We measured the aerobic capacity for exercise in house wren (Troglodytes aedon) nestlings from 3 to 10 days of age. Exercise data were compared with previous measurements of the energy cost of begging (Ebeg) to determine if begging chicks are working at maximal exercise capacity. We also compared exercise with the peak metabolic rate during digestion and, in older chicks, with thermogenic heat production. Rates of oxygen consumption (VO2) during exercise increased rapidly with age, but the factorial aerobic scope for exercise (exercise VO2/resting VO2) averaged only 1.4 (at three to four days) to 1.7 (at 8 to 10 days). The mean Ebeg was consistently lower than exercise VO2, but maximal Ebeg was similar to exercise VO2. The peak VO2 during digestion was higher than exercise VO2 for young chicks (three to six days) but not significantly different for older chicks. In older chicks (8 to 10 days), regulatory thermogenesis at 22°C was significantly higher than exercise VO2. These results suggest that some begging by house wren chicks can be at maximal muscular effort, but the average output during begging probably is not constrained by exercise capacity. The metabolic capacity for muscular activity of any kind is lower than the metabolic capacity for digestion (at least in young chicks), which presumably reflects a high priority for food processing and growth
