Here is your vocabulary word for today: ghrelin. Ghrelin is an amino-acid peptide hormone that affects appetite and the secretion of growth hormone in various organisms. New findings demonstrate that, for migrating birds, ghrelin governs when a bird will continue its trip after stopping en route between northern and southern destinations. People have known about bird migration for centuries, but knowing what causes an individual bird to decide when to resume its long journey after a pit stop along the way has eluded ornithologists until now.

According to a study published in the Proceedings of the National Academy of Sciences, “Twice a year, billions of birds migrate across continents.” We see such migrations in the United States every spring and fall with hummingbirds, swallows, waterfowl and other avian species. The basic explanation for migration is that birds move to warmer feeding areas during autumn to avoid winter cold and depleted food sources. They return to cooler areas in spring and summer to breed when food has become more plentiful. One external factor influencing the onset of migration is the increasing period of daylight in spring and vice versa in the fall. But flying takes calories, so during a stopover, how does a bird know it has acquired enough fuel to continue its journey? A study conducted by Wolfgang Goymann (Max Planck Institute for Ornithology in Germany) and several colleagues may have found the answer.

The researchers studied garden warblers, which migrate across the Mediterranean Sea between northern Europe and central Africa. On the return trip, many of the little birds land on Ponza, a small island off the western coast of Italy. The Mediterranean is much larger than the Gulf of Mexico, across which some U.S. birds migrate, and Ponza is used as a feeding stopover before the birds reach the mainland and continue north. After such a long flight, it is essential for them to linger and refuel. But at some point they must resume their travel. The researchers wanted to find out what prompts them to leave.

Based on measurements of body fat of arriving birds in the spring, the researchers determined that food intake and the amount of stored energy of a garden warbler is controlled by its hormone levels. Field experiments revealed that injections of ghrelin caused birds to decrease their food consumption. Subsequently, the inclination of a bird to continue migrating increased. Furthermore, the higher the fat levels in a bird, the higher the concentration of ghrelin. Researchers found that the hormone controls the amount of food necessary for flight and is also indicative of a bird’s fat levels, a useful tool in research.

The research team also conducted hormonal experiments and found that birds they injected with ghrelin decreased their food intake and increased their drive to continue migration. The interpretation is that a bird’s migratory behavior is regulated by the hormonal system that controls the amount of food consumed and therefore the amount of stored energy required for long distance flying.

Interactions among hormones that affect diet are complex and not yet fully understood, but mammals and birds alike are affected by the presence or absence of ghrelin. For example, previous studies found that the ghrelin hormone stimulated food consumption in rodents but inhibited it in chickens and quail. The garden warbler study established a connection between stored food and migratory behavior.

According to the research publication, the discovery of the role of ghrelin in migratory birds reveals “a hormonal system shared by birds and mammals, whose disruption causes eating disorders, obesity” and complications in metabolism. The implications of the hormone ghrelin for controlling the appetite in people who need some help with that are extraordinary. More research will be necessary to make the use of ghrelin a medical reality for diet control, but the initial work on garden warblers demonstrates the importance of understanding connections between ecology and humans.

Whit Gibbons, professor emeritus of ecology, University of Georgia, grew up in Tuscaloosa. He received bachelor’s and master’s degrees from the University of Alabama and his Ph.D. from Michigan State University. Send environmental questions to ecoviews@gmail.com.