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

SYNOPSIS. Measures of energy expenditure by free-living birdscan provide quantitative testsof a number of ecological theories,regarding such diverse phenomena as foraging strategies, resourcecompetition, or parental investment. Our confidence in thesetests rests heavily on the confidence we have in the estimatedrates of energy expenditure. The most common approach to obtainingsuch estimates is the construction of time-energy budgets, inwhich the durations of ananimal's daily activities are multipliedby the respective energy costs of the activities, and thesecosts 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 ofenergy expenditure obtained using doubly labeled water indicatethat time-energy budgets can yield accurate estimates of energyexpenditure. However, this is likely to be achieved only underfairly rigorous conditions in which resting costs, activitycosts, and thermoregulatory costs are all well described.Evidenceis accumulating to suggest that, under some conditions, energyexpenditure by birds reaches a maximum sustainable level, atwhich point it is limited by the physiological capacitiestoingest and assimilate energy. Under these conditions, behavioralresponses to changing physical environments and resource availabilitymay be critical to the maintenance of energy balance.