Energetics of Occupied Hummingbird Nests

Abstract

Hummingbirds have high weight-specific metabolic requirements combined with the smallest thermal mass among birds (Lasiewski and Dawson, 1967), yet those species which breed in northern climes, such as the ruby-throated hummingbird, Archilocus colubris, are exposed to cold night temperatures. The high energy demands required in cold conditions would be intolerable if the birds did not have well-developed means of conserving energy. Adult hummingbirds apparently go into hypothermic torpor, thereby conserving energy (Lasiewski, 1963; Pearson, 1950; Wolf and Hainsworth, 1971). It is unknown if young birds have similarly variable metabolic rates. Although some studies have dealt with egg development and nest temperatures of birds (e.g., Calder, 1973; Howell and Dawson, 1954; Huggins, 1941; Irving and Krog, 1956; Kashkin, 1961), the nest site has not been related to energy-budget considerations, nor has recent literature included adequate discussion of metabolic rates of nestlings (e.g., Calder, 1973, 1971; Dawson and Hudson, 1970). In this study, a primary objective was to describe the changing microclimate throughout a day at an occupied nest of a ruby-throated hummingbird, and to relate this to the energy budget and metabolic rate of the nestlings, incorporating principles developed and used by Gates (1962, 1970), Porter and Gates (1969), Southwick (1971), and Southwick and Mugaas (1971).