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 predictabl 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.
The relationship between the fruit-bearing plants and the fruit-eating birds benefits both parties. The birds disperse the seeds of the plants in return for a bit of nutritious fruit.
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 habitats. Such ecological release is a huge advantage in competing with native plants with their own herbivores and pathogens.
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 Honor’s 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. The rose and viburnum fruits persisted into the winter, offering wintering birds (primarily waxwings and American Robins) some sustenance.
Brie measured the energetic content of each of the four types of fruits using an instrument called a bomb calorimeter. The fruits of the two native species had higher caloric content than the two invasive species. However, choice experiments with the rose and viburnum fruits showed that birds did not show a preference between the two fruits. Perhaps, Brie thought, 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. They published their work in 2013 in the Northeastern Naturalist. These researchers examined five native plants: three species of dogwoods, arrowwood viburnum and spicebush. In addition to the invasive plants studied by Brie, the RIT team examined buckthorn fruits.
The researchers determined total energy content and fat content of each type of fruit. They found that the caloric content of native plants was slightly higher than the caloric content of the invasives. However, striking differences in fat content emerged. No invasive fruits had higher than 1% fat content while the fat content of native fruits ranged from 6% to 48%.
Migrating birds primarily use fat to fuel their migration. Fats are more energy-dense than carbohydrates or proteins so a gram of fat provides more energy to a migrating bird. One would predict that fall fruit-eating birds would prefer the native fruits.
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.
[Originally published on September 6, 2015]