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2. The Interlaced Biology of Birds and Plants
by Stephen W. Kress
Birds and plants have evolved side by side over thousands of years and interact in remarkable ways that often benefit each other. Birds consume the fruits, buds, flowers, and nectar of plants, and as they do so, they pollinate these plants and disperse their seeds over the landscape. In the eastern deciduous forests, at least 300 trees, shrubs, and vines depend solely on birds to spread their seeds -- usually far from competing parent plants. Unlike rodents, which destroy seeds by gnawing into the seed coat, birds eat only the fleshy fruit; as the seeds pass through the birds' gizzards they remain intact and the protective seed coat is scarified (scratched) -- improving the chances of germination. Bird-distributed seeds also benefit from the high-nitrogen excrement that surrounds seeds when they are "planted."
Fruit appeal
Plants have, not coincidentally, evolved in ways that assure the appeal of their fruits to their co-conspirators in reproduction. Most trees and shrubs have small fruits, no more than three-fifths of an inch in diameter -- just the size of a bird's gape. Many fruits ripen precisely when bird migration reaches its peak. And the majority of plants that depend on birds to distribute their seeds produce brightly colored fruits -- again, no coincidence, as birds live in a highly visual world and almost all, with the exception of owls, Whippoorwill, and their kin, have a keen ability to see color.
Tufted titmouse on Staghorn sumac (Rhus typhina). The fruits of this shrub or small tree feed at least 31 species of birds. (Photo: Stephen W. Kress)
Plants usually put on their colorful fruit display at the same time that they are producing sugars or fatty lipids, nutrients that serve no purpose for the plant other than enticing flocks of ravenous, seed-dispersing birds; the appearance of brightly colored fruit is an instinctive magnet for fruit-eating birds such as thrushes, catbirds, and waxwings. Some plants deploy bright colors, not in their fruits, but rather in other parts of the plant. Virginia creeper, wild grape, sassafras, and gray dogwood have muted blue or white fruits -- but these are either displayed on red stems or ripen just when the leaves attain their most brilliant colors. Virginia creeper and some other vines appear to advertise ripe fruit by prematurely withdrawing chlorophyll from their leaves, revealing red pigments that contrast with the unchanged sea of green foliage around them -- at the same time that the vines' fruits ripen and are ready for dispersal.
Timing is all
In nature, plant fruits ripen in different seasons, providing a nourishing smorgasbord for birds throughout the year. In spring, for example, sweet foods such as serviceberries, wild cherries, mulberries, and strawberries are available to hard-working parent birds. Fall migrants such as thrushes, tanagers, vireos, and warblers require fatty fruits to fuel their long journeys; a few of their favorite fruits are found on spicebush, magnolia, sassafras, and flowering dogwood. The fruits of species such as mountain ash, hawthorn, and cranberrybush viburnum have a low lipid content, making them less desirable to fall migrants -- but also less likely to turn rancid and rot on the plants. Some fruits, like nannyberry (wild raisin), shrivel in place and remain available for wintering robins, bluebirds, and flickers. These persistent foods are especially important when late snowfalls cover the ground, preventing returning robins, thrashers, and sparrows from finding earthworms, insects, and other invertebrates under rotting leaves. Likewise, early spring migrants such as the Yellow-rumped Warbler and the Tree Swallow often survive on persistent fruits such as bayberry when spring cold snaps the flying insects on which they typically feed.
Nourishing natives
Interactions between native birds and native plants have taken thousands of years to evolve. Climate is constantly selecting features of bird behavior and anatomy that fit the habitat -- including preferences for specific plant foods. At the same time, plants that are not adept at spreading their seeds are unlikely to survive, as more aggressive species can soon crowd them out. To protect birds and our native plant heritage, gardeners should favor indigenous (native) species and avoid using plants from other areas -- especially those that are invasive and have the potential to escape into wild habitats, crowding out native plants. What's more, these non-natives may not provide the proper nutrients. Native plants, which have co-evolved with native wild birds, are more likely to provide a mix of foods -- just the right size, and with just the right kind of nutrition -- and delivered just when birds need them.
Woodpeckers and Northern Cardinal are attracted to the fruit of beautyberry (Callicarpa americana). (Photo: Richard Thom)
Over the years, some invasive species have been promoted for their benefits to wildlife. Tartarian honeysuckle (Lonicera tatarica), for example, is an invasive shrub that has spread so aggressively across the eastern United States that it is crowding out native shrubby dogwoods and viburnums. While this shrub produces prolific orange and red fruits that are readily consumed, monocultures of honeysuckle now replace once-diverse communities of native shrubs, limiting the kinds of foods available throughout the year. A recent study also found that Cedar Waxwing that feed on tartarian honeysuckle often have orange-tipped rather than yellow-tipped tail feathers. Plumage colors are badges used for gender and species recognition, so the effects of food on birds' color could be very disruptive.
Much remains to be learned about the interactions between birds and plants -- even the feeding habits of common birds are not well understood. But if you watch the birds that visit your yard carefully, you will see that they use a wide range of plants for feeding, resting, and shelter -- and likewise that many, many plants rely on birds to distribute their seeds. These relationships between birds and plants reflect thousands of years of co-evolution and are dynamic and changing. In light of these complex interactions, our attempts to create bird habitats can be humbling, but -- when successful -- immensely satisfying.
Stephen W. Kress is director of the National Audubon Society's Seabird Restoration Program and manager of the Society's Maine coast seabird sanctuaries. He teaches field ornithology at the Cornell Laboratory of Ornithology, where he is a research associate. He is author of The Audubon Society Bird Garden, The Audubon Society Guide to Attracting Birds, the Golden Guide Bird Life and other publications on birds and their management.