17 September 2009

Migratory birds: supremely designed for travel

Bird migration has no human equivalent. If we doubled our fat in a short time and try to use up the fat while doing a non-stop cross-country run, we would die. But this is exactly what migratory birds do.
The Black-winged stilt at Sungei Buloh Wetland Reserve. SBWR is an important stopover for migratory birds. Photo from Wetlands, a publication of SBWR.

And other marvellous feats too. How will global warming affect them?

Flying Transformers: Birds Gear Up for Migration
Brandon Keim - Wired.com, PlanetArk 17 Sep 09;
As days grow shorter, millions of birds across North America prepare for one of nature's great events: the autumn migration to warm winter climes. Many species fly for thousands of miles, often without stopping.

At their baseline state, migratory birds are already marvels of evolution. They're equipped with hollow bones and optical compasses and lungs that provide purer oxygen than is breathed by any mammal. But when dwindling sunlight sparks their migratory physiology, the birds become truly marvelous.

"This is one of those particularly amazing feats that animals do. It presents it with extreme difficulties. You're basically looking at the limits of animal design," said Scott McWilliams, a University of Rhode Island ecologist who specializes in the physiology of migrating birds.

In just a few days, their food intake rises by multiple orders of magnitude - the equivalent of having a hamburger for lunch on Monday, and 100 hamburgers on Friday. Of course, birds don't eat junk food: even seed-eaters switch to insect-heavy diets rich in energy-dense polyunsaturated fats.

The fat is packed on aerodynamically, tucked in the lower back and wherever else it won't add much drag. Some species, such as warblers, which can weigh less than an ounce but fly 2,500 miles without resting, double their body weights in preparation for the voyage.

"To put it in a human perspective, if you were going to put on a lot of fat and try to live off it while doing a cross-country run, you would die," said Russell Greenberg, head of the National Zoo's Migratory Bird Center. "There's no human equivalent."

In order to keep pace with the dietary influx, the birds' digestive organs expand. Even the cells of their stomachs swell. But shortly before takeoff, with no more need for this extra bulk, their guts shrink back to size.

In the meantime, the birds' pectoral muscles become thicker and denser. The pecs of the red knot, a shorebird that makes 2,000 mile-long migratory flights, swell by 40 percent.

Much of this added muscle mass will be burned for in-flight energy, but most of their fuel comes from fat. Unlike mammals, who fuel endurance exertion with protein and carbohydrates before switching - a transition felt as "hitting the wall" - birds start by burning fat, and use only the minimum of protein needed to keep their brains running. They never hit the wall.

To better turn fat to fuel, their bodies boost production of fat-metabolizing proteins. In sparrows, levels of the proteins double from their usual rates. To further feed their cells, extra oxygen-carrying hemoglobin protein is pumped into the blood, right up to the limit where it would be too thick to flow.

Some species do rest during migration, stopping for a few weeks before crossing some especially vast and barren expanse, such as the Sahara. Even in this brief time, red knots' pectorals will shrink at first, and their legs and stomachs swell. When the birds are done eating and ready to fly again, the process will be reversed. Their hearts grow the whole time.

Scientists don't know when birds originally evolved the ability to migrate. The tendency appears to be cyclical, expressing itself when ecological opportunities are rich and disappearing when temperate areas are limited. Modern migratory birds developed their habits over the last 10,000 years, since glaciers retreated at the end of the last Ice Age.

On an evolutionary scale, such plasticity is as radical as migratory birds' morphing muscles and metabolisms. It raises hope that the birds will weather their latest challenge.

"We're talking a lot about global warming these days. The environment is changing under these animals as we speak," said McWilliams. "Is an animal like a migratory bird, which has a tremendous capacity for phenotypic change, going to be less affected by climate change? Maybe. Maybe not."

Arctic Geese Skip Migration as Planet Warms
Michael Reilly, Discovery News 16 Sep 09;
In the Fall of 2007, tens of thousands of small arctic geese called Pacific brant (Branta bernicla nigricans) decided not to go south for the winter.

For these long-haul migratory birds, it was a dramatic choice -- they usually spend the cold months munching their favorite eel grass in the waters off Mexico's Baja peninsula. But changes in Earth's climate have so affected them that the barren windswept lagoons of western Alaska are looking more and more appealing.

The trend is likely to continue, according to a new study, affecting not only brant but a host of migratory birds around the globe.

David Ward of the United States Geological Survey in Anchorage has been studying brant behavior for nearly three decades.

When he began back in the 1970s, only around 4000 birds toughed out the winter in Izembek Lagoon, a 25-mile long stretch of protected water on the Alaska Peninsula. Two autumns ago, the number had climbed to 40,000 -- nearly 30 percent of the total population.

"The birds normally wait for a storm system to come down through the Aleutians," Ward said. "They catch the tail winds down south. But the track of storm systems is a little different now."

Changing winds have been accompanied by warmer weather, which means less ice covering Izembek's eel grass-rich waters. It's a buffet for the brant, which can feast through the winter without having to make the arduous journey several thousand miles south and back.

Come spring they are the first birds back to the breeding grounds, and often the most successful at raising their young.

In fact, conditions are so good that the geese run the risk of overpopulating, according to Robert Trost of the U.S. Fish and Wildlife Service in Portland, Ore. The Pacific brant population hasn't grown much in size over the years, but an increasing food supply could lead to an explosion of birds in the next few years.

"Throughout North America and parts of Asia, geese are most influenced by springtime conditions," he said.

As spring thaws creep earlier in the calendar, geese will be able to raise larger clutches of young.

The honeymoon isn't likely to last. Brant and many other species that live on coastlines could soon see their habitats flooded by sea level rise and swallowed by rampant erosion, two consequences of human-induced global warming.

"Right now it's conjecture to say what the long-term impact will be, but the prognosis is not so good," Trost said.

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