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Peer Reviewed Literature

Erich R. Eberts, Morag F. Dick, and Kenneth C. Welch Jr.

January 15th, 2019

During the day, hummingbirds quickly metabolize floral nectar to fuel high metabolic
demands, but are unable to feed at night. Though stored fat is the primary nocturnal metabolic
fuel, it has been suggested that hummingbirds store nectar in their crop to offset fat expenditure
in the night or to directly fuel their first foraging trip in the morning. We examine the use of
crop-stored sugar in the nocturnal energy budget of ruby-throated hummingbirds (Archilochus colubris)
using respirometry and 13C stable isotope analysis. Hummingbirds were fed a 13C-enriched sugar
solution before lights-out and held in respirometry chambers overnight without food. Respirometry
results indicate that the hummingbirds metabolized the sugar in the evening meal in less than
2 h, and subsequently primarily catabolized fat. Breath stable isotope signatures provide the key
insight that the hummingbirds converted a substantial portion of an evening meal to fats, which
they later catabolized to support their overnight metabolism and spare endogenous energy stores.
These results show that the value of a hummingbird’s evening meal depends on how much of this
energy was converted to fat. Furthermore, this suggests that evening hyperphagia is an important
energy maximization strategy, especially during energetically expensive periods such as migration
or incubation.