Study Shows Songbirds Switch From Bugs To Berries To Fuel Fall...( insect-rich diets. Parrish also con...)

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-
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Contact: Scott Turner
Scott_Turner@Brown.edu
401-863-2476
Brown University
12-Sep-1996