Chappell MA, Bachman GC (1998) The exercise capacity of House Wren nestlings: begging chicks are not working as hard as they can. The Auk 115:863-870.
ABSTRACT -- We measured the aerobic capacity for exercise in 3 to 10 day old House Wren nestlings (Troglodytes aedon). The exercise data were compared to previous measurements of the energy cost of begging (Ebeg) to determine if begging chicks are working at maximal exercise capacity. We also compared exercise to the peak metabolic rate during digestion and, in older chicks, to 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 (in 3-4 day old chicks) to 1.7 (in 8-10 day old chicks). 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 (3-6 days) but not significantly different for older chicks. In 8 to 10 day old chicks, regulatory thermogenesis at 22 °C was significantly greater than exercise VO2. These results suggest that some begging by House Wren chicks can be a maximal muscular effort, but the average output during begging is probably 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.