Some birds have developed amazing physical adaptations to assist them during long-range migrations (Whimbrel photo by Paul Konrad).

Some birds radically change their bodies and their metabolism for the major migrations they undertake. They undergo physical transformations, shrinking their internal organs, rapidly gaining and absorbing fat deposits, barely sleeping, and more. These are incredible adaptations, but they come with trade-offs. The energy required to fly hundreds or thousands of miles during a short time span leaves birds with little room for error during migration, while they are especially vulnerable to natural and human-based threats.

Migrations are an especialy perilous time, when birds fly across expansive landscapes and waterscapes, survive storms and cold snaps, navigate skyscrapers and windows, avoid predators, and successfully forage for food - or else fail to complete their journeys. Migration is perilous, but it’s also wondrous; consider some of the incredible ways birds’ bodies have adapted in extreme ways for migration:

Some Birds Double Their Body Weight - A bird’s first inkling that it’s time to shift into migration mode comes from seasonal changes in its surroundings, explained Paul Bartell, professor of avian biology at Penn State University. When the days become progressively shorter during the end of summer, birds undergo hyperphagia: They eat excessive amounts of food for 2 weeks or more to store fat before migration. During this time, many birds gorge on high-energy berries and fruits loaded with carbohydrates and lipids, which are stored as fat.

Birds’ reliance on fat is unusual in the animal kingdom. “It’s remarkable that they’re using fats as fuel,” says Scott McWilliams, professor of wildlife ecology and physiology at the University of Rhode Island. People, for example, rely primarily on carbohydrates for endurance activities because our physiology can efficiently convert carbs into usable energy. But for birds, fat makes sense. Fat is lighter and less bulky than carbohydrates and protein, which is important for lightweight small-framed bodies that must stay aloft by beating their wings. Plus, fat contains more energy than carbs. “You get the most energy per gram of fat that you store, if you want to fly long distances,” Dr. McWilliams explained. As they fly, birds can replenish fat by taking breaks to feed. That’s why it’s so important that we grow native plants that produce the lipid-rich berries that birds need.

Ruby-throated Hummingbirds are best known for packing on grams of fat: Most of these tiny birds at least double their body weight in fat before embarking on migrations. Some even gain close to half that in just 4 days! They need this increase because their metabolism is one of the most extreme of any animal on earth. Ruby-throats require the human equivalent of more than 150,000 calories every day to power their fast-moving heart and wings, which can beat 1,000 and 3,000 times per minute respectively. The fat accumulated before migration is burned in a steady release of energy, ideal for the 2,000-mile journey many Ruby-throated Hummingbirds make twice each year.

All that fat added on to a small bird’s frame can’t be deposited just anywhere - it must be distributed properly. To make it all fit, many birds’ bodies are able to shrink and grow their internal organs. Bar-tailed Godwits, for example, are among the bird world’s most intense migrators, flying 6,800 miles non-stop from Alaska to New Zealand each fall. To make room for energy-rich fat, these godwits absorb into their body 25 percent of the tissue comprising their liver, kidneys, and digestive tract. That happens through a natural cellular process in which their body recycles and cleans its cells and tissues through autophagy (which means “self-eating” in Greek). Bar-tailed Godwits also increase the size of their heart and chest muscles to distribute extra energy and oxygen to these highly active areas during mid-flight.

“Flying is the most energy-intensive form of locomotion,” Dr. McWilliams explained, “but it’s also more efficient if you want to go farther, faster. A flying bird actually gets better energy economy when using fat.”

Birds also undergo organ transformation during hyperphagia. While gorging on berries and bugs to gain weight, songbirds like Blackpoll Warblers expand their digestive tract to process more food, and quickly shrink and re-absorb parts of the same digestive system during migration as they burn fat. According to McWilliams, this keeps energy focused on the most important flight muscles, reducing any need for frequent breaks to feed at stopover sites.

Birds Greatly Can Reduce Their Sleep Time - The need for sleep among some birds seems as though it’s just another physiological rule to break. During migration, a neurological shift initiated by the changing season forces birds to adapt to nocturnal habits and sleep less. How do birds rest while in mid-air? Very quickly. Swainson’s Thrushes, a species that migrates as much as 3,000 miles from wintering areas in Central or South America to nesting sites in northern Canada and Alaska, enter a sleep-like state for about 9 seconds at a time. They keep half of their brain awake to avoid predators or mid-air collisions while the other half of their brain rests.

The neurological change from wintering- and nesting-season sleep behavior to migration sleep is as crucial to bird endurance as metabolic changes, said Dr. Bartell. “Birds are somehow resilient to all the increased fat and the detrimental effects of staying up almost all night. And they actually perform better than if they were in a non-migratory state.”

Some Birds Consume Their Own Muscles - As a last-ditch backup, some birds can absorb their muscles for energy, a mode that some birds use to their advantage. Experiments conducted in a wind tunnel revealed that Swainson’s Thrushes that typically fly up to 200 miles in a single stretch during migration, even absorb muscle unnecessarily so they can fly farther and reach the most beneficial stopover sites. It is risky, though, if they can’t recover that muscle after a migration.

Revert to Their Previous Form - When birds reach their destination, they need to quickly regain their organ function and shapes, and rebuild emaciated fat deposits. It’s an urgent task during spring migration because as soon as they reach their nesting range, birds must do the hard work of nesting - attracting a mate and producing and raising young, all while still caring for themselves. “Essentially, birds need to quickly start establishing territories and reproducing,” Bartell said. “If they can’t get food within a couple hours or are delayed after landing, they can actually starve.”

The stakes are lower during fall migration because birds don’t need to nest when they reach wintering areas; plus the warmer tropical areas to the south tend to have more food available. “All they’re usually trying to do is to maintain their body mass at a certain level and get through the winter,” McWilliams added. Birds on average need to restore between 17 and 23 percent of their body weight in fat when they arrive at wintering areas, while accounting for significant protein and water loss depending on the species and migratory pattern.

It’s all an astounding physiological balance to maintain while undergoing spectacular migrations across the planet. But if we’ve learned anything about birds and migrations at this point, it’s that we shouldn’t be surprised by anything.

To refer to the original Audubon article by Kevin Johnson, see https://www.audubon.org/news/five-incredible-ways-birds-change-their-bodies-spring-and-fall-migration