Bird Migration

Text by Liz McKenzie      Photos and Sounds by Richard Nelson


"Even the smallest migratory hummingbird,
with a brain scarcely larger than a dried pea,
can plot a flawless course spanning immense distances."

– Scott Weidensaul, Living on the Wind

Champion Migrants

In the avian world there are athletes of all stripes—high altitude fliers, darting aerial gymnasts, masters of desert heat or arctic cold. But perhaps the most remarkable of all are the long distant migrants. Some are exceptional for their diminutive stature and the ardor of their journey; some are outstanding for the distance they cover; and some are best described as miraculous--for the amount of time they spend on the wing without stopping for rest or food.

Consider ruby throated hummingbirds—tiny feathered bundles that migrate every spring from South America to the southern U.S., a 500-mile flight across the Gulf of Mexico, nonstop, entirely alone, and sometimes battling against storm winds.

Recent advances in scientist's abilities to measure bird's migratory paths, especially with satellite tracking and geolocation, have revealed many other astonishing feats of endurance and faith.

Let's look at four of these grand champions of bird migration.


Blackpoll Warbler


Blackpoll Warbler. Photo by Bob Armstrong.


Migration Routes for Alaska's Blackpoll Warblers

These strikingly marked, black-on-white songbirds are just slightly bigger than hummingbirds, measuring about 5 ½ inches long and weighing about as much as a U.S. fifty cent coin. They might flit unnoticed in neighborhood parks, but their story teaches us not to judge anything by its size.

Blackpolls have a remarkable, multistage migration. In summer, they breed all across the boreal forests of Alaska and Canada, and the first leg of their fall migration sees them moving eastward to the maritime areas of eastern Canada and New England. For birds that started their journey from Alaska, already they've made a transcontinental journey of 3,000 miles.

After arrival on the east coast, blackpolls spend several weeks feeding and fattening, which nearly doubles their weight. They will need all that extra fuel for the next leg of their journey—a transoceanic flight, aided by autumn tailwinds, covering another 2,000 miles. This they accomplish in about 40 or 50 hours of non-stop flight, flapping their wings 1,200 times per minute (which equals 20 wing beats every second) all the way to Venezuela or Guyana in tropical South America. Some blackpolls go even farther, adding another 1,500 miles, to Bolivia or Brazil.

The blackpolls' spring migration takes them on a different route, first across the Gulf of Mexico or the Caribbean Sea, and then up through the interior of the continent, back to the northern boreal forests. For warblers who summer in Alaska, this makes a round trip journey of up to 12,000 miles—the longest migration of any North American songbird.


Sooty Shearwater and Arctic Tern


Satellite Tracking of Sooty Shearwater Epic Migrations
Shaffer, Scott A. et. al. Migratory shearwaters integrate oceanic resources across the Pacific Ocean in an endless summer. PNAS 2006 103 (34) 12799-12802. Copyright 2006 National Academy of Sciences, U.S.A.

Every year, ocean diving sooty shearwaters make an epic, ocean-spanning flight from their nesting areas in New Zealand up to rich feeding areas in the North Pacific, off the coasts of California, Canada, and Alaska.

Scott Shaffer and his colleagues at the University of California, Santa Cruz, attached very small, light-based geolocators to the legs of sooty shearwaters to track their migration—and what their data revealed is extraordinary: a round trip migration of 40,000 miles.

This journey rivals that of another remarkable migratory seabird—the arctic tern. Both shearwaters and terns can rest on the water and catch their food at sea, so they are well equipped to sustain these epic transoceanic journeys.

It has long been known that arctic terns migrate every year from nesting grounds in the Arctic to wintering areas at the other end of the world—in the waters off Antarctica—but the full extent of their travels was not discovered until scientists fitted arctic terns in Greenland with geolocators.


Arctic Tern Migration Paths
Greenland Institute of Natural Resources

These tiny devices weigh less than a few grains of rice, making it possible to track small birds that could not fly with the heavier satellite devices. Geolocators record sunrises and sunsets, and with complex computer programs scientists can figure out where the birds have traveled.

In 2008, Danish researcher Carsten Egevang and colleagues recaptured 10 Arctic terns fitted with geolocators. Many months of calculations later, the results were in: The terns had flown from their summering areas near Greenland, southward to the coast of North Africa where they lingered for several weeks. Then they continued south all the way to the icy waters near Antarctica, where the southern hemisphere gave them another summer.

The spring migration route for the terns took them up the Atlantic coast of either South America or Africa, and finally back to the Arctic for the northern summer.

All told, the terns covered a huge expanse of the globe. Their total round trip mileage—an astonishing 49,700 miles—is the longest migration ever recorded for any living creature.

And because arctic terns move between summers in both the northern and southern hemisphere, they spend much of their time in 24-hour daylight. So these birds enjoy more light than any other animal on earth—and this gives them more time for fishing and fueling up for these unimaginably long migrations.


Bar-tailed Godwit


Bar-tailed Godwit. Photo by Bob Armstrong.


Bar-tailed Godwit Migratory Routes

Perhaps the most astonishing of all bird migrations is the one recorded in 2007, by the team led by biologist Robert Gill of the USGS Alaska Science Center in Anchorage. Gill used satellite tracking to follow the migration of shorebirds called bar-tailed godwits—and most notably a female godwit simply called "E7".

Bar-tailed godwits are powered migrants—which means that, like blackpoll warblers, they must flap their wings the whole way, without the luxury of soaring or gliding. Not only this, but unlike arctic terns or sooty shearwaters, powered migrants can never stop to feed or rest at sea.

On August 30, E7 took off from the coast of Alaska and without ever stopping to rest or refuel, she landed in New Zealand nine days later.

Bar-tailed godwits time their departure to coincide with favorable weather patterns, so E7 intentionally took advantage of tailwinds. These helpful winds, combined with her own uninterrupted wing beats, allowed her to travel at an average of 35 miles per hour.

The total distance—7,270 miles flown entirely over the immensity of the open Pacific Ocean. With stored body fat as her only fuel, this shorebird weighing less than a pound made the longest recorded nonstop flight of any bird.


The Impossible Made Possible

We still have much to learn about how birds are able to accomplish such feats of endurance and strength. We do know that over the millions of years of bird evolution, they've been specially equipped to undertake the most arduous of travels.


Equipment

Birds have a variety of wing shapes—and those of longer distance migrants tend to be longer and more pointed than the wings of birds who make shorter trips or who don't migrate at all. The advantage of long and pointed wings is that they create less drag, while still maintaining lift.

As we've seen, blackpoll warblers are among the champion bird migrants, and not surprisingly they have longer wings than any other species in the very large family of North American warblers.

Hermit thrushes are short distance migrants—wintering in the U.S. and Mexico—and their wings are shorter and more rounded compared to the wings of Swainson's thrushes that migrate all the way to South America.

Also important is the type of flight undertaken—if a bird has to move its wings, it's using precious energy. Warblers and bar-tailed godwits, as we've seen, are powered migrants, meaning they have to flap their wings continuously to fly. Others, like shearwaters are able to pump their wings and then soar or glide. Some birds, like hawks alternately soar and flap their wings to move between thermals, which carry them effortlessly upward, as if they are riding on pillows of heated, rising air. After gaining altitude, they fly until they find the next thermal. And albatrosses, the champion soarers, often keep their long, bladelike wings extended without flapping for hours, using the constant wind and the updrafts created by ocean swells to keep them aloft.


Navigation

We don't fully understand how birds stay true to the compass on journeys covering hundreds or thousands of miles, but we do know that they are masters of navigation.

Migrating birds use a number of senses and skills to stay on track. Sometimes they navigate by sight, following landmarks such as mountains and coastlines and rivers. They also use the position of the sun as a natural compass…but this means they have to compensate for the sun's changing position throughout the day.

Many migrants fly at night, taking advantage of cooler temperatures, higher humidity, less turbulent air and safety from predators—so they need other ways to stay on course. The moon and stars help birds to orient in darkness. This is why the bright lights of cities are a problem for birds navigating at night—it may be harder for them to see the dim pinpoints of stars and they might also become disoriented because the land beneath them is swarming with unnatural lights.

Birds also have an almost magical ability to detect the earth's magnetic field, so they literally have a built-in compass to help them stay on course. In fact, biologists have found that some bird species have magnetite in their nasal cavities. As they fly above the earth, they might be perceiving changes in the magnetic field, which they use like relief maps to follow their route.

Birds lifting off at sunset may be taking advantage of an ability to detect polarized light in one part of the sky. This helps them to head off in the right direction for their flight through the darkness.

Some migrants may also use their sense of smell. For example, when a bird gets close enough, perhaps it can smell its home river, or a bird heading up the coast can smell the sea and may know to keep flying parallel to the coastal waters.


Fuel

Perhaps the most crucial factor for long distance migrants is their ability to gain a huge amount of body fat very quickly to fuel their ultra marathon flights.

We know that on average, before birds start migrating, fat makes up a very high percentage of their total body weight-- about 15% for hawks, 66% for shorebirds, and a whopping 70% for songbirds.

Before bar-tailed godwits take off from Alaska on their nine-day, nonstop flight to New Zealand, their bodies are 55% fat.

The red knot is an even more impressive example. When these robin-sized shorebirds start their flight from Brazil all the way to the arctic, they are about 66% body fat! Partway along their journey, red knots land on the coast of North America at Delaware Bay for a rest and refueling stopover. By this time they are exhausted and starving—and down to only 3% body fat. In just two weeks, they must gain back their reserves before heading north again for thousands of miles to their Arctic feeding and nesting grounds.


An Act of Faith

When birds migrate—heading north or south, east or west—for most there is no turning back. And for birds who migrate long distances, especially those who commit to flying over open ocean, it is critical to find food and rest when they reach their stopover or final destination. They are exhausted and starving when they arrive—and they must refuel or they won't survive.

Some of these critical fueling stations are disappearing.

Red knots, for example, depend on horseshoe crab eggs to replenish their depleted bodies at the stopover in Delaware Bay, but the horseshoe crab population has been severely reduced by commercial fishing. The number of red knots had also dropped so low that in 2005 the Audubon society and other groups asked the US Fish and Wildlife Service to list the red knot as an endangered species. But their bid was unsuccessful.

After leaving New Zealand on their return to the arctic, bar-tailed godwits depend on wetlands in coastal Asia. About half of those wetlands have been lost to development and as much as 40% of the remaining wetlands are scheduled for development. Bar-tailed godwit populations have fallen to about half of their numbers in the 1990's.

The loss of stopovers may be a key factor in such migratory bird declines. We can understand this in human terms. If a person's survival depended on driving across the continent every year, but the gas stations started closing so the gaps between refueling stops grew larger and larger, eventually it would become impossible to reach the destination. This is exactly what some bird species face today, as fueling stations like marshes and coastal wetlands are lost to development.

Migratory birds face many other new challenges, such as nighttime collisions with tall buildings, cell phone towers, and power lines; poisoning by pesticides and herbicides; and falling prey to free-ranging house cats.

According to the Fish & Wildlife Service, about a quarter of all migratory bird species in the U.S. are declining fast enough to be considered in trouble.

Every time a bird takes wing on the annual migration, it is an act of faith—that the same tropical forest will be there to provide food and a home for the winter, the same shoreline to rest and feed at a critical stopover, the same stretch of boreal forest for nesting and raising the next generation. It is these acts of faith and endurance that give us champion migrants like the red knots and Swainson's thrushes, blackpoll warblers and bar-tailed godwits, arctic terns and sooty shearwaters.

Migratory birds bring flashes of beauty and song into our daily lives, and more than ever, the fate of these miraculous creatures is in our hands.

Learn more about the miracle of bird migration.

Learn about the challenges facing migratory birds and what you can do to
help
.


Works Cited

Weidensaul, Scott. Living on the Wind: Across the Hemisphere with Migratory Birds.
     New York: North Point Press, 1999. Print.


For Further Reading:

Living on the Wind by Scott Weidensaul
The Migration of Birds: Seasons on the Wing by Janice M. Hughes
Make Prayers to the Raven: A Koyukon View of the Forest by Richard Nelson

For Listening:

Bird Migration
Boreal Forest
Gray Jay
Hummingbird
Sandhill Crane
Shearwater
Shorebirds
Thrush

Important Links:

Alaska Department of Fish and Game
American Bird Conservancy
Audubon
Audubon Water bird Conservation: Issues and Answers
ebird
H.R. 1643 Federal Bird Safe Building Act of 2011
Migratory songbirds and cats
Nature Conservancy
Nature Conservancy's Migratory Bird Program
Shade Grown Coffee and Migratory Birds
The State of the Birds 2011
US Fish and Wildlife Service
USFWS Migratory Bird Program

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