Data obtained from field measurements was used in conjunction with physically based energy balance equations to estimate the metabolism of nestling Ruby-throated Hummingbirds (Archilocus colubris). Data is presented which suggest that the adult female selects a particular microclimate for nest construction. She seeks out a site optimizing environmental conditions such that the energetic cost to the exposed nestlings is reduced to a minimum. Partitioning of the energy exchange routes revealed the relative importance of radiant exchange, convective exchange, and evaporative heat loss in the energy balance of the nestlings at the nest site.