Energetics of hummingbird foraging at low ambient temperature

Because of their small size and the high energetic casts of hovering and fo rward flight, hummingbirds achieve the highest mass-specific metabolic rate s known among vertebrates. Rufous hummingbirds (Selasphorus rufus) stop to refuel on floral nectar in subalpine meadows as they migrate south from Bri tish Columbia to Mexico. In such habitats they face the challenge of achiev ing daily net energy gain despite the high energetic costs of night and the rmoregulation at near-freezing morning temperatures. Hummingbirds provided with 15 or 20% sucrose while subjected to these conditions for 4 h in the l aboratory did not remain in energy balance and lost mass. However, they ach ieved energy balance or net energy gain on 30% sucrose. Because these sucro se concentrations are within the range observed in the nectar of hummingbir d-visited flowers, the results suggest that the energetic cost of thermoreg ulation may influence the coevolution of hummingbirds and flowers. Hummingb irds maintaining energy balance at low ambient temperature via high foragin g frequencies and high rates of energy intake can sustain average metabolic rates of about 250 W/kg over a 4-h period. These are the highest metabolic rates known among vertebrates at which rates of dietary energy intake equa l rates of energy expenditure.