Brood Size and Thermal Environment Influence Field Metabolism of Nestling Yellow-Eyed Juncos

Abstract

We used the doubly-labeled-water method to examine the effect of thermal environment (mean shade air temperature) and brood size (two to six young) on field met- abolic rate of nestling Yellow-eyed Juncos (Junco phaeonotus). The relationship between brood size and nestling field metabolic rate was curvilinear, such that nestling energy expenditure was lowest in broods of four (the most common clutch size) and highest in broods of two (the smallest brood size). Mean air temperature accounted for 21.0% of the variation in nestling field metabolic rate, whereas brood size accounted for 17.5% of the variation. Although brood reduction would decrease the brood's field metabolic rate, the reduction is not proportional to the decrease in brood size. Received 15 November 1990, accepted 27 June 1991. LACK (1947) proposed that clutch size in al- tricial birds is determined by the maximum number of young that parents can feed ade- quately. Whereas some studies have supported this contention by documenting increased nest- ling mortality or decreased nestling mass in en- larged broods, in many species the most pro-