J Comp Physiol B (2001) 171: 701±709
DOI 10.1007/s003600100230
O R I GI N A L P A P E R
Alexander S. Kitaysky á Evgenia V. Kitaiskaia
John C. Wing®eld á John F. Piatt
Dietary restriction causes chronic elevation of corticosterone
and enhances stress response in red-legged kittiwake chicks
Accepted: 31 July 2001 / Published online: 30 August 2001
Ó Springer-Verlag 2001
Abstract Release of corticosterone in hungry kittiwake
chicks facilitates begging and allows them to restore
depleted energy reserves by increasing parental food
provisioning. However, in order to avoid detrimental
eects of chronic elevation of corticosterone, chicks
might suppress adrenocortical activity in response to
prolonged food shortages. In this study we examined
temporal dynamics of corticosterone release in red-legged kittiwake (Rissa brevirostris) chicks exposed to
prolonged restrictions in energy content and/or nutritional quality (low versus high lipid content) of their
food. Starting at the age of 15 days, chicks were fed
either high- or low-lipid ®sh at 40%, 65%, and 100% of
ad libitum energy intake. Body mass measurements and
baseline plasma samples were taken on a weekly basis
after beginning of the treatment. After 3 weeks of
treatment, chicks were exposed to a standardized acute
handling and restraint stress protocol, where in addition
to a baseline sample, three plasma samples were taken at
intervals up to 50 min. We found that food-restricted
chicks had lower body mass, chronically (during 2±
3 weeks) elevated baseline and higher acute stress-induced levels of corticosterone compared to chicks fed ad
libitum. Low lipid content of food further exacerbated
these eects. An increase in baseline levels of corticosterone was observed within a week after energy requirements of food-restricted chicks exceeded their daily
energy intake. A tendency for suppression of adrenocortical activity was observed in treatments fed low-lipid
diets only at the end of the experiment. We suggest that
Communicated by L.C.-H. Wang
A.S. Kitaysky (&) á E.V. Kitaiskaia á J.C. Wing®eld
Department of Zoology, University of Washington,
Box 351800, Seattle WA 98195, USA
E-mail: kitaysky@u.washington.edu
J.F. Piatt
Alaska Biological Science Center, USGS,
1011 E. Tudor Rd., Anchorage AK 99503, USA
nest-bound chicks, if food-stressed, might suer deleterious eects of chronic elevation of corticosterone.
Keywords Corticosterone á Food-stress á
Nest-bound chicks á Seabirds
Introduction
Nest-bound chicks of several species of seabirds respond
to food shortages by increasing corticosterone secretion
(e.g., blue-footed boobies Sula nebouxii, Nunez-de la
Mora et al. 1996; black-legged kittiwakes, Rissa tridactyla, Kitaysky et al. 1999a). Release of corticosterone (at
concentrations similar to baseline levels of foodrestricted individuals and/or stress-induced levels of
corticosterone in individuals fed ad libitum) in hungry
kittiwake chicks allows them to restore depleted energy
reserves by facilitating begging and thereby increasing
parental food provisioning (Kitaysky et al. 2001).
However, chronic elevation of corticosterone at stressinduced levels can be detrimental (Sapolsky et al. 1986;
Sapolsky 1992; recent review in Welberg and Seckl
2001). In order to avoid possible detrimental eects of
chronic elevation of corticosterone, nest-bound seabird
chicks might suppress adrenocortical activity in response
to prolonged food shortages.
As a group, seabirds have evolved multiple behavioral and physiological traits allowing them to cope
with high variability in food resources (Lack 1968). It
might be bene®cial for seabird chicks to decrease corticosterone secretion in response to prolonged food
shortages. Seabird parents are not always able (or
willing) to compensate for a decrease in food availability by increasing their eort in foraging for the
young and chick provisioning with food may remain
low for extended periods of time (e.g., Kitaysky 1996;
Kitaysky et al. 2000). When this is the case, seabird
chicks are able to adjust their developmental rate to the
rate of energy provisioning by parents and ¯edge successfully despite severe food shortages (Kitaysky 1999).
702
It is not known whether reduced production and/or
release of corticosterone is characteristic for slowly developing under-nourished seabird chicks, but a recent
study of the domestic turkey (Meleagris gallopavo)
suggests that a long-term (4 weeks) nutritional stress
might cause a diminished adrenocortical activity (Carsia
and McIlroy 1998). A suppression of adrenocortical
response to an acute stressor was also documented in
adult black-legged kittiwakes breeding during long-term
food shortages (Kitaysky et al. 1999b). Elevated secretion of corticosterone can change metabolic pathways
so that nutritionally stressed individuals rely on catabolism of proteins to fuel their activities (Axelrod and
Reisine 1984; Le Ninan et al. 1988; Cherel et al. 1992).
As a short-term bene®t, a reduced production of corticosterone would allow under-nourished seabird chicks
to avoid loss of skeletal muscle proteins. As a long-term
bene®t, a reduced production of corticosterone would
allow slowly developing individuals to elude lasting
detrimental eects of chronically increased corticosterone such as a suppressed immune system and neuronal
cell death (Sapolsky et al. 1986; Sapolsky 1992; also
reviewed in Sapolsky et al. 2000).
The temporal dynamics of corticosterone release in
response to prolonged food shortages are not well studied. In the king penguin, Aptenodytes patagonica, 3- to 4month-old chicks are able to tolerate 100 days of natural
winter fast without a substantial increase in corticosterone secretion (Le Ninan et al. 1988). However, in seabirds
that provision their chicks with food in a continuous
manner, elevated levels of corticosterone in chicks were
documented within two days of fasting (Nunez-de la
Mora et al. 1996), and after 3 weeks of moderate food
restriction (Kitaysky et al. 1999a). Although increased
corticosterone secretion was shown to be strongly negatively correlated with endogenous fat reserves at the end
of 3 weeks food restriction in kittiwake chicks (Kitaysky
et al. 1999a), it remains to be shown whether nest-bound
seabird chicks experience chronic elevation of corticosterone during prolonged food-shortages.
The functional relationship between diet composition
and the physiological condition of young seabirds can be
measured using well-characterized responses of adrenocortical hormones to stress (Kitaysky et al. 1999a).
Controlled experiments have shown that food-related
stress can account for the lower body mass, depleted fat
reserves, and elevated baseline levels of corticosterone of
birds at ¯edging when compared to birds raised on ad
libitum nutritional regimes (Boag 1987; Kitaysky 1999;
Kitaysky et al. 1999a; Romano 2000). If baseline levels
of corticosterone appear normal, the pattern and extent
of corticosterone secretion following the application of a
standardized stressor, such as capture, handling and
restraint, indicates increased potential for stress (Wing®eld 1994). For example, in black-legged kittiwake
chicks, prolonged dietary restrictions enhance adrenocortical response to a standardized stressor even though
baseline levels of corticosterone appear normal (Kitaysky et al. 1999a).
In this study we examined the relationship between
energy intake, diet quality, baseline levels of corticosterone and standardized acute stress-response (sensu Wing®eld et al. 1992) in captive red-legged kittiwake (Rissa
brevirostris) chicks. The major goal of this study was to
assess temporal dynamics of corticosterone release in relation to the long-term alterations in quality or energy
content of diet and short-term energy demands of growing chicks. For comparison with a natural system, we also
measured adrenocortical response of wild red-legged
kittiwake chicks exposed to a standardized stressor.
Materials and methods
Partially incubated eggs of free-living red-legged kittiwakes were
collected on St. George Island in the south-eastern Bering Sea and
transported to the University of Washington facilities. Eggs were
incubated using Lyon incubators at 37.2°C and 56% relative humidity until hatching. Chicks were raised in individual nests at
30°C and 80% relative humidity (newly-hatched to 10 days posthatch), at 25°C and 70% relative humidity (10±25 days posthatch), and at outdoor ambient temperatures and humidity until
¯edging (42±55 days post-hatch). Chicks were hand-fed a mixture
of high-lipid forage ®sh given ad libitum until the experiment.
Starting at the age of 15 days post-hatch, either low quality ®sh,
rainbow smelt, Osmerus mordax (lipid to protein ratio, LPR=0.61,
hereafter called ``low-lipid diet''), or high quality silverside, Menidia menidia (LPR=1.47, hereafter called ``high-lipid diet'') were fed
to chicks for 21 days (Table 1). The energy content of silverside
was calculated from the results of proximate analyses of whole ®sh
(for details of the analysis see Romano 2000, and Kitaysky et al.
1999a). The energy content of rainbow smelt was also calculated
from the results of proximate analyses of whole ®sh (as reported by
a commercial provider, Shoreline). Chicks received either
565.1 kJ day±1 of high-lipid diet (80 g day±1, wet ®sh biomass),
353.2 kJ day±1 of high-lipid diet (50 g day±1), 356.3 kJ day±1 of
low-lipid diet (80 g day±1), or 222.7 kJ day±1 of low-lipid diet
(50 g day±1). Daily food intake of 7- to 8-week-old captive redlegged kittiwake chicks (n=5) fed high-lipid diet ad libitum was on
average 83.912.54 g day±1 (meanSD; A.S. Kitaysky et al.,
unpublished observations). Therefore, in this study chick energy
intake was at 40%, 65%, and 100% of the ad libitum ration. To
control for possible eects of dietary restrictions other than lipids
to protein ratio and daily energy intake, all chicks received an
excess of multi-vitamin/mineral supplement (one tablet of USP
quality Kirkland Signature on daily basis) in their food. To control
for possible eects of captivity on the stress-response of the experimental birds, free-living red-legged kittiwake chicks (similar to
the experimental birds' ages) were taken from their nests at St.
George Island and exposed to handling stress, and blood samples
were taken as described below.
Body mass of post-absorptive chicks was measured every
5 days. To compare chick's energy requirements with its daily energy intake during treatment, we used the equation [log10resting
metabolic rate (kJ day±1)=0.87+0.70log10body mass (g, wet)] experimentally derived for growing post-absorptive seabird chicks
(Kitaysky 1999). Metabolizable energy of forage ®sh used in this
study was assumed at 87% of total energy intake, as determined
earlier for growing black-legged kittiwake chicks (Romano 2000).
A blood sample of undisturbed (collected immediately after
taking a chick from the nest) post-absorptive (after overnight fast)
chicks was taken on weekly basis starting at the beginning of the
experiment. Blood samples were collected by puncturing the alar
vein and collecting blood in 100-ll heparinized hematocrit tubes.
At the end of the experiment, post-absorptive chicks were exposed
to a standardized acute handling and restraint stress protocol,
where in addition to the ®rst blood sample (baseline) three blood
samples (acute stress response) were taken at intervals of 10 min,
703
Table 1 Experimental protocols and body mass of red-legged kittiwake chicks (LPR lipid to protein ratio). LPR ratio for lantern®sh
(Stenobrachius sp.) from Van Pelt et al. (1997)
Diet composition
Silverside
Rainbow smelt
Silverside
Rainbow smelt
Lantern®sha
Lantern®shb
LPR
1.47
0.61
1.47
0.61
2.96
Food intake
(wet, g d±1)
80
80
50
50
?
Energy intake
(kJ d±1)
565.1
356.3
353.2
222.7
?
Body mass at 15 days old
Body mass at 35 days old n
Mean
SE
Mean
SE
181.8
177.2
185.3
187.4
±
12.4
13.0
10.9
9.6
±
338.0
237.5
262.0
188.9
339.2c
373.5c
3.5
4.9
3.7
2.0
42.2
11.5
5
5
6
5
5
2
a
30-day-old wild chicks infested with ticks
30-day-old wild chicks with no ticks
It was not possible to control for the post-absorptive condition of wild chicks and their body mass re¯ects weight of the bird as well as
mass of ingested food and feces
b
c
30 min, and 50 min after capture. All captive and wild chicks were
bled between 1000 hours and 1400 hours according to the following standardized technique. After each sample, blood ¯ow was
stopped by the application of cotton, and birds were placed individually into cloth bags. After blood collection, hematocrit tubes
were emptied into 0.5-ml vials, which were stored on ice (in the
®eld) or in a refrigerator at 4°C (in the laboratory). Blood samples
were centrifuged and plasma collected within 6 h.
Plasma samples were frozen at ±20°C until radioimmunoassay
analyses (for details see Wing®eld and Farner 1975; Wing®eld et al.
1992). The concentrations of corticosterone in plasma samples were
measured using 20 ll of plasma. Steroid concentrations were
measured in duplicate for each sample after extraction in 4 ml of
dichloromethane. Recovery values (ranging from 85% to 97%)
following extraction were used to adjust assayed concentrations of
steroids. All baseline plasma samples were processed in one assay,
and all plasma samples collected during stress-series were processed
in the second assay. Intra- and inter-assay coecients of variation
were 5% and 8%, respectively.
All chicks survived the experiment, were fed ad libitum after
dietary restriction was lifted, and 2 months after ¯edging were
transferred to Alaska SeaLife Center (Seward, Alaska).
were log10-transformed and examined again. All computations
were performed by using SYSTAT statistical package (Wilkinson
1992). Statistical signi®cance was assumed at P<0.05.
Results
Body mass and baseline levels of corticosterone were
similar among treatments prior to the experiment (F3, 17
=0.15, P=0.930 for body mass, Fig. 1; and F3, 17
=1.73, P=0.199 for baseline corticosterone, Fig. 2,
upper panel).
Eects of food-stress on growth of body mass
The diets signi®cantly aected body mass of chicks at
the end of treatment (daily food intake eect:
Statistical analyses
Body mass and baseline levels of corticosterone prior to the experiment were tested with one-way ANOVA, with treatment as a
factor. The eects of the experimental treatments on body mass of
chicks at the end of treatment were examined with two-way
ANOVA (followed by Tukey Post Hoc test for pairwise comparisons), where daily energy intake and quality of food (LPR) were
used as factors. The eects of daily energy intake and quality of the
food on temporal dynamics of baseline levels of corticosterone were
examined with repeated measures two-way ANOVA, where daily
energy intake and LPR were used as factors and a duration of
treatment as a repeated measure. The eects of the experimental
treatments on acute stress-induced levels of corticosterone were
examined with repeated-measures ANOVA, where experimental
treatments were used as factors and samples obtained from the
same chick during the acute stress protocol as repeated measures.
The eects of chicks' body mass on baseline levels of corticosterone
and stress response to handling (expressed as a maximal level of
corticosterone achieved during 50 min of restraint) were examined
by using regression analyses.
Wild chicks were infested with hard ticks (Exodes sp.) making a
statistical comparison with not-infested captives invalid. An eect
of tick infestation on baseline levels of corticosterone in wild chicks
was examined by using linear regression analysis where number of
ticks (found feeding on the individual chick) was used as an independent variable.
During statistical analyses, initial data were tested for assumptions required by parametric statistical tests according to
Sokal and Rohlf (1981). If these data violated assumptions they
Fig. 1 Growth rate of body mass (means, SE) of red-legged
kittiwake chicks in relation to daily energy intake (40%, 65%, and
100% of the ad libitum), diet composition (lipid to protein ration,
LPR: low-lipid=0.61, high-lipid=1.47), and duration of treatment
704
Fig. 2 Upper panel: baseline
levels of corticosterone (means,
SE) of red-legged kittiwake
chicks in relation to daily
energy intake (40%, 65%, and
100% of the ad libitum), diet
composition (LPR: low-lipid=0.61, high-lipid=1.47), and
duration of treatment. Lower
panel: estimated (see Materials
and methods) chicks daily
energy requirements (means,
SE; symbols are as in upper
panel) versus chicks daily
energy intake (indicated with
lines for each experimental
treatment, as in upper panel)
F1, 17=282.10, P<0.0001; LPR eect: F1, 17=547.70,
P<0.0001, Table 1). Also, there was a signi®cant eect
of interaction between daily food intake and quality
(LPR) of food on chick's body mass (F1, 17=13.58,
P<0.002). In particular, chicks that received
356.3 kJ day±1 of low-lipid diet were signi®cantly lighter
at the end of treatment compared to chicks fed
353.2 kJ day±1 of high-lipid diet (Tukey post hoc test:
P=0.001; Fig. 1).
Daily energy requirements versus daily energy intake
Energy requirements of chicks in the dierent treatments
exceeded their energy intake at dierent times after the
beginning of treatment (Fig. 2, lower panel). In particular, metabolizable energy of 50 g day±1 low-lipid diet
was lower than the chick's energy requirements immediately after the start of treatment (Fig. 2, lower panel).
Energy requirements of chicks fed either 80 g day±1 of
low-lipid or 50 g day±1 of high-lipid diets ®rst exceeded
metabolizable energy of their food during the 2nd week
of treatment (Fig. 2, lower panel). In contrast, metabolizable energy of 80 g day±1 high-lipid diet exceeded
energy requirements of controls during the treatment
(Fig. 2, lower panel).
Eects of food-stress on temporal dynamics
of baseline levels of corticosterone
The experimental treatments had a highly signi®cant
eect on baseline levels of corticosterone during treatment (daily food intake eect: F1, 17=14.26, P=0.002;
LPR eect: F1, 17=51.27, P<0.001; daily food in-
take´LPR interaction term: F1, 17=0.212, P=0.651,
Fig. 2, upper panel). Baseline levels of corticosterone
were signi®cantly elevated during the experiment in all
food-restricted chicks compared to controls (treatment
duration eect: F2, 34=11.39, P<0.001, Fig. 2, upper
panel). However, the elevation in baseline levels of corticosterone was signi®cantly higher in chicks that were
fed low-lipid diet than in chicks that were fed high-lipid
diet (treatment duration´LPR interaction term:
F2, 34=5.04, P=0.012, Fig. 2, upper panel). Also, an
elevation of baseline levels of corticosterone appeared
signi®cantly earlier in chicks that received 50 g day±1 of
low-lipid diet compared to chicks that were fed either
80 g day±1 of low-lipid or 50 g day±1 of high-lipid diets
(treatment duration´LPR´daily food intake interaction
term: F2, 34=3.71, P=0.035, Fig. 2, upper panel). In
particular, elevation in baseline levels of corticosterone
occurred during the 1st week in the most restricted
(50 g day±1 of low-lipid diet) treatment, and during the
2nd week in the two other food-restricted treatments
(Fig. 2, upper panel).
A decrease in baseline levels was observed between
day 14 and day 21 of the experiment in the low-lipid
treatments but not in the high-lipid treatments (repeated
measures ANOVA on baseline levels at days 14 and 21
of treatment; treatment duration´LPR interaction term:
F1, 18=7.7, P=0.012, Fig. 2, upper panel).
Acute stress-induced levels of corticosterone
in captive chicks
All captive chicks responded to an acute standardized
stressor with a rapid increase in secretion of corticosterone (Fig. 3, left panel).
705
Fig. 3 Adrenal response to a
standardized handling and
restraint stressor in captive
5-week-old red-legged kittiwake
chicks in relation to dietary
treatments (left panel), and in
wild red-legged kittiwake chicks
of ages similar to captives (right
panel)
Captive chicks fed low-lipid diet responded to acute
stress signi®cantly faster and achieved signi®cantly
higher levels of corticosterone than the chicks fed highlipid diet (Fig. 3, left panel; LPR eect: F1, 17=19.46,
p<0.001; acute stress eect: F3, 51=12.29, P<0.001;
acute stress´LPR interaction term: F3, 51=3.48,
P=0.028). The adrenal responses were statistically indistinguishable among the captive chicks that were fed
either 50 g day±1 or 80 g day±1 of the same diet (Fig. 3,
left panel; daily food intake eect: F1, 17=2.22,
P=0.155; daily food intake´LPR interaction term:
F1, 17=1.39, P=0.255; acute stress´daily food intake´LPR interaction term: F1, 17=1.76, P=0.174).
Baseline and acute stress-induced levels
of corticosterone in wild chicks
Degree of infestation with hard ticks was signi®cantly
positively correlated with baseline levels of corticosterone in wild red-legged kittiwake chicks (R2=0.926,
F1,5=62.8, P=0.001, Fig. 4).
All wild chicks responded to a standardized stressor
by an increased secretion of corticosterone (Fig. 3, right
panel), but the adrenal response of ®ve chicks infested
with ticks tended to be higher than that of two chicks
with no ticks (Fig. 3, right panel).
Body mass and circulating levels of corticosterone
Among the captive chicks the baseline and maximal
acute stress-induced levels of corticosterone were signi®cantly negatively correlated with chick body mass
(R2=0.29, F1, 19=7.6, P=0.013 and R2=0.25, F1,
19=6.46, P=0.02 for the baseline and maximal levels,
respectively).
Discussion
In this study we examined whether long-term experimental variations in energy content and quality of food
result in a chronically elevated corticosterone secretion
in red-legged kittiwake chicks.
We found that nutritionally stressed chicks had lower
body mass and chronically higher baseline levels of
corticosterone than chicks fed high-lipid food ad libitum. Baseline levels of corticosterone were elevated for a
longer period of time (3 weeks) in chicks that were restricted in both quantity and quality of food, and for
shorter period (2 weeks) in chicks that were fed either
reduced quantity or quality of food. Timing of an increase in baseline levels of corticosterone in general
re¯ected the time at which energy demands of foodrestricted chicks exceeded their energy intake. In particular, metabolizable energy of low-lipid diet given at
50 g day±1 was lower than the chick's energy requirements immediately at the beginning of treatment (Fig. 2,
lower panel), and a signi®cant increase of baseline levels
of corticosterone was observed during 1st week of the
experiment (Fig. 2, upper panel). Similarly, energy requirements of chicks fed 80 g day±1 of low-lipid and
50 g day±1 of high-lipid diets exceeded metabolizable
energy of their food during the 2nd week of the experiment and an increase in baseline levels of corticosterone
was observed during the 2nd week of treatment (Fig. 2,
upper panel). In contrast, metabolizable energy of highlipid diet given ad libitum exceeded energy requirements
of control chicks throughout treatment, which was re¯ected in consistently low baseline levels of corticosterone of those chicks (Fig. 2, upper panel). We conclude
that increased secretion of corticosterone in kittiwake
chicks re¯ects negative energy balance experienced by
those chicks. Increased secretion of corticosterone
706
Fig. 4 The relationship between infestation with ticks and baseline
levels of corticosterone in 5-week-old wild red-legged kittiwake
chicks (n=7)
probably allows under-nourished chicks rely on protein
catabolism to fuel their increased activity levels (e.g.,
begging rates and aggression; Kitaysky et al. 2001; A.S.
Kitaysky et al., unpublished observations) during food
shortages.
In birds, long-term eects of chronic elevation of
corticosterone during early neonatal development are
poorly known. However, chronic elevation of glucocorticoids during prenatal development has long-term
deleterious eects on the aected mammals (reviewed in
Welberg and Seckl 2001). Speci®cally, chronic elevation
of corticosterone (at concentrations similar to stress-induced levels) is known to suppress memory and immune
systems, promote wasting of muscle tissue, and cause
neuronal cell death (Sapolsky et al. 1986; Sapolsky 1992;
also reviewed in Wing®eld 1994; Sapolsky et al. 2000). It
is not known whether the observed chronic elevation of
baseline levels of corticosterone in food-restricted kittiwake chicks was high enough to cause long-term deleterious eects. However, the baseline levels of
corticosterone observed in low-lipid diet treatments at
the end of the 2nd week of the experiment were similar
to maximal levels of corticosterone achieved during
acute stress procedure by controls. Thus, the observed
chronic elevation of corticosterone was potentially
damaging and could cause long-term deleterious eects
on food-restricted kittiwake chicks. We recently con®rmed this prediction by showing an impaired cognition
of young kittiwakes exposed to nutritional stress and/or
moderate chronic elevation of corticosterone during
early development (A.S. Kitaysky et al., unpublished
observations).
At the end of the food-restriction period, baseline
levels of corticosterone decreased in chicks fed low-lipid
diet. A physiological mechanism(s) that would allow
chronically stressed chicks to reduce baseline levels of
corticosterone is not known. It is possible that either
clearance rate of plasma corticosterone was higher
among chicks on low-lipid diet, or long-term nutritional
stress reduced adrenal capacity or adrenal sensitivity
(e.g., Rees et al. 1985; Romero et al. 1998). However,
although baseline levels of corticosterone decreased between the 2nd and 3rd weeks of the experiment in lowlipid dietary treatments, they were still higher than in
controls. Moreover, chicks on low-lipid dietary treatments also had stronger and quicker adrenocortical
stress-response to a standardized acute stressor than
controls. Therefore, it is unlikely that either adrenal
capacity or sensitivity of chronically food restricted redlegged kittiwake chicks were impaired. Regardless of
causal factors, a temporal decrease in baseline levels of
corticosterone in nest-bound kittiwake chicks exposed to
prolonged food shortages might represent a mechanism
that allows aected individuals to ameliorate deleterious
eects of chronically high concentrations of corticosterone.
Among food-restricted red-legged kittiwake chicks,
low-lipid diet enhanced adrenocortical function compared to high-lipid diet. In particular, diets that were isocaloric but dierent in nutritional quality and fed to
chicks at 65% of ad libitum energy intake, resulted in
higher baseline and acute stress-induced levels of corticosterone among chicks raised on the low-lipid diet
compared to those among chicks raised on the high-lipid
diet. Similar results were obtained in an experimental
study of black-legged kittiwake chicks (Kitaysky et al.
1999a). Several studies of birds have established that the
amount of metabolizable energy is positively correlated
with lipids to proteins ratio of the diet (e.g., Buchsbaum
et al. 1986; Castro et al. 1989; Romano 2000). Romano
(2000) has found that black-legged kittiwake chicks are
able to metabolize 12% more energy from high-lipid
compared to low-lipid forage ®sh and, as a result, they
are able to accumulate large fat deposits if fed high-lipid
diets. Baseline and acute stress-induced levels of corticosterone re¯ect total body lipids in kittiwake chicks
(Kitaysky et al. 1999a); thus, it is not surprising that in
this study we found lower levels of corticosterone in redlegged kittiwake chicks fed high-lipid diet (relatively
high body mass) compared to chicks fed low-fat diet
(low body mass).
The results of this and earlier studies (Nunez-de la
Mora et al. 1996; Kitaysky et al. 1999a) suggest that low
endogenous energy reserves in nest-bound semi-precocial seabird chicks cause an elevation in baseline levels of
corticosterone and enhance corticosterone secretion
following the application of a standardized stressor. This
is in contrast to observations of chicks and juveniles of
altricial species of birds, where a relationship between
body condition and corticosterone secretion was not
found (Romero et al. 1998; Schwabl 1999; Sims and
Holberton 2000). It has been suggested that development of the hypothalamic-pituitary-adrenal axis and
consequently a chick's ability to respond to acute
707
stressors is associated with chick developmental modes
(Schwabl 1999; Sims and Holberton 2000). Speci®cally,
the hypothalamic-pituitary-adrenal axis probably becomes functional later in a chick's life in altricial compared to precocial species of birds. This would explain
the discrepancy in the results obtained for dierent
groups of birds.
Among birds that have precocial young, such as the
domestic fowl (Gallus gallus domesticus) and the mallard
(Anas platyrhynchos), the baseline plasma concentration
of corticosterone in chicks fed ad libitum does not
change signi®cantly during the ®rst 4 weeks after
hatching (reviewed in Holmes et al. 1990). In mallard
chicks fed ad libitum, a restraint for 30 min caused a
signi®cant increase in corticosterone concentration and
the adrenal stress response was stronger in the chicks
during ®rst 2 days post-hatch than between 3 weeks and
4 weeks of post-hatch (Holmes et al. 1990). Similarly, we
did not ®nd a signi®cant change in baseline levels of
corticosterone among black-legged and red-legged kittiwake chicks fed ad libitum between 2 weeks and
5 weeks of postnatal life (Kitaysky et al. 1999a; this
study). Thus, it is likely that the hypothalamic-pituitaryadrenal axis of precocial and semi-precocial chicks is
fully developed shortly after hatching. However, studies
of adrenocortical stress response of semi-precocial seabird chicks at early ages are needed to con®rm this
prediction.
Adrenocortical responses of precocial and semi-precocial chicks to nutritional stress and depleted energy
reserves are not uniform among dierent species. Freeman et al. (1981) found that reducing food intake to
75% of ad libitum causes an increase in plasma corticosterone concentration after 1 week of restriction in the
precocial chicks of domestic fowl. Increased levels of
corticosterone were also recorded in protein-restricted
young of the domestic chicken in vivo, and ACTH
challenged adrenal steroidogenic cells in vitro (McIlroy
et al. 1999; Carsia and Weber 2000). In semi-precocial
nest-bound chicks of blue-footed boobies, a short-term
food deprivation results in an increase of baseline levels
of corticosterone, which was associated with a low body
mass (Nunez-de la Mora et al. 1996). In nest-bound
chicks of semi-precocial black-legged and red-legged
kittiwakes, long-term dietary restriction also causes elevation of baseline levels of corticosterone and enhances
adrenocortical response to acute stress (Kitaysky et al.
1999a; this study). In contrast to the results of these
studies, Rees et al. (1985) found that adrenocortical responses of immature chicken to an acute stressor and
ACTH challenge were reduced by food deprivation.
Similarly, increased adrenocortical response to an acute
stressor was not associated with dietary restrictions in in
vitro study of precocial domestic turkey (Meleagris
gallopavo, Carsia and McIlroy 1998). Furthermore, in
semi-precocial young of American kestrels (Falco sparverius), baseline levels of corticosterone were not aected
and acute stress-induced levels of corticosterone were
reduced by long-term food-restriction (Heath and Dufty
1998). Thus, modulation of adrenocortical activity of
chicks in response to food restriction can re¯ect not only
precocial-altricial spectrum of chick development, but
other ecological, physiological and behavioral traits of a
particular species (e.g., temporal variability in food resources, rate of chick provisioning with food, metabolic
responses of chicks to food restrictions, and/or phylogenetic constraints (e.g., Kitaysky 1999). Also, in the
above-mentioned experimental studies, the duration of
food restriction ranged from 2 days to 4 weeks, and the
severity of dietary restriction ranged from complete food
deprivation to alterations of quality of diet fed ad libitum. As the results of this current study show, the
temporal dynamics of adrenocortical response to nutritional stress are complex, which might account for the
discrepancy in the results obtained by dierent studies of
related/same species of birds.
We are convinced that the results found in this study
were not merely a consequence of captive conditions,
but in fact re¯ect responses we could expect to see in
free-living red-legged kittiwakes. Other studies of adrenal response in young birds to a standardized acute
stressor did not indicate that those birds habituated to
frequent handling (Freeman et al. 1981; Dufty and
Beltho 1997; Heath and Dufty 1998; Kitaysky et al.
1999a). Baseline and acute-stress induced levels of corticosterone recorded for captive chicks in this study were
within the range observed in free-living red-legged kittiwake chicks. Furthermore, baseline and acute stressinduced levels of corticosterone in free-living (not tick
infested) and captive chicks fed ad libitum were similar,
whereas baseline and acute stress-induced levels of corticosterone were similar between nutritionally stressed
captive chicks and free-living chicks infested with ticks.
It is possible that tick infestation causes severe metabolic
challenges to young red-legged kittiwakes, as has been
reported for chicks of other species of birds (reviewed in
Duy 1983), which could result in enhanced adrenocortical function of tick-infested wild chicks similar to
under-nourished captive chicks. Finally, captive chicks
in all treatments were subject to similar controlled conditions. The only dierences between them were the
energy content and nutritional quality of their food.
Thus, it is reasonable to conclude that the results of this
study demonstrate dierences in the functional responses of red-legged kittiwake chicks to variations in
energetic content and nutritional quality of their food.
To the best of our knowledge, this paper represents
the ®rst experimental demonstration of the temporal
dynamics of adrenocortical response of nest-bound
semi-precocial chicks in relation to long-term dietary
restrictions. The results of this study suggest that for a
realistic assessment of adrenocortical response of chicks
to nutritional stress it is crucial to understand the temporal dynamics of corticosterone release in relation to
diet composition and a chick's energy requirements.
Non-manipulative observations and a single assessment
of adrenocortical activity at the end of long-term foodrestriction experiments might lead to equivocal results.
708
Speci®cally, our results indicate that there are three
phases of adrenocortical response of semi-precocial nestbound kittiwake chicks to long-term food shortages,
which are dierent from previously described phases of
fasting in young and adult birds (e.g., Cherel et al. 1988;
Le Ninan et al. 1988). Phase 1 ± initial elevation of
baseline levels of corticosterone, occurs shortly after
chicks started experiencing negative energy balance and
is associated with a decrease in growth of body mass;
phase II ± chronically elevated baseline levels of corticosterone, no change or slight decrease in body mass;
phase III ± a suppression of corticosterone secretion, no
change in body mass. An extent and duration of each
phase probably depends on a chick's ability to accumulate fat reserves prior to a food shortage, how severe
dietary restrictions are, and developmental and physiological responses to food shortages speci®c for a
particular group/species of birds.
Acknowledgements We thank Art Sowls and the members of ®eld
crews at St. Paul Island and St. George Island for their help with the
collection of eggs and blood samples in the ®eld, Kindree Brownbridge for great help with raising chicks, Morgan Benowitz-Fredericks for doing proximate analyses of ®sh. Two anonymous
reviewers and M. Benowitz-Fredericks provided useful comments
on earlier version of the manuscript. Financial support for this study
was provided by the NPMR University Alaska Fairbanks grant to
A.S.K., J.C.W. and J.F.P., by the Exxon Valdez Oil Spill Trustee
Council (Restoration Project no. 00479 to A.S.K. and J.F.P., and by
the U.S. National Science Foundation (grant no. OPP9530826) to
J.C.W. Logistic support was also provided by the Alaska Maritime
National Wildlife Refuge, USFWS. The experimental manipulations with birds were accomplished according to the rules of Laboratory Animal Care and Use Protocol of the University of
Washington, and under the federal and state collection permits.
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