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The impressive coordinated migrations of wildlife like monarch butterflies and salmon have always made me curious. How do they know where they’re going, when to start, what route to take, and when they have arrived at their destination? Those are questions I think about every spring, too, as millions of birds migrate to our area, many traveling great distances under treacherous conditions. And some of them still have a long way to go once they get here.

The tiny blackpoll warbler, for instance, undertakes a two- or three-day flight across open water each autumn from southern New England to the Caribbean with nowhere to land if it gets tired. And the even tinier ruby-throated hummingbird crosses the entire Gulf of Mexico in 24 hours without stopping, their buzzing wings flapping at about 50 beats per second the whole way.

But how are they able to keep going for so long without refueling?

The answer, according to Scott McWilliams, has everything to do with the digestive physiology of the birds. Or, as the University of Rhode Island ornithologist likes to say, “migration takes guts.”

McWilliams studies the physiology of migrating birds. He captures migrant yellow-rumped warblers, hermit thrushes, white-throated sparrows and other species on Block Island or at the Kingston Wildlife Research Station near URI. For most of the birds, Rhode Island is only a rest stop at the halfway point of their seasonal migration. He then evaluates the physical condition of the birds, the food they eat and other factors – sometimes conducting short dietary experiments with them before releasing them again – all to understand the physiological changes that take place when the birds migrate.

What he has found continues to astound me every time I think about it.

McWilliams says it takes a great deal more than an innate knowledge of what route to travel and strong flight muscles to successfully complete their twice-a-year migration. It takes a digestive system that is highly adaptable to different activities and conditions, what scientists call phenotypic plasticity. This physiological flexibility enables some species to double their weight, support long-distance flight, and withstand changing temperatures.

In the days preceding migration, songbirds increase the size of their gut by producing new and larger cells. This makes it possible for the birds to dramatically increase how much food they can consume so they can store up extra energy.

The digestive system, however, uses massive amounts of energy to operate. So once the birds have packed on the calories, they can shut down their digestive tract so the energy that would have been used to process food can be diverted to fuel flight.

McWilliams said that the birds’ guts actually begin to atrophy during migration. Unfortunately, that means that when the birds stop along their migratory route or reach their ultimate destination – a time when they are surely hankering for a meal – they are unable to eat for a day or two until their gut resumes operating normally again.

This discovery explains why birds do not immediately gain weight when they complete their migration. It also provides insight into their dietary requirements prior to departing. Because rather than exclusively consuming high energy foods to sustain their flight, they also must eat proteins to help rebuild their digestive tract at the end of the trip.

Which reminds me that I should probably go for a bike ride. It’s not only birds that can modify their bodies to accomplish formidable feats of endurance. With a little exercise, people can too.

Naturalist Todd McLeish has been writing about wildlife and the environment for more than 25 years. His latest book is “Narwhals: Arctic Whales in a Melting World.”

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