Altitudinal variation in parental energy expenditure by white-crowned sparrows
1 Department of Avian Sciences, University of California, Davis, CA 95616,
USA
2 Department of Animal Science, University of California, Davis, CA 95616,
USA
3 Point Reyes Bird Observatory, Stinson Beach, CA 94970, USA
4 Biology Department, Occidental College, Los Angeles, CA 90041,
USA
* Author for correspondence (e-mail: wwweathers{at}ucdavis.edu)
Accepted 14 June 2002
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Summary |
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Key words: field metabolic rate, reproductive effort, parental effort, white-crowned sparrow, Zonotrichia leucophrys
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Introduction |
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Most DEE studies in which temperature seemed unimportant used air
temperature data obtained from meteorological stations located 8-50 km from
the study site (e.g. Bryant et al.,
1985; Bryant and Westerterp,
1980
,
1983
;
Bryant and Tatner, 1988
;
Moreno, 1989
;
Tatner, 1990
;
Deerenberg et al., 1995
).
Given that environmental temperature can vary by 10°C with minor changes
in location or posture (Mahoney,
1976
; Mugaas and King,
1981
), finding a weak (or no) correlation between DEE and remotely
measured air temperature seems inconclusive. In addition, air temperature
alone may provide an inadequate index of the thermal potential driving heat
exchange (Campbell and Norman,
1998
). Standard operative temperature (Tes),
which incorporates wind and radiation effects on endotherm heat transfer
(Bakken, 1980
,
1990
), should usually provide a
more reliable assessment of a bird's actual thermal environment
(Piersma and Morrison, 1994
),
yet it has seldom been measured in DEE studies.
In this study, we measured DEE during the incubation and nestling stages in
two subspecies of white-crowned sparrow (Zonotrichia leucophrys) that
encounter very different thermal environments. One subspecies, Nuttall's
white-crowned sparrow (Z. l. nuttalli), is a permanent resident of
California's narrow coastal fog-zone between approximately 34 and 40°N
latitude. It inhabits characteristically low, wind-swept terrain, often on
sea-facing hillsides dominated by California sage (Artemesia
californica) and coyote bush (Baccharis pilularis). The other
subspecies, the mountain white-crowned sparrow (Z. l. oriantha), is
an intracontinent migrant that breeds in the high mountains of the western
United States and winters from the extreme southwestern United States and Baja
California south as far as the Mexican states of Michoacan and Tamaulipas
(American Ornithologist's Union,
1998). Compared with the climatically mild coastal environment of
Z. l. nuttalli, the high-altitude, montane habitat of Z. l.
oriantha is harsh and unpredictable. Late spring snowstorms often delay,
disrupt or devastate the breeding attempts of Z. l. oriantha, with
both adults and young often exposed to freezing temperatures, especially at
night. We assessed each population's thermal environment concurrently with our
DEE measurements, and used the field temperature assessments together with
laboratory measurements of resting metabolic rate to evaluate DEE.
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Materials and methods |
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These populations occur at the same latitude (approximately 38°N),
breed about the same time of year (although Z. l. nuttalli usually
starts approximately a month earlier than Z. l. oriantha), but differ
somewhat in the details of their breeding effort. Single brooding is the rule
in Z. l. oriantha (Morton,
1976, whereas Z. l. nuttalli at PRBO are commonly
double-brooded, individuals averaging 2.0 clutches per season
(Mewaldt and King, 1977
;
Baker et al., 1981
). Clutch
size averages 18% higher at TPM (3.86, N=1154);
Morton, 2002
) than at PRBO
(3.27, N=170; N. Nur, G. Geupel and D. DeSante, in preparation). The
difference in clutch size implies higher instantaneous reproductive effort
among Z. l. oriantha; the difference in brood number suggests greater
cumulative reproductive effort in Z. l. nuttalli. Finally, male
Z. l. nuttalli do not regularly begin to assist the female in feeding
the young until they are approximately 4 days old
(Blanchard, 1941
), whereas male
Z. l. oriantha usually begin to assist on the day of hatch, but do
not feed as often as the female until the young are 3 days old
(Morton et al., 1972a
). During
the incubation stage, male white-crowned sparrows sing, engage in other
territorial behaviors and may accompany the female while she forages, but they
do not incubate the eggs. Accordingly, all references to `incubating males'
refer to the stage of the breeding cycle, not the male's specific
activity.
Doubly labeled water
We measured rates of CO2 production and water flux of adult
sparrows incubating eggs or feeding 5- to 7-day-old nestlings using either the
single- or double-sample doubly labeled water (DLW) method
(Webster and Weathers, 1989).
Measurements at TPM were made between 29 June and 15 July 1993 (day of
year=186±6; mean±S.D.) and at PRBO between 8 June and 19 July
1995 (day of year=182±11). These mean day-of-year dates do not differ
(t51=1.49, P=0.14). We captured birds on their
territories with mist nets or Potter-style traps, banded them, weighed them to
the nearest 0.1 g with either a K-tron electronic balance or Pesola spring
balance and gave them an intramuscular injection of 60-70 µl of water
containing 97 atoms % 18O and approximately 0.7-0.8 MBq of
3H. The birds were then either released immediately (single-sample
method) or held for 1 h for isotope equilibration and subsequent blood
sampling (double-sample method). Approximately 1-2 days later, the birds were
recaptured, reweighed and a first or second blood sample was obtained.
For the single-sample method, we estimated initial isotope level and body
water fraction based on 1-h equilibration values determined on 8-11
double-sample birds from each population. Blood samples were kept in
refrigerated sealed glass tubes until they were micro-distilled
(Nagy, 1983) to obtain pure
water, which was assayed for tritium activity by liquid scintillation
spectrometry (duplicate 5 µl samples, toluene/Triton X-100/PPO
scintillation cocktail). The 18O content of triplicate samples was
determined by cyclotron-generated proton-activation analysis
(Wood et al., 1975
) at the
University of California, Davis.
Body water volumes, rates of CO2 production and water efflux
were calculated using the equations of Nagy
(1980,
1983
) and Nagy and Costa
(1980
). We calculated daily
energy expenditure (DEE) from rates of CO2 production assuming an
energy equivalent of 23.3 kJl-1 CO2, based on a diet
containing a mixture of seeds and insects
(Martin et al., 1951
). Maximum
errors in validations of our DLW method were less than 9% for individual birds
and less than 2% for groups of nine birds for both the double-sample
(Buttemer et al., 1986
) and
single-sample (Webster and Weathers,
1989
) techniques.
Environmental temperature
We assessed the birds' thermal environment during DLW measurements at both
TPM and PRBO with centrally placed meteorological stations. At each site, we
determined the following variables in the open 1 m above ground: air
temperature (Ta) (shaded 36-gauge type-T thermocouple),
operative temperature (Te) [3.5 cm diameter metal-sphere
thermometer painted flat gray (Bakken et
al., 1985; Walsberg and
Weathers, 1986
)] and wind speed (u) (Thornthwaite model
901 cup anemometer). We also determined ground-level Te
and solar radiation (LiCor model 200 pyranometer) at the open site and
Te and wind speed (hotball anemometer;
Roer and Kjölsvik, 1973
)
0.5 m above ground inside bushes or shrubs. Sensor outputs were assessed at
10-s intervals, averaged every 10 or 30 min and recorded with Campbell
Scientific 21X data loggers. The cup-anemometers and pyranometers were
factory-calibrated. The various thermocouples were calibrated against a
National Bureau of Standards certified mercury thermometer. We calibrated the
hotball anemometers in a large laminar-flow wind tunnel at the UC Davis
hydraulics laboratory against a HanDar two-dimensional sonic anemometer and a
Campbell CSAT three-dimensional anemometer (both previously calibrated against
primary standards).
The goal of our meteorology measurements was to estimate the standard
operative temperature (Tes) that birds encountered during
DLW measurements using the following equation
(Bakken, 1990):
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Oxygen consumption
We determined resting metabolic rates (RMRs) of fasted adult white-crowned
sparrows by measuring their rate of oxygen consumption
(O2) while they
rested in the dark. We measured six TPM adults during June 1994 and six PRBO
adults between late July and early August 1995. The PRBO birds' plumage was
visibly more worn than that of TPM birds and, although they had begun the
postnuptial molt, very few feathers were in sheaths. The birds used in RMR
determinations were collected with seed-baited Potter traps, transported to
Davis by automobile and housed in individual wire cages on a 15 h:9 h L:D
photoperiod. They were provided with water, oyster shell grit and a
commercially available mixed finch-seed diet supplemented daily with waxworm
larvae (Galleria sp.) ad libitum. Sparrows were allowed 2
weeks to adjust to captivity before metabolic measurements began. Body mass
was measured daily with a calibrated electronic balance. Most birds maintained
their original capture body mass (±2%); one individual exhibited a 4%
mass gain.
O2 was
determined with a positive-pressure open-circuit respiratory system similar to
that of Weathers et al.
(1980
). Each bird was fasted
for a minimum of 3 h and placed inside a metal metabolism chamber (volume 41)
that was painted flat-black inside. The chamber was placed inside a controlled
temperature cabinet (±0.5°C) and measurements were made at
approximately 0, 15 and 30°C during the bird's subjective day (10:30-15:00
h) and night (17:30-22:00 h). Air temperature within the chamber was measured
with a thermocouple suspended approximately 5 cm above the bird. Before
beginning
O2
measurements, birds were allowed to equilibrate to chamber temperature for 1 h
while dry, CO2-free air flowed through the chamber at approximately
800 ml min-1. Flow rate was measured with Gilmont rotameters
calibrated (±0.8%) with a bubble meter
(Levy, 1964
). Atmospheric
pressure during the respirometry measurements and calibrations was measured
with a mercury manometer. The fractional O2 content of dry,
CO2-free influx and efflux air was measured with an Applied
Electrochemistry S3-A analyzer and recorded with Sable Systems software. The
O2 analyzer was calibrated using a metered flow of nitrogen
(Fedak et al., 1981
). Chamber
efflux O2 concentration was monitored for at least 20 min, and
resting metabolic rate was calculated from the minimal stable O2
concentration maintained for at least 3 min using equation 2 of Hill
(1974
). Values were corrected
to STPD and converted to energy units assuming that 1 ml of O2 is
equivalent to 20.1 J of metabolic heat.
Authorizations
White-crowned sparrows were captured and maintained in captivity under
authority of US Department of Interior Fish & Wildlife Service Permits Nos
9316 and 8400 and University of California Animal Use and Care Protocol Nos
3607. Field metabolic rate measurements using tritiated water were authorized
by University of California Radiation Use Authorization No. 0942, State of
California Department of Health Services Radioactive Material License
No.1334-57 and US Forest Service Special Use Permit No. 2720, Inyo National
Forest.
Values are presented as means ± S.D.
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Results |
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Daily energy expenditure and water flux
We injected 47 TPM sparrows with doubly-labeled water (DLW) and recaptured
25 of them within 0.91-1.09 days (0.97±0.05 days) and six within
1.95-2.09 days (1.99±0.05 days). We injected 39 PRBO sparrows and
recaptured 16 within 0.84-1.07 days (0.99±0.05) and six within
1.86-1.98 days (1.91±0.05). With one exception, we recaptured all birds
within ±10% of either a 1- or 2-day measurement interval. Thus, our DLW
measurements approximate daily energy expenditure for most birds of both
populations. We recaptured an additional four PRBO sparrows, but excluded them
from our analyses because we were uncertain whether they were incubating eggs
or feeding nestlings. Brood size (number of nestlings fed) averaged
2.9±0.7 at PRBO and 3.7±0.8 at TPM
(t19=2.38, P=0.03).
Fig. 2 summarizes the doubly-labeled water results, presenting mean values for male and female sparrows arranged by population and stage of the breeding cycle. Pooling data for each population (Table 1) reveals no difference in body mass during DEE measurements, but shows that mass-specific rate of CO2 production and water efflux averaged 26% and 27% higher, respectively, at Tioga Pass Meadow.
|
Although the pooled body mass of the two populations did not differ
(Table 1), there were
significant sex differences in body mass within and between populations
(Fig. 2). In both populations,
the transition from incubating to feeding nestlings was accompanied by a
decrease in body mass that was statistically significant only for females.
Each of these conclusions was drawn from the results of several analyses.
Specifically, the fully parameterized models included terms for the effect of
population, sex and a covariate for the potential contribution of temperature.
Additional models that provided for a different temperature regression
coefficient for each population were also considered. Computations made use of
the general linear model (GLM) procedure of SAS
(2000), following typical
techniques for linear models (e.g.
McCulloch and Searle, 2000
).
Effects where the levels were found not to be significantly different from one
another were deleted from later analyses.
PRBO females averaged 1.9 g (6%) lighter when feeding nestlings (26.6±0.9 versus 28.5±1.3 g); TPM females were 2.1 g (7%) lighter (26.6±0.9 versus 28.7±1.2 g). PRBO males feeding nestlings weighed significantly more than TPM males feeding nestlings (30.1±1.0 versus 28.5±1.6 g). Body mass change during the DEE measurement interval averaged 0% (range -3.7 to 4.2%) at PRBO versus -1.3% (range -5.6 to 3.4%) at TPM (Table 1). Neither mean differs significantly from zero mass change.
Mass-specific rate of CO2 production was significantly higher at TPM than at PRBO for both sexes and reproductive stages (Fig. 2), averaging 26% higher overall. Combining data for males and females, the transition from incubation to feeding nestlings resulted in a significant increase in CO2 production at TPM (6.16±0.48 versus 7.02±0.997 ml CO2 g-1 h-1; t29=3.23, P=0.003) but not at PRBO (5.04±0.55 versus 5.36±0.55 ml CO2 g-1 h-1; t20=1.35, P=0.19).
Daily energy expenditure (DEE, kJ day-1) calculated from CO2 production averaged 24% higher at TPM (Table 1). White-crowned sparrows at TPM worked harder than PRBO sparrows, as judged by their DEE/BMR ratios (2.6 versus 2.1; t51=6.42, P<0.001).
There was no significant correlation between DEE and any measure of environmental temperature for either population considered separately, although the correlation between DEE at TPM and Tes measured in willow thickets during the day approached significance (Table 2). When data for the two populations were pooled, however, DEE correlated significantly with every measurement of temperature (Table 2). Again, a variety of linear models (with and without a temperature covariate) were fitted to these data, such that pooling was only considered across levels of effects that did not differ significantly. For the pooled data, the highest correlation was between DEE and Tes measured 1 m above ground (Fig. 3; r2=0.422), but none of the five measures of temperature differed significantly.
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|
Water efflux differed by population (Table 1) but not by sex within populations for the same breeding stage (Fig. 2). Combining data for males and females, incubation stage water efflux averaged 20% higher at TPM than at PRBO (391±51 versus 470±94 ml kg-1 h-1; t26=2.55, P=0.02). Water efflux when feeding nestlings was 37% higher at TPM (509±51 versus 696±172 ml kg-1 h-1; t23=3.46, P=0.002). Again, using combined data for males and females, water efflux of both populations was significantly lower when incubating then when feeding nestlings: for PRBO, 391±51 versus 509±51 ml kg-1 h-1 (t20=5.45, P<0.001); for TPM, 470±94 versus 696±172 ml kg-1 h-1 (t29=4.64, P<0.001).
Resting metabolic rate
Because the white-crowned sparrow's thermal neutral zone is approximately
23-37°C (King, 1964;
Maxwell and King, 1976
), our
limited RMR measurements adequately describe the birds' thermoregulatory
profile (sensu Scholander et al.,
1950
). Mean daytime and night-time body masses and basal metabolic
rates (BMRs) of TPM and PRBO sparrows differed by less than 3% (all
P>0.53, t-tests). During BMR measurements, pooled body
mass averaged 25.3±2.0 g (N=12). Pooled BMR (N=12)
averaged 3.34±0.23 ml O2 g-1 h-1
during the daytime and 2.91±0.31 ml O2 g-1
h-1 at night (paired t11=3.33,
P=0.007). These values are 91% and 108%, respectively, of those
predicted for passerine birds (Aschoff and
Pohl, 1970
). Daytime BMR averaged 115% of night-time BMR.
Repeated-measures analysis of covariance (ANCOVA) revealed that during the day the subthermoneutral RMR of the two populations (Fig. 4) differed neither in slope (F1,10=0.02, P=0.90) nor elevation (F1,10=0.36, P=0.56). Night-time measurements (Fig. 4) differed significantly in elevation (F1,10=13.39, P=0.004), but not in slope (F1,10=1.50, P=0.25). Effects attributable to individual sparrows were significant for both daytime (F1,10=6.23, P<0.01) and night-time (F1,10=3.83, P=0.02) measurements. We derived the following equations for subthermoneutral RMR (ml O2 g-1 h-1) using a pooled-slopes model: daytime RMR=6.58-0.139Ta (syx=0.69, sb=0.021, r2=0.66, N=24); night-time RMR at PRBO=6.25-0.127Ta (syx=0.13, sb=0.011, r2=0.84, N=12); night-time RMR at TPM=5.57-0.127Ta (syx=0.14, sb=0.011, r2=0.82, N=12), where syx and sb are, respectively, the standard errors of the estimated intercept and slope.
|
The higher night-time RMR in PRBO birds probably resulted from their
sparser plumage. Because the PRBO birds' plumage was sparser, one would expect
the slope of their metabolic rate/temperature relationships to be steeper.
Indeed, their slopes were steeper for both daytime (0.148 versus
0.122 ml O2 g-1 h-1) and night-time (0.141
versus 0.110 ml O2 g-1 h-1)
O2 measurements,
but neither difference was statistically significant (see F values
above). We presumably lack the statistical power to detect the differences
because the sample sizes for these measurements were small. The night-time RMR
of Gambel's white-crowned sparrow (Z. l. gambelii) measured during
the autumn (King, 1964
) is
similar to that of our TPM birds and is described by the equation: night-time
RMR=5.27-0.125Ta.
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Discussion |
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A literature review revealed 21 studies in which temperature significantly affected DEE (Table 3), accounting for up to 85% of the variation in DEE. Clearly, temperature can be a principal determinant of DEE variation under some circumstances, yet measuring temperature accurately can be extremely difficult. In the studies presented in Table 3, Tes or operative temperature (Te) generally explained more variation in DEE than Ta; r2=0.53 (range 0.21-0.85) versus r2=0.24 (range 0.05-0.46) (t20=4.40, P<0.001). In our study, however, Tes was only slightly more effective at explaining interindividual variation in DEE than Ta, implying that we were unable to reliably assess the actual temperature encountered. Furthermore, within white-crowned sparrow populations, DEE was not significantly correlated with any measure of environmental temperature (Table 2). The absence of a significant correlation between DEE and temperature within sparrow populations may derive in part from the limited range in temperatures encountered. Temperatures were less variable and higher at PRBO than at TPM, and the correlations between DEE and temperature at PRBO were generally lower (Table 2). Some correlations approached significance at TPM, where the temperature range was greater, and across sparrow populations Tes explained 42% of the variation in DEE (Table 2).
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An alternative to quantifying temperature is to use a paired experimental
design in which the DEE of an experimental bird (e.g. brood manipulation) and
a control bird are determined on the same day (e.g.
Dickinson and Weathers, 1999).
Yet, even if both birds in such a paired design encounter the same
temperature, inferring RE from their DEE may be confounded. For example,
bluebirds that feed enlarged broods have DEEs equal to those feeding smaller
broods, yet they perch in the sun more often, which reduces the
thermoregulatory component of their DEE and thus masks the energy cost of
greater provisioning (Mock,
1991
). Even in the absence of such behavioral compensation, the
prospects of gaining meaningful insights into RE through DEE are hampered
because the net energy cost of activity is itself temperature-dependent. When
ambient temperature is below the thermal neutral zone, the heat produced as a
by-product of activity can substitute for the heat required for
thermoregulation, effectively reducing the energy cost of activity
(Paladino and King, 1984
;
Webster and Weathers, 1990
).
As an extreme example of this phenomenon, the rate of energy expenditure of
white-crowned sparrows at -10°C is the same whether they are perched in a
bush shivering or hopping on the ground
(Paladino and King, 1984
). At
this low temperature, activity has no net energy cost. Clearly, studies that
hope to gain insight into RE by measuring DEE need to consider the thermal
context within which behavior occurs.
Two alternative explanations exist for observed patterns of parental
investment by altricial birds. The energy limitation hypothesis holds that
food and/or adult working capacity are limited (Lack,
1954,
1968
;
Drent and Daan, 1980
;
Martin, 1987
;
Roff, 1992
) and that
increasing parental effort increases the relative success of a brood, but
simultaneously decreases the parent's survival and/or future reproductive
success (cost of reproduction: Williams,
1966
; Lessells,
1991
; Stearns,
1992
). The alternative view, the predation limitation hypothesis
(Skutch, 1949
), holds that
constraints due to predation risk limit adult activity at the nest, thereby
limiting clutch size and consequently parental effort. In the latter
hypothesis, parent birds may have substantial reserve physiological capacity
but be prevented from working harder by predation pressure. In support of this
hypothesis, both nest visitation rate and nest predation rate have been shown
to decrease with increasing brood size in open-nesting species, once effects
of nest site on predation risk are accounted for (Martin et al.,
2000a
,b
).
These two hypotheses lead to differing predictions about the relationship
between reproductive effort, brood size and parental survival. If parental
investment is primarily limited by nest predation, then parental DEE could be
submaximal and unrelated to either adult survival or future reproductive
success. In this scenario, one might find substantial variation in DEE between
years or populations. Alternatively, if parental investment is limited
primarily by parental working capacity, then DEE should be maximal, invariant
and correlate with adult survival and/or future reproductive success.
Doubly labeled water measurements of parental effort in altricial birds
provide inconclusive support for (or even contradict) the energy limitation
hypothesis (Bryant, 1988,
1997
). In many species, DEE is
well below the maximal sustainable level of approximately 5-6 times BMR
(Masman et al., 1989
;
Weathers and Sullivan, 1989
;
Bryant, 1977
), yet it is often
unrelated to manipulated brood size
(Bryant and Westerterp, 1983
;
Ricklefs and Williams, 1984
;
Williams, 1987
;
Moreno, 1989
;
Moreno et al., 1995
;
Deerenberg et al., 1995
),
implying a `ceiling' on parental effort. Moreover, DEE is consistent across
populations in some species but variable in others. There is no difference in
DEE among populations of least auklet (Aethia pusilla;
Obst et al., 1995
), but DEE
differs by up to 60% among populations of Leach's storm petrel
(Oceanodroma leucorhoa;
Montevecchi et al., 1992
) and
by up to 43% among great tit populations (Parus major;
Sanz et al., 2000
). Similarly,
in female great tits tending manipulated broods, maximal DEE varies by as much
as 38% between years (Tinbergen and
Verhulst, 2000
). Such disparity in results implies that the
primary limit on reproductive effort may be energy in some species but
predation risk in others.
In our white-crowned sparrows, DEE was submaximal in both populations
(2.1±0.2 times BMR at PRBO versus 2.6±0.3 times BMR at
TPM; t51=6.48, P<0.001), suggesting that
neither energy availability nor parental working capacity is the primary limit
on reproductive effort in this species. Interestingly, the two populations'
nesting success (expressed as the proportion of nests fledging at least one
young) is not significantly different; averaging 47.3% for 1331 Z. l.
oriantha nests over 22 years (Morton,
2002) and 53.7% for 255 Z. l. nuttalli nests (N. Nur, G.
Geupel and D. DeSante, unpublished observations) (
2=1.89,
P>0.05, d.f.=1). Presumably, equivalent nesting success in the two
populations is attained by greater nest predation at PRBO offsetting greater
weather-induced nest failure at TPM.
If nest predation is the principal limit on white-crowned sparrow
reproduction, then adult survival should be unrelated to DEE. There is some
support for this notion. Late in the breeding season, TPM females often feed
nestlings and fledglings by themselves while in molt and, although molt is
delayed somewhat in these `hard-working' females, they are able to catch up by
shortening the molting period. Furthermore, this cohort of females returns to
the study area the following year at the same rate as females that raise their
young early in the season, have continuous help from their mates and molt at a
slower pace (Morton, 2002).
This observation suggests that adult survival in white-crowned sparrows may be
only weakly related to DEE if at all.
Although there is some support for a link between DEE and fitness traits,
as required if DEE is to denote the cost of reproduction
(Bryant, 1988;
Deerenberg et al., 1995
;
Golet et al., 2000
), the
results here are also inconsistent. Female yellow-eyed juncos (Junco
phaenotus) with a relatively low DEE renest faster and are more likely to
breed in multiple years (Sullivan et al.,
1999
), providing a link between DEE and a fitness trait. Yet, in
black-legged kittiwakes (Rissa tridactyla), DEE is the same in Alaska
(61°N) and Norway (70°N), although adult mortality is 2.3 times higher
in Norway (Golet et al.,
2000
). Clearly, more studies that explicitly evaluate the
relationship between DEE and survival are needed to determine the utility of
DEE as a fitness index.
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Acknowledgments |
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References |
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