Altitudinal variation in parental energy expenditure by white-crowned sparrows.

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 birds' 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 birds' thermal environment, including standard operative temperature (T(es)).