Running a marathon is a great test of human endurance. World-class marathoners raise their metabolic rate five times their resting metabolism as they traverse the 26.2 miles in about two hours.

In what seems to be a never-ending effort to test endurance limits, even more arduous competitive events have been created. Toss in a 2.4 mile swim and a 112-mile bike ride and you have the Ironman Triathlon. Other athletes run ultramarathons, long-distance races up mountains and races to see how far one can run in 24 hours.

For all of these events, the metabolic rate tops out at five times the resting rate so energy stores are depleted. These athletes are aided along the way with water and food (orange slices are particularly popular).

As astounding as these athletic feats are, they pale in comparison to bird migrations. A migrating bird burns energy more quickly than a marathoner. Flight is tremendously difficult and  a flying bird raises its metabolism at least eight-fold compared to its resting rate.

Some birds like Bar-tailed Godwits, Semipalmated Sandpipers and Blackpoll Warblers migrate over long expanses of ocean, flying non-stop for three days or more. These birds have to carry all the food and water they require. There no race volunteers handing out cups of water or food morsels. If these migrating birds hit the water, they die. Their energy stores are depleted.

Let’s get some perspective by considering the non-stop flight of a Semipalmated Sandpiper from Cobscook Bay to the mouth of the Amazon River in Suriname. That distance is about 2500 miles, almost 100 hundred marathons in three or four days. Even that feat pales in to Bar-tailed Godwits that fly non-stop for 7200 miles from Alaska to New Zealand.

Birds that migrate over land can stop and replenish their fat stores but they still do the equivalent of five or ten marathons in a night. Not too shabby!

How are birds able to do these extraordinary flights?

First, birds have much more efficient respiratory systems than mammals, reptiles and amphibians. The flow of air through our lungs is a two-way flow. The problem is that we are never able to fully evacuate the air in our lungs that has been depleted of its oxygen as we exhale. When we inhale a new breath with plenty of oxygen, it will mix with residual oxygen-poor air that we couldn’t exhale.

Birds are unique in having a one-way flow of air through their lungs. Thanks to two sets of air sacs, a parcel of air passes first into posterior air sacs, then across the lung, then into anterior air sacs and finally out the trachea.

To fuel a migratory leg, birds pack on fat. The break-down of fat releases energy as well as water. It’s not unusual for some birds to double their lean weight by gluttonously storing fat.  The fat is stored mostly below the skin of the breast and belly. Some ornithologists can assess the fat load of a bird by silhouette. Really fat birds jiggle when they walk.

We’re learning more about changes to body organs over the course of migration. Before migration, some birds increase the length of their gut. This modification allows them to put on weight more rapidly.

After fattening adequately, some shorebirds reduce their intestines and gizzards. Since the birds will not feed during a long flight, shrinking the gut lowers the weight.

At the same time, the flight muscles on the breast and the heart increase in size. Some birds also increase the size of their lungs.

Once birds arrive at their destination, they restore their organs to their original size.

How do birds flying non-stop for days deal with sleep deprivation?  Birds can shut down one side of the brain for a few seconds at time. The brain switches sides back-and-forth, essentially providing hundreds of mini-naps as the birds migrate.