Calculating the Nocturnal Energy Expenditure of an Incubating Anna's Hummingbird

Abstract

Recently, attention has been focused on the bioenergetics of incubating female hummingbirds (Howell and Dawson 1954; Calder 1971, 1973). Hummingbirds are homeothermic with large surface area to volume ratios and have high rates of heat dissipation. During the night when cooler air temperatures prevail and feeding is prohibited, several species become torpid, lowering body temperatures and oxygen consumption (Bartholomew et al. 1957; French 1959; Pearson 1960; Lasiewski 1963; Calder 1971). This physiological state may be carefully regulated, as for Eulampis jugularis (Hainsworth and Wolf 1970). For incubating female hummingbirds, high nest temperatures maintained throughout the night indicate a nontorpid condition (Howell and Dawson 1954; Calder 1971). Pearson (1954) estimated that without torpor male Anna's Hummingbirds (Calypte anna) would require approximately 30% more metabolic energy during a 24-hr period and over five times more at night. Calder (1971) reviewed investigations on the nesting behavior of female hummingbirds and discounted increased feeding before roosting, decreased activity due to roosting, and lower metabolic rates for females as possible energymaintaining mechanisms. He concluded that for the Calliope Hummingbird (Stellula calliope) nest construction and the large overhanging limb which shields the nest from cold night skies must be important factors in nighttime energy conservation. The purpose of this investigation was to quantify the energy conservation properties of an Anna's Hummingbird nest and nest site and provide data on feeding behavior during incubation for comparisons with similar investigations. The analysis was accomplished by simultaneously measuring nesting behavior, nest temperatures, and microclimate for a nesting C. anna. A model was formulated to predict the nocturnal heat transfer of the bird and nest combination and used to compute the relative thermal advantages of nest properties and nest site.