Abstract
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.
Highlights
Small birds often face energy balance challenges because they metabolize the energy that they consume at very high rates, are unable to store large amounts of energy rich tissue, and quickly lose heat to their environment [1,2,3]
Hummingbirds may endure overnight fasts by fueling their nocturnal metabolism with any nectar stored in their crop at the beginning of the night, and with endogenous fat synthesized during the day
We fed hummingbirds an isotopically-labeled evening meal and tracked how this evening meal was metabolized during the night
Summary
Small birds often face energy balance challenges because they metabolize the energy that they consume at very high rates, are unable to store large amounts of energy rich tissue, and quickly lose heat to their environment [1,2,3]. These organisms, such as hummingbirds, are often further energetically stressed at night, when they are unable to feed, often experience low ambient temperatures, and expend energy quickly [3,4,5]. They may reduce their overnight energy expenditure by using torpor when they have low energy stores or when it is advantageous to conserve
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