Survival of the Fittest - and the Least Stressed
Biologist Rebecca Holberton's research focuses on hormones as indicators of avian and environmental health

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At just about 4 1/2 inches long, weighing in under 4 ounces, the blackpoll warbler looks like any other songbird as it summers in northern coniferous forests of Canada and Alaska.

But around this time of year, when nature calls it back to its winter home in South America, the blackpoll warbler turns into "the athlete of the bird world," flying nonstop across the open waters of the Atlantic, from New England and the Maritimes to the Caribbean, then on to the Amazon River Basin in Venezuela — approximately 2,300 miles in almost 90 hours.

It is believed to be the only Neotropical migratory songbird to make such a spectacular journey.

For years, scientists have known the birds do this, but had few clues as to how it was physically possible. Until now.

University of Maine biologist Rebecca Holberton has discovered that the blackpoll warbler prepares for its marathon migration by bulking up with the help of a natural steroid in its body.

In her research, funded by the National Science Foundation, Holberton has shown that a blackpoll warbler preparing for migration can virtually triple its body weight in 72 hours, going from its average weight of .385 ounces to 1.12 ounces. It also is more efficient than other songbirds in how it stores body fat and uses muscle.

"Imagine if you woke up three days from now weighing three times your normal body weight," Holberton says. "These birds don't eat any more than other songbirds at this time of year, but they undergo a huge change in physiology.

"If we can understand that physiology, perhaps we can better understand metabolic systems in humans."

"(Blackpoll warblers) don't eat any more than other songbirds at this time of year, but they undergo a huge change in physiology. If we can understand that physiology, perhaps we can better understand metabolic systems in humans." — Rebecca Holberton

Holberton's background is in physiology, or bodily processes. In her work, physiology is the link between animal behavior and the environment. She uses birds as "a model system" by which to understand how external and internal factors affect individual survival.

Currently, her research focuses on how birds meet challenges to their energy demand. Holberton is studying endocrinology, or the hormonal response to any behavior or physiology — one of the least understood areas of avian biology.

"I'm trying to understand how things work, then trying to apply that knowledge to better the environment, and human and non-human life.

"While we can ask the same questions of reptiles or zebras, birds are my choice. They are on every continent, totaling more than 9,000 species," says Holberton, who fields more than 50 inquiries annually from prospective graduate students similarly interested in pursuing research on Neotropical migrant birds.

When she was a graduate student, Holberton worked first with dark-eyed juncos in the Northeast. Post-doctoral research took her twice to Alaska to study several species of sparrows, and once to the Antarctic to study seabirds, including penguins and many relatives of the albatross. At the University of Mississippi, where she was a faculty member for seven years, she studied the endocrinology of birds and red-eared slider turtles.

She has collaborated with researchers working in Jamaica and Belize, Mexico and Puerto Rico to look at the health of habitats and birds that winter there. For the past five years in Manitoba, Canada, she has studied the breeding grounds of blackpoll and yellow-rumped warblers. In New Zealand, Holberton has collaborated with researchers in the Kakapo Recovery Programme.

In an ongoing collaboration with the Smithsonian Environmental Research Center, Holberton and her colleagues were the first to show that endocrine or hormone measures can assess how well birds survived in their wintering grounds, which can affect how likely some birds will survive to breed.

Now in UMaine's 4,200-acre Penobscot Experimental Forest, Holberton is studying the transition from spring migration to breeding. She is interested in how hormones affect the energetic condition leading to reproduction in several species of migratory and resident songbirds, such as magnolia warblers, ovenbirds and hermit thrushes, chickadees, sparrows and blue jays.

Concurrently, Jason Johnston, a Ph.D. student in biology, is researching bird diversity in the Penobscot Experimental Forest. Zoology graduate student Brent Horton is studying "why some fathers are better than others" among white-throated sparrows.

"We have been studying in the Penobscot Experimental Forest for a couple of years now. This year we expect some important answers," Holberton says.

Holberton is completing the final leg of a three-year, $270,000 National Science Foundation project to study the difference in long- and short-distance migrations of the blackpoll warbler and yellow-rumped warbler. Ornithologists already knew about the unusual migration strategies of the blackpoll warbler. These birds wait for autumn's west-northwest winds, then follow the cold front out over the North Atlantic. After about a day-and-a-half over open water, the birds fly through the front, using the strong northeast winds on the other side of it to turn them south.

"They put on fuel reserves and take off," Holberton says. "Once they're committed to taking off, they're stuck."

The key to the physiological change needed for such an arduous migration is corticosterone, a steroid hormone secreted by the adrenal gland. Corticosterone is important in protein and carbohydrate metabolism. Holberton has found that if a bird's ability to increase corticosterone is blocked prior to migration, it is unable to fatten for its flight.

With the discovery of a direct relationship between corticosterone and fat in the blackpoll warbler, Holberton and graduate student Jennifer Long are now studying the hormone at the molecular level. The goal is to better understand corticosterone's cellular mechanisms, including enzymes it is regulating.

In September at the 3rd North American Ornithological Conference, Holberton will present research results that support the theory that a single hormone can have varied physiological responses with different cell receptors.

"Corticosterone rises to low or moderate levels on a short-term basis for migratory fattening or during other natural, non-stressful life stages, like natal dispersal. It reaches high levels when birds are stressed and meeting emergencies.

"In such a chronic stress response, the hormone is believed to be signaling a different set of receptors than during the normal migratory period. That's what's challenging people. We're working to understand how one hormone can signal different physiological effects."

Corticosterone is essential to birds' survival, whether they are going through the natural life stages or fighting to stay alive in stressful conditions, such as habitat deterioration or other human pressures. At abnormally high levels, the hormone acts to obtain energy in the form of glucose from skeletal muscle when fat stores are depleted. Extended periods of elevated hormone levels due to chronic stress can affect growth, development, reproduction and, ultimately, survival.

For Holberton, migratory birds are bioindicators. Their health, including endocrine disruption, is directly related to environmental health, and all life stages of migratory birds are inextricably linked.

"With migratory songbird populations declining, people pointed to conditions of wintering and summering grounds. But I was seeing birds in trouble during migration. That's why it's important to link all stages of birds' annual cycles. You can save habitat on either end of the migration route, but if you don't have the necessary habitats en route, birds will have problems."

The need for such a holistic approach is clear in the research of graduate student Deb Perkins, who is in the High Arctic studying ruddy turnstones, a long-distance migratory shorebird. Her work focuses on the birds' health following an exhaustive intercontinental migration from southern South America to their Arctic nesting grounds.

Once female turnstones lay eggs, they all but exhaust their energy reserves, leaving males to incubate the eggs and rear chicks. But what if the males also are weak because of declining habitats at stopover sites on the migration route? For the ruddies, that critical stopover is Delaware Bay, where development is increasingly diminishing natural habitat.

"The research is getting at a conservation question," says Holberton. "If these birds arrive in poor (health) condition with the (reproduction) strategies they've used in the past, the species will fail.

"We need to understand the mechanisms at work in order to understand what's wrong and how to fix it — in any species."

by Margaret Nagle
September-October, 2002

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