Hummingbirds in flight display the highest rates of aerobic metabolism known among vertebrates. Their flight muscles possess sufficient maximal activities of hexokinase and carnitine palmitoyltransferase to allow the exclusive use of either glucose or long-chain fatty acids as metabolic fuels during flight. Respiratory quotients (RQ = VCO2/VO2) indicate that fatty acid oxidation serves as the primary energy source in fasted resting birds, while subsequent foraging occurs with a rapid shift towards the use of carbohydrate as the metabolic fuel. We suggest that hummingbirds building up fat deposits in preparation for migration behave as carbohydrate maximizers (or fat minimizers) with respect to the metabolic fuels selected to power foraging flight.

In a series of daul choice tests with large volume feeders, rufous hummingbirds preferred sucrose concentrations near those that maximized their instantaneous rates of energy intake. As predicted on theoretical grounds, energy intake rates increased with increasing sucrose concentration to a maximum then decreased above this maximum. Earlier experimental studies suggested that hummingbirds always prefer the highest available concentration. Our results are consistent with the data of these studies, but by using a wider range of concentrations than previous workers, we found that the hummingbirds discriminated against very concentrated solutions.

This field study of female and immature migratory rufous hummingbirds (Selasphorus rufus) reveals that their feeding territories are closely regulated in size to maintain environmental reserves of energy per individual. Columbine (Aquilegia formosa) produces nectar about four times as fast per nectary as Indian paintbrush (Castilleja miniata) but territories have similar daily caloric productivity regardless of their floral species composition.Une étude en nature de femelles et d'oisilons du colibri migrateur Selasphorus rufus démontre que les territoires où ils se nourrissent sont délimités de façon à ce qu'il y ait dans le milieu une réserve constante d'énergie par individu. L'ancolie (Aquilegia formosa) produit son nectar environ quatre fois plus vite par nectaire que la castilléjie Castilleja miniata, mais les territoires ont des productivités caloriques quotidiennes semblables, quelle que soit la composition de la population florale.[Traduit par le journal]

No abstract

Postbreeding migrant rufous hummingbirds (Selasphorus rufus) establish feeding territories in alpine meadows. An inverse hyperbolic relationship between territory size and flower density indicates that territory size is regulated to maintain food supply: individual hummingbirds make daily adjustments in their territories. These adjustments maintain food supplies from day lo day and improve them over what they would be if no adjustments were made. The ability of individuals to maintain territory quality, however, and the length of time they remain in the meadows, is affected by their age and sex and by the level of competition for food by conspecifics. Territoriality, therefore, must be considered in relation to factors operating beyond the local food environment, both in terms of effects on and effects of the short-term dynamics of energy regulation. This has important implications for hummingbird migration.