Comparative Population Bioenergetics of Three Insectivorous Passerines in a Deciduous Forest

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Abstract
Patterns of time and energy allocation were determined for breeding populations of least flycatchers [Empidonax minimus], red-eyed vireos [Vireo olivaceus] and black-throated blue warblers [Dendroica caerulescens] in the Hubbard Brook Experimental Forest, New Hampshire [USA]. Daily energy expenditures ranged from 0.7-1.7 kcal/g per day, varying with the species and phase of nest-cycle. Differences in population energy budgets relate to each species'' life history characteristics, particularly its reproductive pattern, density and foraging mode. Least flycatchers, despite their relatively brief residency in the forest, were the dominant energy users because of their higher densities, greater productivity, higher metabolic responses to cool night temperatures, and more frequent flight associated with aerial foraging. Black-throated blue warblers used the least amount of energy for thermoregulation at night, required intermediate levels of energy expenditure for foraging, but had the lowest levels of production, largely becuase of high predation rates at nests. Red-eyed vireos, although the largest of the 3 spp., were intermediate in population energy flow and thermoregulatory responses. The foraging mode of vireos was the least expensive energetically, but their reproductive success appeared to be sensitive to the abundance of special prey items. Total energy assimilated during their residence in the breeding area was 7045, 4353, and 975 kcal/ha for the flycatchers, vireo and warbler populations, respectively. These values represent 3.8, 2.2, and 1.8 .times. estimated seasonal standard metabolic rate (SMR) values and provide conversion factors for general use in studies of bird community structure and energetics.

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