Nectarivorous hummingbirds subsist almost exclusively on a mixture of sucrose, glucose and fructose found in floral nectar. Previous studies have shown that hummingbirds can fuel hovering flight almost exclusively using recently ingested sucrose. However, the relative capacities for the direct utilization of glucose and fructose by hovering hummingbirds remain unknown. 13C-enriched solutions of glucose and fructose were administered separately. Exhaled breath samples were collected using feeder-mask respirometry and sent for subsequent mass spectrometric analysis. I found hovering hummingbirds transition from exclusively oxidizing endogenous fatty acids when fasted, to oxidizing newly ingested carbohydrates when given access to either glucose or fructose solutions. Interestingly, the amount ingested, fractional turnover of stable carbon isotope signatures, amount oxidized, energy expended and proportion of hovering metabolism supported by each hexose, were each similar between glucose and fructose. These results demonstrate hovering hummingbirds’ ability to utilize fructose and glucose equally.

Nectarivorous hummingbirds subsist almost exclusively on a mixture of sucrose, glucose and fructose found in floral nectar. Previous studies have shown that hummingbirds can fuel hovering flight almost exclusively using recently ingested sucrose. However, the relative capacities for the direct utilization of glucose and fructose by hovering hummingbirds remain unknown. 13C-enriched solutions of glucose and fructose were administered separately. Exhaled breath samples were collected using feeder-mask respirometry and sent for subsequent mass spectrometric analysis. I found hovering hummingbirds transition from exclusively oxidizing endogenous fatty acids when fasted, to oxidizing newly ingested carbohydrates when given access to either glucose or fructose solutions. Interestingly, the amount ingested, fractional turnover of stable carbon isotope signatures, amount oxidized, energy expended and proportion of hovering metabolism supported by each hexose, were each similar between glucose and fructose. These results demonstrate hovering hummingbirds’ ability to utilize fructose and glucose equally.