The fall migration is well underway. Swallows are scarcer than hen’s teeth now in Maine. We will bid adieu to most warblers and vireos this month.

Migration is a tremendously expensive and arduous undertaking for a bird. As warm-blooded vertebrates, birds have a high metabolic rate. Smaller birds have a tougher time of it than larger birds. On a per-gram basis, it is much more expensive to be a hummingbird than a robin.

To complete a migratory journey, birds require predictable food all along the way. Many migratory songbirds rely on fruits to help meet their fueling requirements. Before Europeans settled in North America, migratory songbirds took advantage of fruits like winterberries, pin cherries, mountain ash fruits.

Human colonization has resulted in both the intentional and accidental introduction of exotic plants. I am writing this column in Lubec, and I can look out the window and see large banks of the invasive Japanese knotweed across the way. Bittersweet, Japanese barberry, Tartarian honeysuckle, Morrow honeysuckle, autumn olives, multiflora rose and two species of buckthorns are other well-established invasive plants in our state.

Invasive plants often outcompete native plants. An invasive plant is usually free of herbivores and pathogens that it has to contend with in its native habitat. Such ecological release is a huge advantage in competing with native plants.

We know that invasive plants are causing reductions in the abundance of native fruit-bearing plants. What implications do these changes have for migratory birds?

Brie Drummond addressed this question for her honors thesis at Colby College in 2003 and published her work in the Northeastern Naturalist in 2005. She studied two representative introduced plants – Tartarian honeysuckle and multiflora rose – and two native plants – a viburnum species and silky dogwood.

She found that the fruits of the honeysuckle and dogwood degrade quickly. All were either eaten or rotted by the end of November. Rose and viburnum fruits persisted into the winter, offering wintering birds (primarily waxwings and American Robins) some sustenance.

Drummond measured the energetic content of each of the four types of fruits using a bomb calorimeter. The fruits of the two native species had higher caloric content than the two invasive species. However, experiments with the rose and viburnum fruits showed that birds did not show a preference between the two fruits. Perhaps, thought Drummond, birds are selecting fruits based on carbohydrate or fat content rather than total energy content. Digestibility of the fruits may play a role as well.

Susan Smith and colleagues at the Rochester Institute of Technology did a more detailed analysis of fruit quality, determining total energy content and fat content of each type of fruit. They found that the caloric content of native plants was slightly higher than that of invasives. However, striking differences in fat content emerged. No invasive fruits had higher than 1 percent fat content, while the fat content of native fruits ranged from 6 to 48 percent.

Migrating birds primarily use fat to fuel their migration. Smith and colleagues measured preferences of fall migrants and found results that fit their predictions: fall frugivorous birds prefer native dogwood fruits to the fruits of the four invasive species.

Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at:

[email protected]

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