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Traditional methods to determine total body fat are undesirable under many circumstances because they require sacrificing individual birds. Walsberg (1988) recently introduced the total body electrical conductivity (TOBEC) method for total body fat to ecological studies. In this paper we expand on Walsberg' s (1988) seminal paper and show that: (1) TOBEC is accurate under a narrow array of body masses, and is therefore appropriate for intraspecific studies; (2) dead birds exhibit significantly different TOBEC than live birds; and (3) the use of metal bands does not affect TOBEC measurements. We conclude that TOBEC provides an accurate estimate of total body fat, that it is preferable to traditional methods because it does not require sacrificing the bird, measurements can be taken quickly in the field, and (most importantly to field ornithologists) body fat can now be tracked through time for individual, free-living birds.

We examined ecophysiological responses of Sanderlings (Calidris alba) to their nonbreeding environments in New Jersey (USA), Texas (USA), Punta Chame (Panama), and Puerto Viejo (Peru). Daily energy expenditure (DEE; measured using doubly labeled water) was strongly influenced by the thermal environment, being higher at colder locations, and equivalent to 4.2 times the basal metabolic rate of New Jersey, 2.8 in Texas, 2.1 in Panama, and 2.7 in Peru (200, 135, 100, and 129 KJ/d; n = 10, 5, 10, and 16, respectively. Time budgets varied among locations, but in all cases feeding and roosting accounted for °90% of daylight hours. Feeding times were 55% in New Jersey, 90% in Texas, 40% in Panama, and 45% in Peru, suggesting that Sanderlings required more time to satisfy their food requirements in Texas than in the other locations. Total body mass and body fat increased linearly with decreasing long—term mean January air temperature but, because of the synchronous increase in energy expenditure, predicted survival times during food deprivation increased only slightly. Individual Sanderlings experience different living conditions throughout their non—breeding distribution, with DEE and nutritional reserves being determined primarily by temperature. Time budgets vary accordingly to accommodate these demands.

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