The integration of energy and nitrogen balance in the hummingbird Sephanoides sephaniodes
Center for Advanced Studies in Ecology & Biodiversity and Departamento de Ecología, Facultad de Ciencias Biológicas, Pontificia Universidad Católica de Chile, Santiago 651-3677, Chile
* Author for correspondence (e-mail: fbozinov{at}genes.bio.puc.cl)
Accepted 25 June 2003
![]() |
Summary |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The maintenance nitrogen requirement of green-backed firecrowns determined by regression was 1.42 mg N day-1, yet they required nearly 10 mg N day-1 to maintain body mass. When arthropods were available, we observed that hummingbirds required approximately 150 fruit flies to maintain body mass, which corresponds to a 5% nitrogen diet. Interestingly, when nectar was restricted (to 4 ml day-1), or was absent, arthropods alone were not able to satisfy the body mass balance requirements of hummingbirds, suggesting that arthropods are not adequate as an energy source. In the group offered an 11.1% nitrogen diet, the size and surface of the small intestine, and liver and kidney mass increased in comparison with the control group (non-reproductive field hummingbirds) or the nitrogen-free group, suggesting a nitrogen overload. Our results are in agreement with other studies showing low nitrogen requirements by nectarivores. An important point to stress is that nitrogen digestibility declined in the 11.1% nitrogen diet, which strongly supports our nitrogen absorption saturation hypothesis.
Key words: nitrogen balance, energy balance, food quality, arthropod consumption, hummingbird, green-backed firecrown, Sephanoides sephaniodes, Chile
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Robbins (1993) calculated
the minimum maintenance nitrogen requirements (MNR) of domestic and wild birds
(430 mg N kg0.75 day-1). Recently, MNR values for three
small (Brice and Grau, 1991
;
McWhorter, 1997
;
McWhorter et al., in press
)
and two medium sized (McWhorter et al., in
press
) species of hummingbirds have been reported. Interestingly,
hummingbirds appear to exhibit lower MNR values than predicted by Robbins
(1993
). These low requirements
appear to be correlated with food habits, as similar low values have been
observed in nectarivorous honeyeaters and sunbirds
(Paton, 1982
;
Bradshaw and Bradshaw, 2001
;
Roxburgh and Pinshow, 2000
) as
well as in frugivorous birds and small mammals
(Howell, 1974
;
Smith and Green, 1987
;
Izahaki, 1992
;
van Tets and Nicolson, 2000
;
Roxburgh and Pinshow,
2000
).
Several studies have demonstrated that hummingbirds regulate their energy
balance on a daily scale and that any energy imbalance immediately affects
their behavior and body mass maintenance
(Hainsworth, 1978;
Wolf and Hainsworth, 1980
;
Calder et al., 1990
;
Martínez del Río and
Karasov, 1990
; McWhorter and
López-Calleja, 2000
;
Fernández et al., 2002
;
López-Calleja and Bozinovic,
2003
). Nevertheless, few studies have examined the protein balance
of hummingbirds over a longer period than a daily time scale. Brice and Grau
(1991
) observed that Costa's
hummingbirds had a reduced body mass after 5 days without feeding on a protein
source, suggesting that a lack of protein affects hummingbirds more slowly
than does a lack of energy.
In this study, we tested the effects of nitrogen intake on nitrogen
balance, and the relationship between nitrogen and energy balance in a medium
sized (ca. 6 g) hummingbird, the green-backed firecrown Sephanoides
sephaniodes. The green-backed firecrown is a migratory hummingbird that
visits the semi-arid Mediterranean-like environments of central Chile during
the austral fall and winter (Goodall et
al., 1956). The sugar concentration in floral nectar consumed by
this species ranges from 0.3 to 1.2 mol l-1
(Belmonte, 1988
;
Smith-Ramirez, 1993
), which is
within the range of previously reported sugar concentrations of nectars in
typical hummingbird flowers (Baker,
1975
; Hainsworth and Wolf,
1976
; Pyke and Waser,
1981
). Our study focused on four objectives: (1) to determine if
the MNR of this species is similar to that of other hummingbirds; (2) to
analyze the effect of different dietary nitrogen concentrations on nitrogen
and energy balance; (3) to determine if a diet composed exclusively of
arthropods allows hummingbirds to maintain energy balance; and finally (4) to
document the effect of varying dietary nitrogen concentration on digestive
anatomy and body composition.
![]() |
Materials and methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Nitrogen requirements with artificial diet
To determine nitrogen requirements, hummingbirds were randomly assigned to
five different groups, which were provided with the following dietary nitrogen
concentrations: N-0 (N-0), 0.96% (N-1), 1.82% (N-2), 4.11% (N-5) and 11.1%
(N-11). Diets were isocaloric by the interchange of protein and sucrose as
required. The amounts of minerals and vitamins were similar in the diets
(Table 1). During the
acclimation and experimental periods, diets were offered ad libitum
in 10 ml syringes, and were changed twice or three times a day to avoid
protein precipitation. Water was offered ad libitum in 3 ml syringes
and in plates that birds also used for bathing.
|
Food intake and body mass change
To determine food intake, birds were maintained for 11 days on each diet.
Volumes of food and water intake (ml day-1) were recorded 2-3 times
a day. To correct for evaporative losses, control feeders not accessible by
birds but containing the different diets and water were located just outside
the cages and recorded at the same intervals. Body mass changes were recorded
daily (before 07:00 and after 18:00 h) with an analytical balance
(accuracy±0.01 g; Acculab V-200, Edgewood, USA). Both food consumption
and body mass were recorded throughout the acclimation period.
Nitrogen and energy balance
Nitrogen (N) and energy balance were determined during the last 2 days of
the acclimation period, and during the 4th and 5th days
for the free access-diet group (see below). To determine nitrogen and energy
balance, early in the morning, birds were moved to identical experimental
cages, which were lined with plastic sheeting along the bottom and walls to
collect excreta. During daylight periods, excreta were collected hourly and
immediately frozen to minimize nitrogen loss (see below). Excreta from night
time were collected early in the morning with a rubber spatula and a small
amount of water. Food and water intake were also monitored. To determine
nitrogen and energy contents, respectively, samples of food and all collected
excreta were analyzed in duplicate using the micro Kjeldahl semi-automated
method for nitrogen determination and a computerized calorimeter (Parr 1261;
Moline, USA). Two replicates were determined to be ash free and reliable when
the difference between values was less than 1%. Considering that nitrogen
losses, especially gaseous ammonia, could be important during the collection
and drying process of excreta, we followed the protocol developed by Roxburgh
and Pinshow (2000), that is,
fresh excreta samples that were immediately frozen were compared with samples
that were frozen after 1 h of exposure to air on plastic sheets. These latter
samples contained 89.9±1.8% (N=5) of the nitrogen found in the
immediately frozen samples and nitrogen excretion. Data were corrected for
nitrogen losses using this value.
Nitrogen retention (i.e. assimilated nitrogen) was calculated as
Ni - total No (mg N day-1), where
Ni = N intake and No = N output. Assimilated N was
plotted against dietary N intake, and the regression for N balance was
calculated. Food intake and excreta produced during 24 h were measured to
estimate apparent assimilated mass coefficient,
AMC*=(Qi-Qe)/Qi,
where Qi and Qe are dry food intake
and excreta production rates in g day-1, respectively
(Karasov, 1990).
AMC* is apparent because birds mix urinary and fecal products in
the hindgut and eliminate them together via the cloaca
(Robbins, 1993
). We also
calculated digestible energy intake (DEi) and digestible nitrogen
intake (DNi). DEi was calculated as
QixGEixAMC* (kJ
day-1), where GEi is the gross energy content of the
diet, and DNi as
QixGNixAMC* (mg
day-1), where GNi is the gross nitrogen content of
diets. Thus, AMC* was calculated for both energy and nitrogen.
Organ masses
To evaluate the effects of protein intake on digestive organ morphology,
the mass and fat content of internal organs were determined after the nitrogen
balance trials were completed. Morphometric variables of birds from groups
N-0, N-2 and N-11 were compared with birds captured in the field during the
same period (winter). Animals were killed by cervical dislocation. We measured
organ size and mass according to Sabat and Bozinovic
(2000) and Konarzewski and
Diamond (1994
,
1995
). Wet length and nominal
area of the total intestine from the end of gizzard to the cloaca were
measured to the nearest mm. The nominal surface area of the small intestine
was determined by multiplying mean luminal circumference (measured at three
equidistant points) by length. Mesenteries and fat were removed prior to
measurement to ensure maximum extension when suspended from one end. Dry mass
(after removal of adherent fat) of carcass, heart, kidneys, liver, intestine
and gizzard were also determined after drying to constant mass in an oven at
65°C. Adherent and carcass fat of hummingbirds were measured according to
Konarzewski and Diamond (1994
)
except that rather than samples we used the entire bird. Ash of carcass was
determined by burning samples in a muffle oven for 3 h at 500°C. Lean
fresh muscular mass LFMM was calculated as carcass body mass minus the fat and
ash contents.
Nitrogen and body mass balance on simulated natural diets
To determine the effects of variation in nectar quantity on the number of
arthropods consumed we used fruit flies Drosophila melanogaster,
which are similar in size to insects consumed by hummingbirds in the field (M.
V. López-Calleja, unpublished data). We designed a sequential protocol
with three different experimental situations. First (Nect+FF group), over a
period of 5 days we determined the nectar/protein ratio used by six
green-backed firecrowns given fruit flies and N-0 nectar ad libitum.
During the pre-acclimation period, the photoperiod and thermal conditions were
similar to the artificial diet experiments described above. On the last day we
collected excreta using the same protocol as that in the artificial diet
experiment. In the second sequence (Nect4+FF group), we attempted
to determine if Mb balance would be maintained with a
restricted nectar diet. We used the same birds acclimated to eat fruit flies
and nectar, and during one day we offered a limited amount of nectar (3 ml
during early morning and 1 ml in the afternoon). In the final sequence (FF
alone group), and after birds had recovered Mb (2 days
later), we explored whether hummingbirds were able to maintain
Mb eating only fruit flies. Throughout all of these
sequences we recorded variation in Mb, the number of fruit
flies and volumes of nectar and water consumed, and finally the presence of
torpor during night. To measure fly consumption, we collected 500 live fruit
flies per hummingbird and released them into the experimental cages before the
light turned on in the morning. When the light turned off, we moved each bird
to its nocturnal cage and counted the remaining fruit flies.
Statistics
Statistical analyses were performed using the STATISTICA (1997) release 5
for Windows 95 (third edition; StatSoft, Inc., Oklahoma, USA). We used two-way
as well as repeated-measures analysis of covariance (ANCOVA) with body mass
(Mb) as a covariate. To satisfy the assumptions of these
parametric statistical methods, we transformed digestibility and fat data to
arcsine square root values (Zar,
1997). During the acclimation period, paired sample Student's
t-tests for related measurements with Bonferroni corrections were
used to test for daily changes in Mb, as well as in food
and energy intake. All results are reported as mean ± 1 standard error
(S.E.M.).
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
|
The volume of food (ml day-1) and energy (kJ day-1) intake were similar between birds on different diets at the beginning of the acclimation period (ANCOVA; F4,30=0.63, P=0.64, and F4,30=0.82, P=0.52, respectively), with a mean of 11.24±0.40 ml day-1 or 46.66±1.66 kJ day-1. At the end of the experiment, food and energy intakes showed significant variation as a function of dietary nitrogen content (Table 2).Hummingbirds in groups N-11 and N-5 consumed more food and gross energy in comparison to birds in the N-1 and N-0 groups. Energy digestibility was different between diets, with the N-11 diet exhibiting the lowest digestibility (Table 2). Digestible energy intake (DEi) followed the same general pattern as gross energy intake. Only the extreme diets were significantly different (N-0 versus N-11, Table 2). Interestingly, N-0 individuals reduced their food and energy intakes with respect to the observed initial values (Z=2.57, P=0.009 for both variables) and those of the N-11 group, despite the fact that they presented the highest energy digestibility (Table 2).Nitrogen intake and assimilation were significantly different between diets at the end of the acclimation period (Table 2), but nitrogen digestibility decreased in the most concentrated diet (Table 2).
The observed increase in Mb during daylight hours changed during the experimental period according to diet. Body mass increased on all diets during the first day (ANCOVA; F4,30=1.18; P=0.31), mean ± S.E.M. = 0.84±0.06 g day-1. At the end of the acclimation period, however, the diurnal increase in Mb was significantly different between groups (ANCOVA; F4,30=4.52; P=0.006). Body mass of birds in the N-11, N-1 and N-0 treatment groups increased at a higher rate than those in groups N-5 and N-2 (Table 2).
Body mass and food intake variation: diets with natural nitrogen
sources
All groups fed fruit flies showed similar Mb at the
beginning (Table 3), but this
pattern changed during the 5 days of experimentation. Hummingbirds in the
group fed N-0 nectar plus fruit flies ad libitum (Nect+FF) maintained
constant Mb (paired sample Student's t-test,
t5=0.77, P=0.49) and nectar intake (paired sample
Student's t-test, t5=-1.78, P=0.27)
during the experiment. Fruit fly intake increased during the second day
(paired sample Student's t-test, t5=-9.06,
P=0.001), but declined and stabilized from the third until the fifth
day (paired sample Student's t-test, t5=-2.86,
P>0.01). At the end of acclimation, nectar intake was
10.1±0.3 ml day-1, and fruit fly consumption was
148±73 flies day-1, representing an energy intake of 45.45
kJ day-1. Fruit flies represented approximately 9% of the total
energy consumed.
|
Hummingbirds in the Nect4+FF group decreased in Mb during daylight periods, however daily (24 h) changes in Mb were not significantly different from the Nect+FF group (Table 3). All hummingbirds assigned to the Nect4+FF group entered into torpor at night, and consumed more fruit flies in comparison to the Nect+FF group, but consumed less energy (Table 3).
Individuals maintained only with fruit flies (FF alone), actively caught and ate insects during the first hours of the experiment. After 4 h, birds changed their activity pattern, remaining perched with feathers ptiloerected, a behavior presumably to reduce thermoregulatory costs. Considering only the time while the birds were active, the number of fruit flies eaten per hour was 14.8±3.5, a value similar to that observed in the other two natural diets (Nect+FF and Nect4+FF, see Table 3). Nevertheless, hourly energy intake was significantly lower for FF group; in fact birds consumed only 12% of the energy consumed by individuals assigned to the Nect+FF treatment (Table 3).
Fig. 2 shows Mb change and nitrogen assimilation in both experimental and natural dietary treatments. Body mass changes in the Nect+FF group were lower in comparison to the N-0 experimental group (ANCOVA F5,34=8.24, P<0.0001, Fig. 2A). Nitrogen assimilation for the Nect+FF group was 4.55±0.24 mg N day-1 (or 148±7 fresh fruit flies), a value similar to that observed in individuals assigned to the N-2 and N-1 dietary groups (ANCOVA F5,34=65.10, P<<0.0001; see Fig. 2B for post hoc comparisons). Nitrogen digestibility on the natural diets was 78.8±6.4%, higher than that observed in all the experimental groups (see Table 2). The energy intake of the Nect+FF group was 45.45±1.15 kJ day-1 (Table 3), similar to that observed on the N-2, N-5 and N-11 experimental diets (nectar intake: ANCOVA, F6,34=5.28, P=0.001; energy intake: ANCOVA, F6,34=2.60, P=0.04; Table 3).
|
Nitrogen balance
The amount of N required for hummingbirds to maintain a positive N balance
was determined by a regression between the apparent N retention on N intake
for all 31 birds from all artificial dietary treatments
(Fig. 3). Considering that
nitrogen assimilation was significantly different between diets, and that
birds presented an apparent saturation of intake capacity on the highest N
diet (N-11, Table 2), we
selected the best curve fitting all data. Birds were in positive balance (MNR)
at an average of 1.42 mg N day-1
[y=-1.19+(59.16x/69.22+x),
r2=0.96, P=0.001] or 67.92 mg N
kg-0.75 day-1, corresponding to the experimental
nitrogen content of the N-1 diet (1.2% of dietary nitrogen).
|
Total endogenous nitrogen losses (TENL), measured directly (i.e. on a N-0
diet) were -1.33±0.16 mg N day-1 or 67.86±8.33 mg N
kg-0.75 day-1 (Table
2). In most studies, endogenous nitrogen losses are not measured
directly because few animals will consume nitrogen-free diets. The equation of
regression between the apparent N retention on N intake from all artificial
dietary treatments predicted a TENL of 1.19±0.51 mg N day-1
or 55.20±23.66 mg N kg-0.75 day-1. This value is
in agreement with the value measured directly. Nevertheless, when changes in
body mass during the entire experiments were regressed against daily nitrogen
intake (sensu Murphy,
1993), we estimated that 5.83±1.4 mg N day-1 was
necessary to maintain body mass for Sephanoides sephaniodes
(r2=0.93, P=0.001). This value was higher than
the values obtained by MNR or TENL, but is consistent with the observation
that animals in the N-2 group maintained body mass.
Energy balance
Digestible energy intake (DEi) was significantly different
between artificial and natural diets (Tables
2,
3). Animals assigned to N-0 and
N-1 decreased DEi and Mb. To confirm that the
volume of N affected DEi and Mb, we conducted a
linear regression of DEi against overall change in
Mb (Fig.
4), thus estimating that S. sephaniodes needs 43 kJ
day-1 to maintain Mb. This value is not
statistically different to the one in the experiments with fruit flies
(Nect+FF) and artificial diets N-2 and N-5.
|
Morphological effects of nitrogen intake
Body and organ masses are presented in
Table 4. The N-0 group clearly
exhibited lower body, carcass and free-fat carcass masses than birds in the
N-11 and control groups. Fat as a percentage of total body mass was higher in
birds in N-0 than N-11 and control groups. Feather mass was similar among all
groups. The masses and sizes of several organs changed significantly between
the different artificial dietary treatments as well as in comparison to the
control group (Table 4).Kidney,
gizzard and small intestine (mass and length) decreased in individuals
assigned to the N-0 diet in comparison to controls
(Table 4). An increase in liver
mass was observed only in hummingbirds from the N-11 group. Kidneys of N-0 and
N-2 individuals were lower in mass in comparison to controls
(Table 4).
|
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Nitrogen requirements
The minimal nitrogen requirement of Sephanoides sephaniodes
recorded in this study was 1.42 mg N day-1 or 65.87 mg N
kg-0.75 day-1. These values are similar to the measured
TENL (1.32 mg N kg-0.75 day-1 or 67.86 mg N
kg-0.75 day-1), which is the combination of endogenous
urinary and metabolic fecal nitrogen loss. These values represent the minimum
amount of nitrogen that an animal would need to replace in order to maintain a
positive nitrogen balance.
Both MNR and TENL values are approximately 15% of the values predicted by
Robbins (1993), 430 mg N
kg0.75 day-1 or 9.27 mg N day-1 for a bird of
this body size. Also, MNR and TENL were significantly lower than the N
requirements for Mb maintenance estimated according to
Murphy (1993
). Sephanoides
sephaniodes maintained Mb when fed either Nect+FF
(with fruit flies to fulfil their nitrogen requirements), or the artificial
N-2 diet. Interestingly, both diets exceed their MNR requirements. This
situation is similar to that reported in other studies. Brice and Grau
(1991
), Murphy
(1993
) and Roxburgh and
Pinshow (2000
) reported that
small nectarivorous birds were only able to maintain Mb on
diets that substantially exceed their MNR. Apparently a fair proportion of the
amino acid content of the food consumed does not exactly match the animal's
amino acid requirements (Robbins,
1993
). Birds are unable to synthesize 10 of the 20 or so
obligatory amino acids (Murphy,
1996
). Thus, diets with low nitrogen concentration may not match
the specific nutritional requirements of the species. As an example of this
mismatch, Bradshaw and Bradshaw
(2001
) proposed that a dietary
amino acid deficiency may explain the reduced rate of reproduction observed in
the honey possum Tarsipes rostratus. Alternatively, the difference
between MNR and Mb maintenance requirements could be
explained by sloughed skin or feathers or ammonia lost over respiratory
surfaces as suggested by Brice and Grau
(1991
) and Roxburgh and
Pinshow (2000
).
TENL and MNR values in S. sephaniodes are very similar to those
determined in other species of nectarivores (see
Roxburgh and Pinshow, 2000;
McWhorter et al., in press
).
Robbins (1993
) hypothesized
that animals feeding on liquid diets, which are also low in fiber and lipids
(such as floral nectars), probably have low nitrogen losses due to lower
secretion of protein digesting enzymes and bile acids, reduced sloughing of
intestinal epithelial cells and smaller populations of gut microorganisms.
Post-renal nitrogen recycling may explain the low MNR if hummingbirds are
reducing losses of nitrogen in urate excretion and by recycling the protein
associated with urate. Reduced urate in excreta may be the result of the
breakdown of urates by uricotelic bacteria
(McNabb et al., 1973
;
Dawson et al., 1991
;
Janes and Braun, 1997
).
Indeed, Roxburgh and Pinshow
(2002
) described a decrease in
urate concentration and proportionally more ammonium in the excreta in the
Palestine sunbird Nectarinia osea feeding on low nitrogen diets.
Other workers, by contrast, have related the change in the ammonia:urate ratio
to energetic constraints such as low temperatures rather than low dietary
nitrogen concentrations (e.g. Calypte anna,
Preest and Beuchat, 1997
;
Pycnonotus xanthopygos, van Tets
et al., 2000
). This explanation is based on a potential reduction
in the metabolic cost of nitrogen excretion by excreting primarily
ammonia.
The apparent nitrogen retention observed in S. sephaniodes feeding
on the more N-concentrated diets suggests a saturation in nitrogen absorption
capacity. The study of McWhorter et al.
(in press) documents a
nitrogen retention plateau in small hummingbirds feeding on high N diets. It
is unclear if this is a byproduct of experiments where captive hummingbirds
were fed artificial diets, because field data show that arthropods may be
intensively consumed during short foraging bouts
(Wagner, 1946
;
Hainsworth, 1977
;
Paton, 1982
;
Gass and Montgomerie, 1981
).
In the nectar of our experimental diets, nitrogen and carbohydrates are
combined, unlike the diet in the field. If a very high nitrogen content were
to increase gut transit time, then the daily volume of nectar that a
hummingbird could eat would be reduced, with potentially negative effects on
the daily energy balance. This explanation is in agreement with our
experimental observations. Indeed, hummingbirds preferred liquid diets with
lower nitrogen concentrations over more concentrated diets (M. V.
López-Calleja, unpublished data). The adaptation to liquid diets, low
in fiber and lipids, probably evolved together with a rapid gut transit time
and high digestive efficiency that are typical of hummingbirds
(Karasov, 1990
;
López-Calleja et al.,
1997
; McWhorter and
López-Calleja, 2000
). In general, the flow of the nectar in
feeding birds is directly from proventriculus into duodenum, bypassing the
gizzard altogether, whereas arthropods are diverted to the gizzard for
mechanical maceration and peptic digestion
(Klasing, 1998
). Moreover,
green-backed firecrowns have a large thin-walled crop, which always contained
more arthropods than in other sections of the digestive tract (M. V.
López-Calleja, unpublished data). We suggest that in natural conditions
hummingbirds can transport arthropods gradually into the stomach, thereby not
affecting carbohydrate absorption during the day.
Morphological effects
Body mass was lower in individuals from the N-0 group compared to
individuals in control and N-11 groups (see
Table 4), which may be
explained by a significant decrease in muscular mass (fat-free carcass). Fat
mass remained unchanged among groups, indicating that the decrease in
Mb and food consumption at the end of our experiments may
be explained by a nitrogen deficiency and not by an energy constraint.
Hummingbirds acclimated to N-0 diets or those with little nitrogen (N-0 and
N-2) had shorter and lighter small intestines than birds in N-11 and control
groups (see Table
3).Information about the effects of low dietary protein levels on
the morphology of digestive organs in birds is scarce
(Karasov, 1996). Nevertheless,
according to Karasov (1990
),
Dykstra and Karasov (1992
),
Piersma et al. (1993
) and
McWilliams et al. (1999
,
2001), the main adjustments in the digestive system that compensate for
decreases in digestive efficiency are an increase in gut length, mass and/or
volume (see also Bozinovic et al.,
1990
; Bozinovic,
1993a
,b
,
1995
). Thus individuals
maintained on a high dietary protein load, such as those in the N-11 group,
had longer and heavier intestines than the control group (field-caught
hummingbirds). Hummingbirds were presumably consuming both nectar and
arthropods before being captured, and arthropods contain indigestible matter,
such as chitin, which may reduce digestive efficiency. It is possible that the
dietary N levels experienced by the N-11 group reduced digestive efficiency to
a greater extent than natural arthropod diets.
The increases in liver and kidney masses in hummingbirds exposed to high
dietary nitrogen could be explained by an increase in the production and
excretion of nitrogenous waste products resulting from a high protein load.
Kidney hypertrophy has been documented in S. sephaniodes when exposed
to chronic cold environments and diluted diets of high protein concentration
(López-Calleja and Bozinovic,
2003), and in rodents confronted with high levels of protein in
their diets (Klahr, 1989
;
Hammond and Janes, 1998
).
Energy requirements: arthropods and nectar
Arthropods apparently cannot replace nectar as an energy source for
hummingbirds. Our results demonstrated that when nectar was available in low
abundance or was absent, S. sephaniodes consumed more fruit flies,
but the total energy obtained was significantly lower than that when nectar
was available (5-47% lower, see Table
3). Apparently, net energy gains when foraging for small
arthropods are lower than when feeding on nectar diets. If arthropod
consumption is limited by foraging constraints, then more larger prey items
could possibly contribute enough energy. Dietary information about trophic
preferences of S. sephaniodes during winter time indicates that the
sizes of their prey are similar to the fruit flies offered in our experiment.
Considering the energetic value of fruit flies, a green-backed firecrown would
need to consume nearly 1700 flies in order to meet the energetic requirements
observed in this study (more than 41 kJ day-1). Nevertheless, in
summer and at the southern limit of its distribution (near Puerto Williams,
Chile, 54° 56'S, 67°37'W), S. sephaniodes does
consume larger prey items than fruit flies (insects of 10-15 mm; R. Rossi,
unpublished data). Everything else being equal, this size of prey would
theoretically reduce the number of prey items required by hummingbirds in
order to meet their energetic requirements. Several authors have documented
that nesting hummingbirds consume more arthropods than they need to meet their
nitrogen requirements, and suggested that these are used for long-term heat
production during night time when food intake is suspended
(Montgomerie and Redsell,
1980), probably avoiding torpor during incubation. Our results,
however, indicated that S. sephaniodes obtains nearly 80% of its
energy requirements from nectar carbohydrates during the non-reproductive
period.
As Murphy (1996) indicated,
the amino acids obtained from food are used mainly for replacement of basal
losses or for synthesis of new tissues. Since the amino acids that are not
used immediately are quickly catabolized
(Heger and Frydrych, 1989
),
protein synthesis will require continuous availability of amino acids. Dietary
deficiencies of amino acids have severe effects in birds, including reduced
rates of protein synthesis and/or accelerated degradation, increased rates of
oxidation, and reduced rates of ingestion of food deficient in amino acids
(Murphy and Pearcy, 1993
). In
general, these limitations affect mass balance, and during chronic situations
compromise survival (Murphy,
1996
). In the N-0 treatment, S. sephaniodes dropped
nearly 15% in Mb and reduced food intake at the end of the
acclimation period. This pattern was not observed in previous work with
hummingbirds (Brice, 1992
;
Brice and Grau, 1991
) or other
nectarivores (Roxburgh and Pinshow,
2000
), where rates of nectar intake during a similar time span
remained constant even when the protein concentration was reduced to zero. In
both cases, birds lost 10-15% of Mb. Moreover,
hummingbirds in short term N balance studies maintain or increase
Mb (McWhorter et al.,
in press
). Why S. sephaniodes decreased food consumption
is not clear, but information from granivorous or carnivorous birds also
indicated a decrease in food consumption for individuals on protein-diluted
diets (Robbins, 1983; Murphy,
1996
). We hypothesize that nitrogen balance in hummingbirds is
regulated over a time scale of several days, as opposed to energy balance that
is regulated on a day by day basis (Wolf
and Hainsworth, 1980
). Skeletal muscle is likely to be the first
source of proteins used by birds exposed to a long-term absence of nitrogen
(at least 10 days). A loss in Mb attributed to muscle mass
would directly affect daily activity patterns, thereby compromising foraging
ability and energy balance in hummingbirds.
Finally, animals in general, and hummingbirds in particular, can face unpredictability in food availability and quality at different times and periods. Indeed, the availability of both nectar and arthropods change seasonally as well as daily. We show that during the non reproductive - non growing period, S. sephaniodes require nearly 150 fruit flies per day to satisfy their full nitrogen balance and to maintain body mass; however that amount of flies represents a negligible energy supply.
An important point to stress is that nitrogen digestibility declined in the N-11 diet, which strongly supports our nitrogen absorption saturation hypothesis. Clearly, more laboratory and field oriented studies are necessary to understand how and when arthropods are relevant in the physiological and behavioral ecology of hummingbirds.
![]() |
Acknowledgments |
---|
![]() |
References |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Baker, H. G. (1975). Sugar concentration in nectar from hummingbird flowers. Biotropica. 7, 37-40.
Baker, H. G. (1977). Non-sugar chemical constituents of nectar. Apidologie 8, 349-356.
Baker, H. G. and Baker, I. (1973). Nectar constitution and pollination - plant coevolution. In Plant Animal Co-Evolution (ed. L. E. Gilbert and P. H. Raven), pp.100 -140. Austin: University of Texas.
Bell, G. P. (1990). Birds and mammals on an insect diet: a primer on diet composition analysis in relation to ecological energetics. Stud. Avian Biol. 13,416 -422.
Belmonte, E. (1988). Características de la secreción de néctar en Eccremocarpus scaber R. et P. (Bignoniaceae) en relación a los hábitos de sus polinizadores.Tesis , Facultad de Ciencias, Universidad de Chile, Santiago, Chile.
Bent, A. C. (1940). Life Histories of North American Cuckoos, Goatsuckers, Hummingbirds and Their Allies. Washington DC: US National Museum Bulletin.
Bozinovic, F. (1993a). Fisiología ecológica de la alimentación y digestión en vertebrados: modelos y teorías. Rev. Ch. Hist. Nat. 66,375 -382.
Bozinovic, F. (1993b). Nutritional ecophysiology of the Andean mouse Abrothrix andinus: energy requirements, food quality and turnover time. Comp. Biochem. Physiol. 104A,601 -604.[CrossRef]
Bozinovic, F. (1995). Nutritional energetics and digestive responses of an herbivorous rodent (Octodon degus) to different levels of dietary fiber. J. Mamm. 76,627 -637.
Bozinovic, F., Novoa, F. F. and Veloso, C. (1990). Seasonal changes in energy expenditure and digestive tract of Abrothrix andinus in the Andes range. Physiol. Zool. 63,1216 -1231.
Bradshaw, F. J. and Bradshaw, S. D. (2001). Maintenance nitrogen requirement of an obligate nectarivore, the honey possum, Tarsipes rostratum. J. Comp. Physiol. B 171, 59-67.[Medline]
Brice, A. T. (1992). The essentiality of nectar and arthropods in the diet of the Anna's hummingbird (Calypte anna). Comp. Biochem. Physiol. 101A,151 -155.[CrossRef]
Brice, A. T. and Grau, C. R. (1991). Protein requirements of Costa's hummingbirds, Calypte costae. Physiol. Zool. 64,611 -626.
Calder, W. A., Calder, L. L. and Fraizer, T. D. (1990). The hummingbird's restraint: A natural model for weight control. Experientia 46,999 -1002.
Dawson, T. J., Maloney, S. K. and Skadhauge, E. (1991). The role of the kidney in electrolyte and nitrogen excretion in a large flightless bird, the emu, during different osmotic regimes, including dehydration and nesting. J. Comp. Physiol. B 161,165 -171.
Des Granges, J.-L. (1978). Organization of a tropical nectar feeding bird guild in a variable environment. Living Bird 17,199 -236.
Dykstra, C. R. and Karasov, W. H. (1992). Changes in gut structure and function of house wrens (Troglodites aedon) in response to increased energy demands. Physiol. Zool. 65,422 -442.
Fernández, M. J., López-Calleja, M. V. and Bozinovic, F. (2002). Interplay between foraging energetics and thermoregulatory cost in hummingbirds. J. Zool. (Lond.) 258,319 -326.[CrossRef]
Gass, C. L. and Montgomerie, R. D. (1981). Hummingbird foraging behavior: decision-making and energy regulation. In Foraging Behavior: Ecological, Ethological and Psychological Approaches (ed. A. C. Kamil and T. D. Sargent), pp.159 -194. New York: Garland STMP Press.
Goodall, J. D., Johnson, A. W. and Phillippi, R. A. (1956). Las Aves de Chile, su Conocimiento y sus Costumbres. Buenos Aires: Platt Establecimientos gráficos.
Gottsberger, G., Schrauwen, J. and Linskens, H. F. (1984). Amino acids and sugars in nectar, and their putative evolutionary significance. Plant Syst. Evol. 145, 55-77.
Hainsworth, F. R. (1977). Foraging efficiency and parental care in Colibri coruscans. Condor 79,69 -75.
Hainsworth, F. R. (1978). Feeding: models of costs and benefits in energy regulation. Am. Zool. 18,701 -714.
Hainsworth, F. R. and Wolf, L. L. (1976). Nectar characteristics and food selection by hummingbirds. Oecologia 25,101 -114.
Hammond, K. A. and Janes, D. N. (1998). The
effects of increased protein intake on kidney size and function. J.
Exp. Biol. 201,2081
-2090.
Heger, J. and Frydrych, Z. (1989). Efficiency of utilization of amino acids. In Absorption and Utilization of Amino Acids (ed. M. Friedman), pp.31 -56. Boca Raton, Florida: CRC Press.
Howell, D. J. (1974). Bats and pollen: physiological aspects of the syndrome of chiropterophyly. Comp. Biochem. Physiol. 48A,263 -276.[CrossRef]
Izahaki, I. (1992). A comparative analysis of the nutritional quality of mixed and exclusive fruit diets for Yellow-vented Bulbuls. Condor 94,912 -923.
Janes, D. N. and Braun, E. J. (1997). Urinary protein excretion in red jungle fowl (Gallus gallus). Comp. Biochem. Physiol. 118A,1273 -1275.[CrossRef]
Karasov, W. H. (1990). Digestion in birds: chemical and physiological determinants and ecological implications. Stud. Avian Biol. 13,391 -415.
Karasov, W. H. (1996). Digestive plasticity in avian energetics and feeding ecology. In Avian Energetics and Nutritional Ecology (ed. C. Carey), pp.61 -84. New York: Chapman and Hall.
Klahr, S. (1989). Effects of protein intake on the progression of renal disease. Annu. Rev. Nut. 9, 87-108.[CrossRef][Medline]
Klasing, K. (1998). Comparative Avian Nutrition. New York: CAB International.
Konarzewski, M. and Diamond, J. (1994). Peak sustained metabolic rate and its individual variation in cold-stressed mice. Physiol. Zool. 67,1186 -1212.
Konarzewski, M. and Diamond, J. (1995). Evolution of basal metabolic rate and organ masses in laboratory mice. Evolution 49,1239 -1248.
Law, B. S. (1992). The maintenance nitrogen requirements of the Queensland blossom bat (Syconycteris australis) on a sugar/pollen diet: is nitrogen a limiting resource? Physiol. Zool. 65,634 -648.
López-Calleja, M. V. and Bozinovic, F. (1995). Maximum metabolic rate and aerobic scope in the small-sized Chilean hummingbirds Sephanoides sephanoides. Auk 112,1034 -1036.
López-Calleja, M. V., Bozinovic, F. and Martínez del Río, C. (1997). Effects of sugar concentration on hummingbird feeding and energy use. Comp. Biochem. Physiol. 118A,1291 -1299.[CrossRef]
López-Calleja, M. V. and Bozinovic, F. (2003). Dynamic energy and time budgets in hummingbirds: a study in Sephanoides sephaniodes. Comp. Biochem. Physiol. 134A,283 -295.
Martínez del Río, C. (1994). Nutritional ecology of fruit eating and flower visiting birds and bats. In The Digestive System in Mammals: Food Form and Function (ed. D. J. Chivers and P. Langer), pp.102 -127. Cambridge: Cambridge University Press.
Martínez del Río, C. and Karasov, W. H. (1990). Digestion strategies in nectar- and fruit-eating birds and the sugar composition of plant rewards. Am. Nat. 136,618 -637.[CrossRef]
McNabb, F., McNabb, R. and Steeves, H. R. (1973). Renal mucoid materials in pigeons fed high and low protein diets. Auk 90,14 -18.
McWhorter, T. J. (1997). Energy assimilation, protein balance, and water absorption in broad tailed hummingbirds, Selasphorus platycercus. MS thesis, University of Wyoming, Laramie, USA.
McWhorter, T. J. and López-Calleja, M. V. (2000). The integration of diet, physiology, and ecology of nectar-feeding birds. Rev. Ch. Hist. Nat. 73,451 -460.
McWhorter, T. J., Powers, D. R. and Martínez del Río, C. (in press). Are hummingbirds facultatively ammonotelic? Nitrogen excretion and requirements as a function of body size. Physiol. Zool.
McWilliams, S. R. and Karasov, W. H. (2001). Phenotypic flexibility in digestive system structure and function in migratory birds and its ecological significance. Comp. Biochem. Physiol. 128A,579 -593.
McWilliams, S. R., Caviedes-Vidal, E. and Karasov, W. H. (1999). Digestive adjustments in cedar waxwings to high feeding rates. J. Exp. Zool. 283,394 -407.[CrossRef]
Montgomerie, R. D. and Redsell, C. A. (1980). A nesting hummingbird feeding solely on arthropods. Condor 82,463 -464.
Murphy, M. E. (1993). The protein requirement for maintenance in the white-crowned Sparrow, Zonotrichia leucophrys gambelii. Can. J. Zool. 71,2111 -2130.
Murphy, M. E. (1996). Nutrition and Metabolism. In Avian Energetic and Nutritional Ecology (ed. C. Carey), pp. 31-60. New York: Chapman and Hall, ITP.
Murphy, M. E. and Pearcy, S. D. (1993). Dietary amino acid complementation as a foraging strategy for wild birds. Physiol. Behav. 53,689 -698.[CrossRef][Medline]
Paton, D. C. (1982). The diet of the New Holland honeyeater, Phylidonyris novaehollandiae. Aust. J. Ecol. 7,279 -298.
Peaker, M. (1990). Nutritional requirements and diets for hummingbirds and sunbirds. Int. Zoo. Yb. 29,109 -118.
Piersma, T., Koolhaas, A. and Dekinga, A. (1993). Interactions between stomach structure and diet choice in shorebirds. Auk 110,552 -564.
Preest, M. R. and Beuchat, C. A. (1997). Ammonia excretion by hummingbirds. Nature 386,561 -562.[CrossRef]
Pyke, G. H. (1980). Why hummingbirds hover and honeyeaters perch. Anim. Behav. 29,861 -867.
Pyke, G. H. and Waser, N. M. (1981). The production of dilute nectars by hummingbirds flowers. Biotropica 13,260 -270.
Remsen, J. V., Stiles, F. G. and Scott, P. E. (1986). Frequency of arthropods in stomachs of tropical hummingbirds. Auk 103,436 -441.
Robbins, C. T. (1993). Wildlife Feeding and Nutrition, 2nd edition. New York: Academic Press.
Roxburgh, L. and Pinshow, B. (2000). Nitrogen requirements of an Old World nectarivore, the orange-tufted sunbird Nectarinia osea. Physiol. Biochem. Zool. 73,638 -645.[CrossRef][Medline]
Roxburgh, L. and Pinshow, B. (2002). Ammonotely
in a passerine nectarivore: the influence of renal and post-renal modification
on nitrogenous waste product excretion. J. Exp. Biol.
205,1735
-1745.
Sabat, P. and Bozinovic, F. (2000). Digestive plasticity and the cost of acclimation to dietary chemistry in the omnivorous leaf-eared mouse Phyllotis darwini. J. Comp. Physiol. 170,411 -417.[CrossRef]
Smith, A. P. and Green, S. W. (1987). Nitrogen requirements of the sugar glider (Petaurus breviceps), an omnivorous marsupial, on honey-pollen diet. Physiol. Zool. 60, 82-92.
Smith-Ramirez, C. (1993). Los picaflores y su recurso floral en el bosque templado de la Isla de Chiloé, Chile. Rev. Ch. Hist. Nat. 66,65 -73.
Stiles, F. G. (1995). Behavioral, ecological and morphological correlates of foraging for arthropods by the hummingbirds of a tropical wet forest. Condor 97,853 -878.
van Tets, I. G. and Nicolson, S. W. (2000). Pollen and nitrogen requirements of the lesser double-collared sunbird. Auk 117,826 -830.
van Tets, I. G., Korine, C., Roxburgh, L. and Pinshow, B. (2000). Changes in the composition of the urine of Yellow-vented Bulbuls (Pycnonotus xanthopygos): The effects of ambient temperature, nitrogen, and water intake. Physiol. Biochem. Zool. 74,853 -857.[CrossRef]
Wagner, H. O. (1946). Food and feeding habits of Mexican hummingbirds. Wilson Bull. 58, 69-132
Wolf, L. L. (1970). Impact of seasonal flowering on hummingbirds. Condor 72, 1-14
Wolf, L. L. and Hainsworth, F. R. (1980). Economics of foraging strategies in sunbirds and hummingbirds. In Behavioral Energetics: The Cost of Survival in Vertebrates (ed. W. Aspey and S. I. Lustick), pp.223 -264. Ohio State University Press, Columbus, Ohio.
Zar, J. H. (1997). Bioestadistical Analysis. 3th edition, New Jersey: Prentice Hall Inc.
Related articles in JEB: