Maximum energy intake rate is proportional to basal metabolic rate in passerine birds

Abstract

A high energy intake rate may be important for many animals, but little is known of factors that determine intake capacity. Birds in migratory disposition presumably eat at their maximum capacity and therefore form a good model for studying the energy intake capacity of animals. We measured the maximum daily gross energy intake (GEI$\_{\max}$) and basal metabolic rates (BMR) in 22 species of migrant passerines during autumn. Both GEI$\_{\max}$ and BMR scaled to body mass (W) close to W$^{0.70}$. More importantly, species with high GEI$\_{\max}$ for their body mass also had relatively high BMR. BMR may reflect the size of the metabolic machinery involved in energy uptake. The maximum daily metabolizable energy intake (DME$\_{\max}$) was estimated from our GEI$\_{\max}$ data. The average ratio DME$\_{\max}$/BMR was 4.6. In comparison, daily energy expenditure of reproducing passerines have been reported in the literature to be 3.6 times BMR. This suggests that energy intake rates may not normally limit breeding performance in passerines. Earlier studies have shown that BMR of homoeotherms reflect the rate of energy expenditure during reproduction. Our study shows that the energy intake capacity also correlates with BMR, which gives new perspectives on the ecological significance of BMR.