Migration Takes Guts: Birds Modify Digestive Physiology During Migration

Several studies conducted by URI physiological ecologist Scott McWilliams have shown that birds have a flexible digestive system that they modify to meet the changing energy demands of migration.

"The gut of a migratory bird is a really dynamic organ. In preparation for migration, the gut increases in size tremendously over several days," McWilliams said. "It expands, its cells get larger and it produces new cells so the bird can dramatically increase its food intake and store up energy for the long flight."

But because the digestive system is one of the most metabolically active tissues in the body and it consumes a great deal of energy, it shuts down during migration so more energy can be diverted to fueling flight. This partial atrophy of their digestive system affects birds when they stop to feed at sites along their migration route. McWilliams says that because their digestive system is shut down, the birds must eat less until their gut becomes acclimated and can operate efficiently again.

"We've known for many years that birds recovering from a migration flight do not immediately regain body mass, but we didn't know why. Now it's clear that this digestive constraint is responsible for the delay and likely affects the pace of a bird's migration," said the Kingston resident.

One important result of McWilliams' research is a new understanding of the protein requirements of migratory birds. Ornithologists have long believed that a diet high in energy was all that was necessary to sustain migratory flight. But the URI researcher said that proteins are also needed to build the digestive tract. This need for protein may have a significant impact on habitat management at key migratory stop-over sites.

"To build their digestive tract, birds need foods available in the environment that have sufficient protein," McWilliams said. "When birds feed only on fruits that are high in fat and low in protein, they may have to delay their migration. To help birds ensure a successful migration, we need to ensure, for example, that shrubs along their migratory routes have fruits with higher protein amounts."

Little is known about the nutrients in wild fruits, so McWilliams' current research is aimed at identifying the shrub species that bear fruit with high protein and energy content.

Funded by the National Science Foundation and the U.S. Department of Agriculture, and with logistical support provided by The Nature Conservancy, McWilliams' field studies have involved both free-living and captive birds, mostly white-throated sparrows, red-eyed vireos and yellow-rumped warblers. He and his students work each fall on Block Island, R.I. measuring the dynamics of body mass, body composition and gut size of wild songbirds as they stop there during migration. Using experiments with captive birds, he has also examined the physiological effects of short-term fasting, which most birds experience during migration, and then observed the feeding delay that occurs when the birds were then allowed to eat as much as they wanted.

McWilliams began studying the physiology of birds as a graduate student at the University of California at Davis in the early 1990s. He joined the faculty of URI's Department of Natural Resources Science in 1998 after a three-year postdoctoral fellowship at the University of Wisconsin at Madison.

"Migration is very costly to birds," said McWilliams. "But just like people who exercise to modify aspects of their bodies, birds can modify their bodies too so they can accomplish the formidable feats of endurance required by migration."