Autumn is a stressful time for birds as they prepare for winter. Whether they’re hunkering down for frigid weather or migrating to a warmer climate, fueling up is a top priority. Perhaps birds could have coined the term “stress eating.”
“Birds are under enormous pressure to pack on fat in fall,” said Amanda Gallinat, a visiting assistant professor of environmental studies at Colby College in Waterville, Maine, who has noticed the weight gain while bird banding. “We do a fat score, and birds quickly store up pads of fat on their chest and under their wings. Then they’re ready to migrate.”
Migrating is just one of many challenges facing birds in the fall. Fledglings are trying to get their bearings, and many birds molt before migrating. Birds that stay put also need to bulk up to buffer against the frigid, lean season ahead.
“They really don’t have much wiggle room,” Gallinat said.
Researchers are discovering that birds’ menu choices change dramatically when they flip into fall mode.
Many birds that spend the majority of the year eating insects, grubs and caterpillars turn to calorie-packed berries in autumn to furnish the energy boost they need.
“Berries are really important food sources, especially at stopover sites,” said Susan Smith Pagano, associate professor of biology at the Rochester Institute of Technology in New York. But it’s not just any berries; Gallinat and other researchers have found that, when possible, songbirds prefer native berries. That’s where humans come in. Gardeners can help birds make smart food choices when it matters most by furnishing the right feast.
Pagano’s research has shown that native berries such as dogwoods, spicebush (Lindera benzoin) and viburnums naturally provide the high fat and calorie content that birds need. Native perennials such as pokeweed and Virginia creeper are also good sources of healthy fuel.
Nonnative shrubs, on the other hand, are more like fast food: prevalent, but not a good fit for native songbirds’ nutritional needs. Pagano’s studies showed that nonnative and invasive bush honeysuckle (Lonicera tatarica), autumn olive (Elaeagnus umbellata) and common buckthorn (Rhamnus cathartica) fruit, for example, have low levels of nutrients, fats and proteins. Multiflora rose and Asian bittersweet (Celastrus orbiculatus) berries, which persist through winter, also provide low calorie and fat content. Pagano found that “the high water content in some nonnative fruits can dilute nutrients.”
By analyzing the contents of bird poop, Gallinat has learned that, when given a choice between native and nonnative berries, birds generally shun the exotic feast, going for the healthier options. “Birds just know what’s good for them,” Gallinat said.
That goes beyond the initial fuel-up. Johnny Randall, the retired director of conservation at North Carolina Botanical Garden, has studied bird preferences for stopover spots during migration.
Native viburnums, including arrowwood (Viburnum dentatum), dockmackie (Viburnum acerifolium) and downy arrowwood (Viburnum rafinesquianum), are popular pit-stop menu items. The same goes for native dogwoods, native hollies (particularly Ilex decidua, Ilex verticillata and Ilex opaca) and magnolias.
But when birds find themselves in their version of a food desert, forced to choose between consuming what is available or starving, they go into survival mode, often resorting to nonnative berries.
“There’s a strong correlation between what’s available in the landscape and what birds consume,” says Curtis Smalling, interim executive director of Audubon North Carolina.
In other words, if invasive autumn olives and porcelain berry are the only game in town, birds, “especially thrushes,” will go for them, Smalling says. Because they’re desperate, they may overindulge in these fruits. In some rare cases, the results can be lethal. American robins, for example, sometimes binge on the invasive heavenly bamboo (Nandina domestica), “which can be toxic to birds in high concentrations,” says Smalling.
A scarcity of appropriate native food sources can also delay stopovers during migration, slowing birds down while they search for fuel, Pagano found.
Feeding birds nutrient-rich berries from native plants isn’t just good for avian species; it also helps promote the growth of those plants, which many say are better for the environment because they have co-evolved with the overall ecosystem.
“Native plants depend on birds,” Smalling says. “Many berries germinate better when they’ve gone through a bird’s digestive system. It puts the plants on the landscape.” Nonnative seeds are also readily distributed via airmail. Privet, nandina, Bradford pear, porcelain berry and Asian bittersweet are all invasive berried plants that have spread courtesy of bird dispersal. Essentially, birds often help spread whatever you plant, so why not give the ecosystem the best shot at a positive impact.
“The takeaway is that you should plant a suite of native species that optimize fruit availability and bolster fruit diversity across the seasons,” Gallinat says.
- – -
Tovah Martin is a gardener and freelance writer in Connecticut. Find her online at tovahmartin.com.