High-Powered Hummers
Southeastern Alaska sometimes deserves the envy of the rest of the state. Its scenery is everywhere magnificent, it has mild winters, plenty of fish, ships full of cash-carrying tourists, and hummingbirds.
Granted, not many people find hummingbirds to be as impressive as tidewater glaciers or monster king salmon, but a resident hummer zipping around the flowers is a marvelous addition to any garden. It's easy to see why hardware stores from Cordova to Ketchikan stock red-nozzled nectar dispensers for luring these shimmering little buzzbombs.
And they are little. A full-grown rufous hummingbird, the one species venturing into Alaska, measures about 3 1/2 inches from bill-tip to tail-end, and flared tail feathers and needle-thin bill take up much of those few inches.
Despite their puny size, rufous hummingbirds make long migrations. The hummingbird flaunting his fox-colored feathers in a springtime Juneau garden probably spent the winter well south of the Mexican border.
Physiologists have puzzled over hummingbirds' ability to balance the need to store energy for migration with fueling daily foraging. The birds are tiny and they hover, energy-expensive features that in combination demand a blast-furnace metabolism. Still, every spring and fall pudgy hummers flit nearly the length of North America.
By dint of careful observation and clever experimentation, researchers have established that hummingbirds use an uncommonly efficient dual fuel system. The dual-fuel aspect is familiar enough: humans, like hummingbirds, oxidize glycogen (a carbohydrate stored in liver and muscle tissue) for immediate energy. We burn fat, a more inefficient process for providing energy, after glycogen stores have been consumed.
Hummingbirds fine-tune the system. According to a report by Jared M. Diamond in the British journal Nature, the details were deciphered with the help of some tricky apparatus. Physiologists induced rufous hummingbirds to hover at a nectar dispenser that incorporated a flow-through gas mask. The device enabled the researchers to measure the birds' respiratory quotient or RQ, which is the ratio of carbon dioxide produced compared to oxygen consumed during breathing.
Since fat oxidation is the less efficient process, the RQ is lower when the bird's metabolism is fueled by fat (0.70) than when it is fueled by carbohydrate oxidation (1.00). Thus, by measuring the contents of the birds' breath, the researchers had a good gauge of which interior fuel source the hummers were using,
At the avian equivalent of breakfast time, when the birds had empty stomachs but were well rested, the hummingbirds had RQs near 0.72. During their first meal, their RQs rose to about 0.81. With subsequent bouts of hover-feeding, their RQs rose to 1.0. They were shifting from burning fat to burning carbohydrate as they foraged and ate. After a few meals, they were burning only some of the sugar in the meal at hand---or at beak---and stashing the rest, mostly as fat.
The hummingbirds did use some of the sugars they ingested to replenish their glycogen supply, but that didn't take much sugar. They maintained only small stores of glycogen in their liver and flight muscles, enough to fuel about five minutes of flying. But, as the scientists observed, hummingbirds usually make brief flights. Thus, except when they began the day's activity, they had glycogen to burn. The hummers did not need to tap into their fat reserves until they set off on migration.
Although burning fat is not the most efficient way to generate energy, fat is an extremely efficient way to store energy. Its calorie yield per gram is much higher than that from carbohydrate, so a fat hummer carries less weight on migration than would a bird with energy stashed as glycogen.
Rufous hummingbirds, according to Diamond, may operate very near the limits of the metabolically possible. Like a prospector at the end of his grubstake, these little birds give nearly all they've got to reach Alaska.
