations:
Rate of weight loss before a meal (g/h) = 4.22 x 10-3 (initial weight) - 0.601 (F1,54 = 12.4, p <.001, r2 =.19)
Rate of weight loss after a meal (g/h) = 7.27 x 10-3 (size of meal) - 2.137 (F1, 4 = 4.5, p <.05, r2 =.34)
We corrected all nightly weight gains by the amount predicted to have been
lost according to these equations, assuming that feeds occurred halfway
between weighings (Bolton,
1995b; Hamer and Thompson,
1997).
Experimental feeding and food deprivation protocol
Chicks were randomly assigned to each of two treatment groups (n =
16 in each) at the start of the study period. Between days 1 and 7 (hereafter
designated as "control period"), we simply recorded chick weight
variation, as obtained from the 6-h periodic weighing. From day 8 to the end
of the study (hereafter referred to as "treatment period"), one
group of chicks was given a daily supplemental feed (under license). The feed
consisted of 30 ml (28.3 ± 4.0 g, n = 96, calculated from
chick weight before and after the supplementation) of a homogenized mixture of
120 g tinned sardine in oil, with 50 ml of vegetable oil, administered
directly into the proventriculus using a flexible 5-mm thick catheter. The
feeding procedure took about 30 s to complete, and none of the chicks
receiving the supplement regurgitated any food. The food supplement
corresponded approximately to 11 g of fat and 5 g of protein
(Bolton, 1995a) and represented
about 40-60% of the mean nightly food delivery
(Granadeiro et al., 1998b;
Hamer and Hill, 1993). The
amount of food supplemented to chicks was calculated so as to induce some
improvement in body condition without overburdening their digestive capacity
(Bolton, 1995a), which could
affect the ability to accept more food from their parents.
In the second group of chicks (hereafter referred to as
"deprived") we removed (under license) up to a maximum of 30 g of
food from the proventriculus (28.3 ± 8.6, n = 96, calculated
as above), also using a flexible catheter and a disposable syringe. The food
collected consisted mostly of stomach oil, but occasionally included a mixture
of partly digested fish and squid. Food deprivation was undertaken each day
after the morning weighings (0700 h), and supplement-fed chicks were given the
food supplement after the mid-day weighing (1300 h). During the control
period, neither the periodic weighing nor the installation of the logging
equipment produced any detrimental effect on the behavior of adults
(Granadeiro et al.,
1998b).
We calculated daily chick body condition by a linear regression of weight
at 1900 h upon age during the control and treatment periods (application of
nonlinear growth models did not significantly increase the variance
explained). The residuals, expressed as proportions of the predicted values,
were used as an index of body condition
(Bolton, 1995b;
Hamer and Hill, 1993).
Chick begging behavior
During both the control and treatment periods, we placed a small wire at
the entrance of study burrows, holding closed a coil-operated switch connected
to a portable audio tape recorder. As the birds entered their burrows the wire
was displaced, activating the recording devices. These audio recorders were
fitted with battery-amplified microphones, placed inside the burrow, and
recorded the sounds produced during the 45 min after activation, this limit
being set by the tape length. The mechanism was installed immediately after
the 1900-h weighing, when chicks had not yet been visited by any parents. Six
tape recorders were set each night (randomly, by treatment group), but not all
provided relevant data (some nests were visited by prospecting birds, which
triggered the system, and a few others were not visited at all). We also
installed small vertical wires at the entrance of some of the nests that were
not equipped with tape recorders so as to detect visits by any bird. At the
0100-h weighing, we changed all tape recorders that had been triggered to
other nests where entrance wires had not been displaced. This means that we
maximized the use of the available tape recorders, ensuring that the begging
behavior of chicks was only recorded during the first feeding visit. Feeding
events were easily identified by characteristic sounds, including persistent
begging calls and adult bill clapping, but especially the clearly identifiable
sound of a parent regurgitating food to the chick. Visits by prospecting birds
were easily distinguished from parental visits as they were generally short,
elicited only brief (< 1 min) begging behavior, and did not result in any
weight increase of chicks. Such visits were not recorded by the logger, and
these tape recordings were excluded from the analysis. During the course of
this study, we were able to record 39 feeding events from 23 different chicks.
All feeding episodes recorded in this study took more than 8 min to complete,
and chick begging was almost continuous while receiving food (see Results). We
calculated begging rate as the average number of calls per minute, during the
first 5 min after the start of begging behavior, only when there was
unequivocal evidence of feeding.
Data analysis
Most statistical tests were undertaken on a pairwise basis, using chicks as
their own controls to enhance power (Zar,
1996). The majority of the results presented consist of repeated
observations over the same individuals. To avoid the potential bias due to the
lack of independence of such data
(Hurlbert, 1984), where
appropriate the statistical tests were applied to mean values calculated for
each individual chick during the control and treatment periods. Throughout
this paper, we present means ± standard deviation, except where
otherwise stated.
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RESULTS |
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Growth and condition of chicks
There were no significant differences among mean weights of chicks in control, fed, and deprived groups before the manipulative protocol (one-way ANOVA: 19 August, F2,59 = 1.8; 24 August, F2,59 = 1.5; 28 August, F2,59 = 2.3; Figure 1). Subsequently, on 1 September (day 11), there was a significant difference among fed, deprived, and control mean weights (one-way ANOVA, F2,59 = 3.5, p =.035), deprived chicks being significantly lighter than both fed and control chicks as assessed by Student-Newman-Keuls post-hoc tests (Figure 1).
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During the control period (days 1-7), fed and deprived chicks did not
differ in their rates of mass acquisition
(Table 1). Overall, growth rate
in this period was 10.8 ± 20.7 g/day (n = 32 chicks). After
the onset of the treatment (days 8-13), the weight of chicks in the fed and
deprived groups diverged (Figure
1), indicated by a significant difference in rates of weight
increase (Table 1). We further
examined differences in group responses by excluding the first 2 days after
the start of the treatment (day 8 and 9), therefore allowing time for the
experimental protocol to influence the condition of chicks (following
Bolton, 1995a, and
Weimerskirch et al., 1997b).
In the last 4 days of the treatment period (days 10-13), there was no
significant difference between groups in chick growth rates
(Table 1).
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The experimental protocol resulted in an increase in the condition of supplement-fed chicks, and there was no change in the index of body condition of experimentally deprived chicks (Table 2), which suggests that adults attending deprived chicks were able to compensate for the effect of deprivation.
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Food delivery and feeding frequency
Between day 1 and day 7, there was no significant difference between days
in the proportion of nests visited (
2 = 5.4, df = 6). During
this control period, single and double feeds (control period only, data from
fed and deprived chicks pooled) were significantly larger than those
previously reported at Selvagem by Hamer and Hill
(1993; single meals: 73.7
± 26.4 g, n = 236, t test pooled variance estimates,
t = 8.0, df = 266, p <.0001; double meals: 142.6 ±
32.8 g, n = 44, t test separate variance estimate,
t = 2.7, df = 5, p <.05).
After the start of the treatment, deprived chicks experienced a significant increase in parental visiting frequencies, average nightly food delivery, and average meal size, whereas no changes were observed in fed chicks (Table 3). If the first 2 days after the onset of the treatment are excluded from the analysis, the average nightly provisioning rates of deprived chicks are about twice those of fed chicks (deprived: 120.4 ± 54.5 g, n = 16; fed: 62.4 ± 42.6 g, n = 16; t = 3.35, df = 30, p <.005). Visiting patterns of adults with experimentally deprived chicks varied significantly between control and treatment periods, whereas supplement-fed chicks did not experience any significant change (Table 4).
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Chick begging behavior
Feeding episodes, defined as the time elapsed between the start of active
chick begging and the end of solicitation behavior, took between 8 and 29 min
to complete (average = 15.6 ± 5.5 min, n = 39). During the
control period there was no relationship between begging rate and chick age
(Spearman rank correlation, r = -.46, df = 16). After the start of
the supplementary feeding protocol, there was a significant reduction in
begging rate of supplement-fed chicks. Deprived chicks did not alter their
begging behavior in response to the treatment
(Table 5). There were no
significant relationships between begging rate and either chick condition or
size of meal delivered in the same or in the next night (Spearman rank
correlation, r39 = -.15, r39 =.18 and
r39 =.12, respectively).
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DISCUSSION |
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Natural variability in provisioning strategy
In 1997, chicks at Selvagem were fed less frequently but received larger meals when fed than Cory's shearwater chicks studied previously (Granadeiro et al., 1998b
;
Hamer and Hill, 1993). Growth
rates reflected this irregular feeding; feeding rates were lower and more
variable between individual chicks than those reported in other years and from
other colonies (Granadeiro,
1991; Hamer and Hill,
1993; Klomp and Furness,
1992). The highly variable provisioning rates found in this study
resulted from the fact that parents were switching between short and long
foraging trips, as reported for some Procellariiformes in the southern oceans
(Chaurand and Weimerskirch,
1994; Weimerskirch et al.,
1994,
1997b;
Weimerskirch, 1998). On
Selvagem, Cory's shearwaters delivered more food after long trips than after
short trips (Granadeiro et al.,
1998b). By switching between these two modes of foraging, adults
were able to maintain an average nightly food delivery similar to that found
in other studies (e.g., Hamer and Hill,
1993), and probably foraged over a more extensive area. However,
this provisioning pattern implies a high variability in feeding rates, and
consequently chicks were exposed to increased risks of starvation. These
observations suggest that in 1997, food availability close to the colony was
below the level that would enable regular feeding visits
(Granadeiro et al., 1998b),
which are the norm in this species
(Granadeiro et al., 1998a;
Hamer and Hill, 1993;
Mougin et al., 1996b).
Adjustment of parental response to the need of the chick
After the onset of the manipulation protocol, adults attending the deprived
chicks exhibited a significant increase in their average nightly food delivery
and consequently in the total amount of food given to chicks (meal sizes).
During the last 4 days of our study, deprived chicks were receiving an average
of about 120 g of food each night from their parents, whereas supplement-fed
chicks were receiving about half of that amount (see Results). The difference
between these two values exactly matches the magnitude of the difference in
the treatment. The increase in chick provisioning rate was due to an increase
in both the frequency of parental feeding visits to the chick, and the amount
of food delivered (meal size; see Table
3). Conversely, adults attending supplement-fed chicks did not
seem to alter their behavior in terms of either frequency of visits or amount
of food delivered.
These observations argue that there is a compensatory response of parents in relation to the nutritional status of the chicks. This is further supported by the fact that experimentally deprived chicks maintained their body condition through increased provisioning by parents, and at the end of the treatment period did not differ in their rate of mass gain. This study was necessarily restricted to a short time period, and further investigation is necessary to determine whether adults would be able to maintain a fully compensatory response over a longer time.
The evidence for the ability of Procellariiformes to adjust their
provisioning rate according to the nutritional status of the chicks is still
equivocal. In these long-lived species, increased provisioning effort is only
likely to occur when parents do not incur increased risk of mortality
(Ricklefs and Schew, 1994;
Stearns, 1992;
Weimerskirch et al., 1997b).
In situations of decreased food availability, Cory's shearwaters exhibit a
dual foraging strategy, which probably represents a parental mechanism to
balance the demands of the chick with the maintenance of their own body
condition (Chaurand and Weimerskirch,
1994; Granadeiro et al.,
1998b; Weimerskirch,
1998; Weimerskirch et al.,
1997a). Because the treatment period was short, we could not
assess changes in the relative proportion of short and long foraging trips
exhibited by the adults. We would expect an increase in the proportion of
short trips by adults attending deprived chicks. These short trips appear to
be more productive to the chick
(Granadeiro et al., 1998b),
but they are probably undertaken at the expense of adult condition.
The results presented in this study indicate that Cory's shearwaters do
possess the ability to respond to experimentally induced increases in the
nutritional requirements of their offspring. That experimentally deprived
chicks consume larger meals could simply result from the deprived chicks'
ability to ingest larger quantities of food than controls and not from a
modification in adult provisioning behavior. However, the finding that adults
increased the frequency of visits to the nest provides good evidence that
adult provisioning behavior may be modified in response to short-term chick
requirements. This finding also implies that the rates of food delivery before
the experimental manipulation were submaximal. However, this conclusion is
difficult to reconcile with the suggestion that food supply close to the
colony was scarce, as indicated by the existence of alternating long and short
foraging trips (Granadeiro et al., 1998). Manipulation of chick condition was
carried out for a relatively short period, and it is not known whether adults
tending deprived offspring would have been able to maintain increased food
delivery rates in the long term. The finding that adults tending supplementary
fed chicks did not reduce their rates of food delivery is at variance with
other studies of closely related species (e.g.,
Bolton, 1995a;
Hamer and Hill, 1994;
Hamer et al., 1998;
Weimerskirch et al., 1997b)
and may be taken as evidence that adults were not having difficulty in
maintaining their own body condition because they did not respond to an
increase in the condition of their offspring by increasing their investment in
their own condition.
Hamer and Hill (1994)
failed to find evidence of regulation of food delivery to nestling Cory's
shearwaters in relation to the short-term requirements of chicks. These
authors also used a supplementary feeding protocol, consisting of the daily
provision of 60 g of fresh sardine fillets, and did not detect any change in
the frequency of visits or in the size of meals delivered. They recorded an
increase in the growth rate of supplemented chicks in relation to a control
group, but the differences in weight between groups were relatively small in
the first 6 days after the start of the feeding protocol
(Hamer and Hill, 1994).
Similarly, in our study the weight of fed chicks on day 11 did not differ from
that of a control group (Figure
1). These observations suggest that chicks given supplemental
feeding decreased their assimilation efficiency and possibly exhibited an
increase in the excretion rates in response to the supplement. Similar
abnormal reaction to supplemental feeding was recorded in great skua
Catharacta skua chicks given a lipid/protein mixture
(Hill and Hamer, 1994) and in
Leach's storm petrels supplemented with a high lipid diet
(Ricklefs, 1992).
Change in begging behavior of the chick
There was a change in the begging rate of fed chicks after the onset of the
experimental protocol, with supplemented chicks decreasing in their begging
rate. In view of the increase in the (total) average nightly food provisioning
from the control to the treatment period, these results probably indicate a
satiation effect (i.e., solicitation by chicks decreased as a response to an
increase in their body condition; see Table
5). However, we were unable to demonstrate a significant
relationship between measures of begging rate and chick body condition among
deprived chicks. The lack of relationship between solicitation and begging in
Procellariiformes had already been proposed by Ricklefs
(1992). Although apparently
absent in some seabird species (Ricklefs,
1992; Welham and Bertram,
1993; this study), the link between solicitation and parental
response has been observed in other seabird species, such as puffins
Fratercula arctica and Larus gulls (e.g.,
Harris, 1983;
Henderson, 1975;
Impekoven, 1971).
The lack of any increase in begging rate of deprived chicks could
alternatively be related to the fact that begging rate was at almost its upper
limit (about 1 call per 1.2 s). This is supported to some extent by similar
data collected in Berlenga Island, located about 10 km off the Portuguese
coast, where birds are known to feed their chicks frequently
(Granadeiro et al., 1998a). On
this island, Cory's shearwater chicks that were subjected to exactly the same
manipulative protocol as that described in this study (but were in
comparatively better condition), called less frequently (fed chicks: 28.6
± 13.1 calls/min, n = 6; deprived chicks: 40.2 ± 11.2
calls/min, n = 8; Granadeiro JP, unpublished data). In this study, we
did not consider the intensity of chick calls, but there were clear
differences between chicks in begging intensity, especially during the
treatment period, which could have been related to their condition.
The paucity of information on parent-chick interactions in
Procellariiformes is striking, especially in view of the fact that
vocalizations clearly represent a major communication channel in this group
(e.g., Bretagnolle, 1989;
Genevois and Bretagnolle,
1994; Naugler and Smith,
1992). Many species exhibit nocturnal habits ashore
(Warham, 1990), and vocal
clues are probably more important than visual information
(Bretagnolle, 1989). To our
knowledge, the only study examining relationships between begging intensity
and chick condition in petrels is that of Ricklefs
(1992) on Leach's storm
petrels. However, the behavior of chicks was obtained in response to handling
and not under natural stimulation, and this behavior probably does not match
the normal reaction of chicks to parental presence. In fact, in chicks older
than 7 days, the vocal response to handling is structurally different from the
begging calls (Naugler and Smith,
1992). Moreover, the rate and intensity of calls of handled chicks
appears to be mainly related to the magnitude of the disturbance rather than
to the nutritional status of the chicks
(Naugler and Smith, 1992).
There is a clear need to further investigate whether the nutrition-dependent
solicitation and solicitation-dependent foraging effort systems
(Hussel, 1991) occur among
Procellariiformes in different food availability contexts in order to
ascertain the extent to which chicks are able to influence parental rates of
food provisioning in this group of seabirds.
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ACKNOWLEDGEMENTS |
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Thanks are due to H. Costa Neves, Director of Parque Natural da Madeira, for granting access to the Selvagem Grande Nature Reserve. The Portuguese Navy kindly provided the transportation to and from the island. We also acknowledge Marco for his help with fieldwork and making our stay on the island very enjoyable. We are very grateful to A. Bruxelas for his enthusiastic help producing the detection units. M. D. Burns gave expert advice and expedited the production of the logging system components. P.Catry provided support and made constructive comments on an early version of the manuscript. This work was partly financed by Junta Nacional de Investigação Científica e Tecnológica, through a research grant to J.P.G. (BD/1283/95). M.S.C. was funded by a research grant from Fundação para a Ciência e a Tecnologia (BD/9356/96). R.W.F. was funded by IFOMA (International Fishmeal and Oil Manufacturers Association).
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REFERENCES |
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