GROWTH AND ENERGETICS OF NESTLING YELLOW-EYED JUNCOS

We calculated the energy budget of nestling Yellow-eyed Juncos (Junco phaeonotus) from measurements of their growth, field metabolic rate (FMR), and resting metabolic rate (RMR). We used the doubly labeled water technique to measure the FMR of 69 nestlings that ranged in mass from 4.1 to 15.3 g. We calculated RMR of fed nestlings during the day and fasted nestlings at night from measurements of their O2 consumption. Nestling FMR (kJ/hr) increased linearly with mass (m, grams) according to the relation: FMR = -0.44 + 0.14m. RMR (kJ/hr) = -0.11 + 0.08m for fed nestlings during the day, and RMR = -0.10 + 0.06m for fasted nestlings at night. The growth of 75 nestling juncos was best described by the logistic equation m = 18.53/(1 + (6.5e-0.471t)); where t is age in days and m is mass in grams. The growth constant (k = 0.471) is not significantly different from that predicted allometrically for an altricial species with an asymptotic mass of 18.5 g. Total metabolized energy of nestlings averaged 322 to 483 kJ/bird over the 9 to 12 day nestling period (depending on age at fledging). Of the total energy metabolized, RMR comprised 46-48%, whereas activity and thermoregulation combined accounted for 30-33%. The energy accumulated in tissue was estimated to be 84-95 kJ, or 20-26% of the total metabolized energy.