Estimates of Daily Energy Expenditure in Birds: The Time-Energy Budget as an Integrator of Laboratory and Field Studies

Abstract

SYNOPSIS. Measures of energy expenditure by free-living birds can provide quantitative testsof a number of ecological theories, regarding such diverse phenomena as foraging strategies, resource competition, or parental investment. Our confidence in these tests rests heavily on the confidence we have in the estimated rates of energy expenditure. The most common approach to obtaining such estimates is the construction of time-energy budgets, in which the durations of ananimal's daily activities are multiplied by the respective energy costs of the activities, and these costs are summed. Our knowledge of the energy costs of activities, particularly locomotion, has greatly advanced in recent years, as has the ability to adequately assess thermoregulatory costs. Comparisons between timeenergy budgets and direct measures of energy expenditure obtained using doubly labeled water indicate that time-energy budgets can yield accurate estimates of energy expenditure. However, this is likely to be achieved only under fairly rigorous conditions in which resting costs, activity costs, and thermoregulatory costs are all well described.Evidence is accumulating to suggest that, under some conditions, energy expenditure by birds reaches a maximum sustainable level, at which point it is limited by the physiological capacitiesto ingest and assimilate energy. Under these conditions, behavioral responses to changing physical environments and resource availability may be critical to the maintenance of energy balance.