College Park Wild: Good berries, bad berries
By Rick Borchelt
A mockingbird staked out a deciduous holly at the corner of the house this week. The holly’s bearing heavily this year, filled with bright scarlet berries, and the mockingbird is going to make sure he has exclusive dibs on it through winter, until the berries are gone. Male mockingbirds set up territories so they can defend and protect bushes with edible berries during winter the same way they defend nest sites and females in the spring — with song and aggression displays, lifting their gray wings to expose the white patches as warning flags to other mockingbirds.
Mockingbirds, which are members of the thrush family, are so protective of holly bushes because the berries are a major food source for them during the winter. Like other thrushes we’re familiar with, they have a fruit-rich diet as adults, which earns them the name frugivore — fruit eater. Some thrushes —notably wood thrushes, veeries and catbirds — are critically dependent on fruit as they fly south for the winter. Thrushes that spend the winter here — mockingbirds, along with robins, hermit thrushes and bluebirds — need berries to get them through the cold season. Native berries provide not just a sugar fix but also complex proteins and fats.
We have both evergreen and deciduous native hollies here. Our evergreen species, the American holly (Ilex opaca), is a common understory tree in Maryland, while our deciduous holly (Ilex decidua) — commonly known here as winterberry — is typically found in wetlands in our area. Berries of the deciduous holly are often featured in floral arrangements and wreaths this season.
But there are other, non-native berry producers that frankly are much more of a problem for frugivores than they are a help.
Chief among these bad berries producers are Asiatic bush honeysuckles, especially Amur honeysuckle (Lonicera maackii). These shrubs have red berries that they often bear in abundance. Many wildlife ecologists fear that bush honeysuckle berries act as bird crack — they are high in sugar but low in nutrition. Migratory birds require a diet higher in fats that fuel their long flights to Central and South America; in contrast to those sugary crack berries, native berries offer balanced fats and fill this bill.
The New York Botanical Garden introduced Asian bush honeysuckles in the U.S. in 1898, and with their early spring flowers and red berries, they became popular — and widespread — during the 20th century. Ironically, Amur honeysuckle was originally touted by state and national wildlife officials as a good option for wildlife cover and food; indeed, the shrub was distributed for free to many homesteads and farms.
Another bad berry that is even more sinister, one that actually poisons birds when they eat it, is the fruit of heavenly bamboo, also simply called nandina (Nandina domestica). Nandina is another Asian species that was deliberately introduced,, in 1804, into the U.S. horticultural trade. It is prized for ferny green foliage, brilliant fall color and abundant clusters of scarlet fruit, and you can readily find the shrub at many plant nurseries in growing zones 6 – 12. But nandina’s fruit and leaves contain cyanogenic glycosides, chemicals that convert to hydrogen cyanide when eaten. (Yes, the same cyanide that figures in so many whodunit crime shows and every other Agatha Christie novel.) Birds tend to bypass nandina berries as long as there are juicy native edibles around, but in poor berry-crop years, or late in the winter when nutritious berries are no longer available, birds will turn to heavenly bamboo berries, even as a large serving can prove to be fatal. Cedar waxwings, also a frugivore, seem to be especially vulnerable to nandina’s poison, though the leaves and berries are also toxic to other birds — and to dogs, cats, horses and even people.
What to do about the bad berries? Amur honeysuckle should be cut back before the berries ripen, at the very least — and pulled or dug up if possible. Nandina can still be enjoyed in the landscape if homeowners are diligent about removing those berry clusters that they probably bought it for in the first place. But all is not lost. The nandina berries make good holiday decorations, and if cut off before the new year, they’re unlikely to present a problem for hungry birds.
Bird migration in the fall is carefully timed so that they can take advantage of autumn berry crops. Early migrants hit the early berries — dogwood and spicebush. Later migrants dine on fruit that ripens later — persimmons, rose hips, viburnums. And the overwintering birds feast through the cold season on persistent fruit, like the hollies’ berries.
This delicate choreography, though, is coming undone because of climate change. Berries are ripening earlier, and birds that still arrive on their customary schedules may now find their usually filled native larders diminished or even fully depleted — so they turn to those less-nutritious, non-native berries. You can learn more about this complex relationship in a recent Audubon magazine article, which you can read at tinyurl.com/8uv83ajy
Have questions for Rick about the world of nature in and around College Park, or suggestions for future College Park Wild columns? Drop him a note at email@example.com.