The nestlings of several species of altricial passerines lack any thermogenic response to low ambient temperatures during approximately the first half of their growth. Apparent improvements in the thermoregulatory capacity during the early phase of growth, as reflected by body temperatures after a standardized cold exposure, are due to the relationship of cooling rate to size and are not due to active metabolic regulation. This pattern of development may influence the parental behavior and nesting ecology of passerines. The present study examines the development of temperature regulation in individual nestling Tree Swallows (Iridoprocne bicolor) under laboratory conditions. These data extend the range of observations of thermoregulatory development in passerines and aid in interpreting the mechanisms underlying the maintenance of homeothermy under field conditions.