The Dusky Flycatcher (Empidonax oberholseri), a small, open-nesting Neotropical migrant, is a relatively common summer inhabitant of subalpine environments in the western and southwestern United States. Nestling growth and development of thermoregulation were studied in a population at Tioga Pass (3,000 m) in the eastern Sierra Nevada of California. Despite the selective advantages that might be expected from accelerating rates of growth and thermoregulatory development, in an environment where the season was relatively short and cold temperatures and inclement weather were common, there was little evidence of adaptive modification in either of those parameters; growth rates were comparable to those of other tyrant flycatchers. Feather eruption began around day 5, brushing in dorsal and ventral tracts began two days later, and brushing of the flight feathers around day 9. Asymptotic body masses were attained around day 12, but nestlings did not fledge for another 4 to 5 days. The physiological development of endothermy was closely correlated with increases in body mass and accompanying decreases in surface-area-to-volume ratio and age, but was not significantly correlated with changes in plumage development, when mass and age were statistically controlled. On exposure to ambient temperatures between −3 and 5°C for 10 min (roughly twice as long as the average female bout of inattentiveness), individual nestlings were unable to prevent deep hypothermia until after day 7. Homeothermy, at 80% of adult levels, was not attained until after day 12, although broods of nestlings maintained relatively stable body temperatures as early as day 5. Whereas clutch sizes in this population generally ranged from two to four eggs, the most stable thermal environments were provided by broods of three or four chicks. Although broods of nestlings were generally able to maintain temperatures within 5°C of adult levels, temperature profiles during storms indicated a remarkable capacity for nestlings to tolerate temperatures in the nest as low as 15°C for periods as long as 4 h without apparent ill effect.