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Heavy Hummingbirds And Their Mighty, Miniature Migrations

The average American consumed 4,500 calories at the Thanksgiving table last week. There’s no statistics available for how many they consumed while sleepily standing in front of the refrigerator later that night, though, and absolutely zero information regarding the amount of pie calories ingested for breakfast the next day.

Thanks to holiday feasts and generalized seasonal overconsumption, some of us might soon notice our bodies storing some of those extra calories away in the form of additional body weight. 

Every fall, ruby-throated hummingbirds do much the same thing in preparation for one of the most remarkable migrations in the bird world. Their journey across the Gulf of Mexico might measure a mere 600 miles (~960 km), not a notable distance in the avian record books, but considering this diminutive bird couldn’t tip a balance versus a pair of pennies, it’s quite the trip.

How they fuel their flight is the real wonder. Hummingbirds need to flap their wings 50+ times per second in order to stay aloft, and their race car-tuned metabolism burns ten times more oxygen than elite human athletes (by weight anyway). This requires that they slurp up an incredible amount of food energy in the form of sugary nectar (supplemented with the occasional protein-rich insect).

The average ruby-throated hummingbird doesn’t carry much mass on the wing, weighing in at just more than a penny. During summer, about 20% of that weight is fat, but in preparation for their southerly journey they really pack on the pudginess, hitting upwards of 40% body fat. To do that, they’ll eat as often as every 15 minutes throughout the day, and as we saw in this video from Smarter Every Day last week, that can lead to some frenzied feeding.

Running out of gas while over open ocean would be disastrous for the birds, of course. But why use fat as long-distance fuel over sugar, their normal daily food source? 

If ruby-throated hummingbirds powered their migration on sugars and carbohydrates, they’d need way more mass to get the same energy. Not only would they need to double their body weight with glycogen, they’d have to store water along with it, quickly adopting the aerodynamic profile of a lead zeppelin.

Although it is metabolized more slowly, fat provides about ten times more energy per gram than sugars, and provides water to the thirsty flyers as a side effect of the chemical reactions.

What’s even crazier is, although the hummingbirds burn through more than a third of their body weight in a single day when crossing the Gulf of Mexico, we burn the same mass of fat climbing a few flights of stairs! The fact that they can cross open ocean while burning just 14 calories blows my mind. That’s what I call good mileage.

Here’s a more in-depth research article about ruby-throated hummingbird metabolism if you want to dig into the science a bit deeper. 

If you want to learn about some other record-setting bird migrations, including how they navigate using magnetic fields and why the flying V formation is the most efficient way to fly, check out this video over on the It’s Okay To Be Smart YouTube channel:

Image at top is not an actual fat hummingbird, just in case you’re wondering :)