PROVIDENCE, R.I.--The early bird gets the worm. The smart bird snares the berry.
A new study by a Brown University researcher suggests that many songbirds migrating south each autumn may switch from typical meals of insects to a berry-rich diet to store sufficient fat to fuel their grueling migration. The study also shows migrating songbirds seek out fruit-laden "refueling" sites where they can fill up on berries for the long haul.
The Brown study indicates that these "pit stops," which are under intense development and ecological pressure, are extremely important to many species of tiny songbirds that travel thousands of miles every fall from breeding sites in the United States and Canada to wintering grounds in Latin America and the Caribbean.
"Many researchers and bird watchers knew that North American migrating birds consumed fruit during autumn, but the extent and importance of that shift wasn't clear," said Jeffrey Parrish, author of the study. "There has been a huge gap in our knowledge about the biological processes of songbirds during migration and how those activities might affect the recently reported declines in their numbers."
During three fall migrations on Block Island, R.I., Parrish and volunteers netted 7,000 songbirds and analyzed more than 1,600 of their droppings to determine their diets. In reports to be published in Studies in Avian Biology and tentatively accepted for publication in The Condor, Parrish found that a dietary switch from insects to fruit occurred to some degree in the majority of the 69 species sampled. Several species followed diets that contained more than 80 percent fruit.
Most migrants--the hermit thrush, red-eyed vireo and yellow-rumped warbler, for example--ate berries primarily. The fruits of native shrubs, including northern arrowwood, bayberry and pokeweed, contributed the majority of fruit consumed. Only a few species of birds, such as the American redstart, remained on strictly insect-rich diets.
Parrish also conducted feeding experiments with five species of migrating birds to test the hypothesis that the diet shift to fruit aided birds' ability to gain mass. These experiments demonstrated that a mixed diet of fruit and insects led to a significantly greater weight gain for birds than did a strictly fruit or insect diet alone. For some species, the more fruit in the mix, the greater the buildup of fat necessary for a successful migration.
Switching from insects to fruit offers migrants advantages beyond increasing their energy intake, Parrish suggests. Because fruit is clumped in specific sites, migrating birds spend less time and energy searching for food. Eating berries is more efficient than chasing insects, which are difficult to catch and less numerous in the fall. Feeding in dense, fruit-laden shrubbery rather than pursuing insects through open air also makes songbirds less likely to be taken by predatory falcons and hawks that also migrate south each fall.
To find out whether fall migrants preferred fruit-laden habitat or selected their habitats at random, Parrish and volunteers removed berries from 30-meter square areas of shrubs and allowed birds to choose between identical habitats with and without fruit. Three times as many songbirds visited berry-rich habitats.
The numbers of many migratory songbird species are declining rapidly worldwide. Fragmentation and destruction of both northern breeding and tropical wintering habitat have been considered the most likely explanations. However, the Brown study suggests important aspects of the birds' migration biology may have been overlooked.
"Many of these migrant birds behave very differently during migration compared to the summer breeding season in our forests and backyards where their actions and needs are more familiar to us," said Parrish. "Their diet and habitat use during migration are two examples of how they become more flexible during the migration period--and how our conservation tactics for them must also change."
While many songbirds breed in forests, they often use coastal shrubland habitats during fall migration. Rich in ripe berries during autumn, coastal shrublands may be especially critical in providing some migrants with the fruit-laden diet they desire. Yet today, these coastal habitats are under intense development pressure. More than half of the U.S. population lives in coastal areas, and that percentage is rapidly growing. Conservation biologists may need to add loss of stopover sites to their list of concerns for migrating songbirds. Parrish's study suggests that the importance of fruit-rich habitats along coastal migration routes should be considered in land-management and conservation decisions.
But Parrish urges private coastal land owners to be good land stewards too. "Conserving native habitat in a backyard is simple and inexpensive," he said. "Native shrubs, such as bayberry, provide natural beauty, thrive in coastal areas and are simple to maintain. Many songbirds will even use small, yard-sized patches of these natural landscapes during migration, providing people with exciting opportunities for bird observation--and providing the birds with the critical fuel for a long journey south."
Parrish recently received his doctorate from Brown in ecology and evolutionary biology. He is now a diplomacy fellow with the American Association for the Advancement of Science and is working on environmental issues in Eastern Europe and the former Soviet Union.