| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
|
|
|
|
|
Horizons Migrating Birds, Fit and Fat
In late summer, on the tundra surrounding Churchill, Manitoba, thousands of songbirds undergo a remarkable transformation. In the few short weeks between fledging their chicks and the arrival of winter snows, blackpoll warblers rebuild their bodies from the inside out to prepare for one of the most exhausting migrations in the avian world. In one 90-hour nonstop flight, they will travel 4,000 kilometers—halfway across the Atlantic Ocean—to winter quarters in the Brazilian Amazon. This epic migration flight puts blackpolls (Dendroica striata) in a class by themselves. “Other songbirds that must make dramatic journeys across hostile territory such as the Sahara can stop at oases, find shade, and eat. But blackpolls go out over the North Atlantic and have no such options. If they take off from the coast, and haven’t prepared for it right, that’s it,” says physiologist Rebecca Holberton of the University of Maine. Even Arctic terns, famed for their pole-to-pole migrations, have it easy by comparison—they can rest and float like ducks on the waves, or nab a fishy snack any time they feel like it. Terrestrial songbirds like the blackpoll must fast and fly for the entire ocean crossing. Every August, Holberton leaves her comfortable office and makes her own migration to Churchill. She goes to determine how a 4.5-inch-long handful of bone and feathers can fly so far in one stretch without collapsing from hunger, thirst, and exhaustion. Discovering how the blackpoll and other songbirds migrate phenomenal distances has not been easy. Though scientists have been scanning the skies for decades, we know more about the daily wanderings of blue whales than backyard birds on migration. Tracking birds with leg bands is a game of diminishing returns—thousands are banded, but just a handful are recaught. Radio transmitters are too heavy for all but the largest songbirds to bear. To make matters worse, most migrants travel at night, invisible to all but the most sensitive radar beams. After a century of careful studies, scientists like Holberton are gaining amazing insights into the symphony of instincts and hormones that change ordinary birds into energy-efficient, Olympic-class athletes. The first order of business for a prospective migrant is making sure it has stowed enough energy on board. Like bears preparing for hibernation, blackpolls balloon into roly-poly butterballs come autumn. But blackpolls will go the bears one better. Ounce for ounce, these songbirds can plump it up faster than the hungriest grizzly. Blow on the breast feathers of a blackpoll preparing to migrate, and the parting feathers reveal a thick white vest of fat beneath a taut membrane of skin. Blackpolls can triple their body mass, and double their fat load, in under a week. “Our champ went from 11 grams to 32 grams in five days,” Holberton says. For a 150-pound human, that would be like gaining 15 pounds of pure fat every day for a week. Then there is the blackpoll’s tundra neighbor, the closely-related yellow-rumped warbler (Dendroica coronata). Like most other songbirds, yellow-rumps fly south for a few hours a night over several weeks. A largely overland route allows them to stop, rest, and feed along the way. In summer, both blackpolls and yellow-rumps scarf down insects, take in a similar number of calories, and maintain nearly identical ratios of fat to lean body mass. Once the days begin to shorten, the scenario changes fast. Blackpolls “don’t eat any more than yellow rumps do,” says Holberton. Yet they consistently grow much fatter. “Clearly something is happening to allow them to get so fat,” Holberton says. Hormones, Holberton found, are the key to the blackpoll’s plumping success. In late summer, blackpolls produce more corticosterone than yellow-rumps. In the lab, blackpolls blocked from secreting the hormone fail to fatten. “Corticosterone is regulating the machinery” that transforms food into fat, says Holberton. Aiding songbirds in their quest for calories is an amazingly cooperative digestive system. Researchers have found that birds preparing to migrate literally grow bigger guts than at other times of the year. The extra alimentary space helps birds process far more calories than usual. Once a bird embarks on its journey, however, that giant gut becomes a liability. The same digestive organs so critical for amassing fat are also the most expensive to maintain. But birds have found a way around this as well. When Scott McWilliams, now with the University of Rhode Island, and Bill Karasov of the University of Wisconsin prevented white-throated sparrows (Zonotrichia albicollis) from eating for several days during migration, the birds’ innards literally shrank. And wild yellow-rumped warblers accidentally killed in mid-migration were found to have the smallest digestive organs ever seen in the species. Fasting while flying hard, they concluded, signals bird guts to atrophy and frees up more energy for flight. Those precious nutrients don’t go to waste. Birds likely recycle protein liberated from the gut to repair and maintain hardworking flight muscles. “Exercise demands protein catabolism. Over a day, the birds are literally having to replace structural muscle,” McWilliams says. Migrating songbirds also draw from protein stores banked in bulked-up leg and breast muscles. But shrunken guts can’t hold much food, either. This can pose a problem for birds counting on meals at rest stops. “Birds caught in nets within 24 hours or so of first landing at a stopover site don’t seem to gain much weight. This finding was somewhat puzzling; they should be packing it on,” McWilliams says. After fasting captive birds for several days and then allowing them to feed at will, he and Karasov showed that birds can enlarge their digestive organs within a matter of hours to take advantage of a stopover smorgasbord. Like athletes on a training regimen, birds will alter their diets to prepare for migration. Warblers that spend their summers chasing after protein-rich worms and insects begin carbo-loading on sugary fruits. At the same time, many normally vegetarian birds expand their diets to include the occasional caterpillar or spider. “These changes help accomplish fattening and increases in protein reserves that are stored in gut and muscle,” McWilliams says. And birds can identify exactly what foods they need to stockpile. “Birds can discriminate between foods different only in fatty acid composition,” says McWilliams, who discovered this ability with graduate student Barbara Pierce. “They have specific hungers for particular amino acids. It’s a way to deal with protein requirements—they can select a food that’s not abundant but contains the specific nutrients needed,” says McWilliams. A balanced diet delivers better flight performance; birds fed both saturated and unsaturated fats, the researchers found, were better able to boost their metabolic rates to peak performance levels than those fed on only one type of fat. While adding on the ounces, birds simultaneously turn into exercise fiends. Instead of tucking head under wing after sundown, many songbirds will gather in flocks to soar repeatedly from tree to tree. The exercise bulks up flight muscles in preparation for the journey to come. Captive birds flap in thrall to the same nervous urges. Pets once content to perch and feed will flutter their wings for hours on end in a weird, stationary pantomime of the entire migration. Species that cross deserts flap for as long as it will take them to reach their oases, rest for a day or two as though they were at an oasis, and resume their frenetic activity until they’ve flapped enough to finish the journey. Known as Zugunruhe, or migratory restlessness, the behavior highlights how hard-wired migration urges are in birds. In the 1930s, ornithologist William Rowan of the University of Alberta discovered there is a seasonal trigger for Zugunruhe. In the middle of November, long after local birds had already completed their migrations, he manipulated the lighting in his aviary to mimic the lengthening days of spring. Rowan was able to trick his captive juncos and crows into resuming their frantic flights. Staying up late helps birds adjust to a nocturnal migration schedule. Migrating by night has many advantages. On clear evenings, birds can navigate by sighting on stars. Darkness helps birds evade the sights of predators, while denser, cooler night air and generally calmer winds make for easier flying conditions. Night flying might also help birds conserve energy. Just this year, Martin Wikelski of Yale University and colleagues tracked energy usage in migrating Catharus thrushes caught in Wisconsin. They injected the birds with heavy isotopes of oxygen and hydrogen, glued transmitters to their backs, and let them go. After a harrowing night barreling down rural highways in hot pursuit, the researchers located their quarry a few hours later in suburban Illinois. Sleepy local residents awoken by frantic knocking let the mad scientists into their backyards to recapture the birds with mist nets. The hard work paid off. Blood samples revealed how much heavy water the birds had metabolized. The researchers found the thrushes used less than 30 percent of their energy on flight. Their remaining energy was spent finding food at stopovers and keeping warm while resting. The findings suggest night migration helps birds dodge the high cost of staying warm while resting during frosty spring evenings. After launching themselves into the heels of an autumn cold front, migrating blackpolls will emerge over the heart of the North Atlantic. From there, they ride prevailing winds south to Brazil, where they disappear into the jungle. After a winter in the tropics, the blackpolls turn north again, lured by the sky-darkening clouds of nutritious tundra mosquitoes. Luckily, the return trip to Churchill is far less perilous. Heading north across the Caribbean, blackpolls can island hop until they reach North America, bringing their prodigious migration full circle. Kathleen M. Wong is Senior Editor of California Wild |
||||
