Abstract
Abstract 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 de...
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