Altitudinal variation in parental energy expenditure by white-crowned sparrows
2002
SUMMARY We used the doubly labeled water technique to measure daily energy
expenditure (DEE) during the incubation and feeding nestling stages in two
populations of white-crowned sparrows ( Zonotrichia leucophrys )—
one montane and migratory, the other coastal and sedentary —
that differ in thermal environment and clutch size. We assessed the birds9
thermal environment by continuously monitoring (among other variables)
operative temperature and wind speed both in the open and within bushes and
willow thickets occupied by sparrows. From these measurements, we derived
several estimates of the birds9 thermal environment, including standard
operative temperature ( T es ). Shade air temperature and
T es averaged 6.6 and 10.3°C lower, respectively, at
the montane study site during DEE measurements. The montane population9s DEE
averaged 24% higher than that of the sea-level population (103.6±12.2
versus 83.7±9.6 kJ day -1 ; means ± S.D.,
N =31 and 22, respectively), reflecting both its larger brood size
(3.7 versus 2.9) and the colder environment. The DEE:BMR ratio was
lowest in the sea-level population (2.1 versus 2.6), but neither
population worked to their physiological capacity to produce young. DEE was
significantly correlated with temperature across populations, with
T es explaining 42% of the variation in DEE. Statistically
removing the effect of temperature by adjusting DEE to a common temperature
reduced the difference in DEE between populations by 34% to 87.7 and 100.8 kJ
day -1 , respectively, for sea-level and montane populations. Basal
and resting metabolic rates were similar in both populations, implying that
greater activity in the montane population accounted for its higher
temperature-adjusted DEE. Our results indicate that the thermal context within
which behavior occurs can significantly affect interindividual variation in
DEE. Attempts to assess reproductive effort by measuring DEE should therefore
account explicitly for the effect of temperature.
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