Exercise capacity of House Wren nestlings: Begging chicks are not working as hard as they can

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 bogging (E-beg) 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 ((V)over dotO(2)) during exercise increased rapidly with age, but the factorial aerobic scope for exercise (exercise (V)over dotO(2)/resting (V)over dotO(2)) averaged only 1.4 (at three to four days) to 1.7 (at 8 to 10 days). The mean E-beg was consistently lower than exercise (V)over dotO(2), but maximal E-beg was similar to exercise (V)over dotO(2). The peak (V)over dotO(2) during digestion was higher than exercise (V)over dotO(2) 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 degrees C was significantly higher than exercise (V)over dotO(2). These results suggest that some begging by House Wren hicks 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 hicks), which presumably reflects a high priority for food processing and growth.