Abstract 1. 1.|To determine the thermoregulatory prowess of altricial nestlings, we conducted both equilibrium and transient analyses of white-crowned sparrow nestings, a representative fringillid. 2. 2.|For an individual nestling at thermal equilibrium, feather development is the major factor reducing heat loss after 2 days of age; tissue- and boundary-layer resistances are of minor importance. 3. 3.|The nest substantially reduces wind speeds near the nestlings. Heat transfer through the nest material is of only moderate importance. Evaporation also appears to be a small proportion of total heat loss during hypothermia in natural environments. 4. 4.|Net long-wave radiant exchange is also minor, but short-wave radiation is potentially a major component of the nestling's energy budget, approaching the magnitude of maximal metabolic heat production. 5. 5.|When nestlings cool, their body mass and metabolic rate are also major importance in determining the rate of cooling, and (for metabolism) the equilibrium temperature as well. 6. 6.|The huddling together of nestlings is perhaps the single most important factor affecting heat transfer. 7. 7.|An older brood actually has more insulation than does an adult in the same microclimate.