Exercise Capacity of House Wren Nestlings: Begging Chicks Are Not Working as Hard as They Can

Abstract

We measured the aerobic capacity for exercise in House Wren (Troglodytes ae- don) nestlings from 3 to 10 days of age. Exercise data were compared with previous mea- surements of the energy cost of begging (Eg) 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 con- sumption (902) during exercise increased rapidly with age, but the factorial aerobic scope for exercise (exercise 902/resting 902) averaged only 1.4 (at three to four days) to 1.7 (at 8 to 10 days). The mean Eg was consistently lower than exercise 902, but maximal E.bg was similar to exercise 902. The peak 902 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 22oC was significantly higher than exercise 'O2. 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 ca- pacity. 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. Received 2 October 1997, accepted 6 February 1998.