Ecology Beyond Cub Scouts

October 15, 2009
birds and fruiting trees Rose-breasted Grosbeak. Photo by Gary Tyson via Birdshare.
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How would you like to stake your claim as a practicing ecologist—without leaving your own backyard or your local stomping grounds?

If so, you might try your hand at a study of songbirds and the fruits they consume this fall and winter. Equipment requirements are minimal: binoculars, a notebook and pen, a point-and-shoot camera, and—depending on your botanical experience—perhaps a field guide to the woody plants of your area.

For guidance and inspiration, read John Baird’s article, “The selection and use of fruit by birds in an eastern forest,” in the March 1980 issue of The Wilson Bulletin(available online). Baird explains his approach and findings in easy-to-follow prose, and his methods could be adopted by any birder wherever fruit grows, which is any green area in North America—or anywhere around the world, for that matter.

We all learned as children that fruits are a plant’s way of getting its seeds distributed. “The bird eats the fruit, flies somewhere, and poops out the seeds,” my Cub Scout leader explained many years ago. “A cherry tree is stuck where it grows. It can’t fly, run, or walk. So, instead, it pays birds to scatter its seeds by giving them a treat. It’s a good trade, both ways.”

Some birders’ understanding of the relationship between frugivores and fruiting plants never progresses beyond a Cub Scout’s, no matter how many decades pass by—or, at least that’s the case of the birder whose perspective I know best.

I have noticed, of course, that some fruits disappear each summer much sooner than others. Blueberries and cherries, for example, are gone from our yard within a few days of ripening. In the first week or two of July, my wife and I race against our backyard catbirds to pick some of our garden’s blueberries before they eat them all—usually while one or the other of the catbirds tsk-tsk-tsk at us. When the cherry trees ripen later in the month, we make no attempt to compete, as the catbirds are joined by robins, mockingbirds, and other resident birds, and the fruits disappear so quickly you have to close your eyes and imagine they once hung there.

At the opposite extreme are certain fruits—bayberry, holly, juniper, greenbrier, and others—that linger through fall, well into winter, or even into spring. “Go eat the cedar berries,” I’ve told the scolding catbirds in our garden more than once. “Beggars can’t be choosers.”

And I’ve also noticed that fruit eating seems to peak here in late September when Wood Thrushes, Hermit Thrushes, and other migrants join the resident fruit-eaters. For a week or two, everywhere you look, especially in the front yard dogwood, it’s a frugivore’s festival.

“What were those migrants eating a week ago up north?” I have wondered. “And aren’t they headed against the tide?” Even a Cub Scout knows that flowers bloom earlier in the South than they do in the North, so common sense tells him that they would fruit earlier there also. A Wood Thrush gobbling down dogwood fruit at 40 degrees latitude in the last few days of September seems headed for trouble when it arrives at 35 degrees in the first few days of October. All the dogwood trees will have been plucked clean by then, no? Wouldn’t resident birds get the vast majority of fruits in any area, simply because they are present when the fruits are first available?

Baird’s elegant methodology enables any interested birder to investigate these phenomena.

He marked off three tracts of forest (in the Institute Woods at Princeton, New Jersey), identified the 23 species of fruiting plants that grew there, and on twice-weekly trips counted the number of fruits remaining on the plants and recorded the birds he observed feeding on them. For poison ivy and other plants with too many fruits to count, he took photographs of the same patches each week and estimated the numbers and percentages remaining. When migration ended in late November, he began a once-weekly census of the birds along a walking route, counting all birds within 30 meters of the path through mid-March. He did not attempt to count how many fruits were eaten by any individual bird. If a bird was seen eating one or more fruits, he counted it as one observation of fruit eating.

That’s it—a study so simple it begs to be tried elsewhere.

See the article for Baird’s full analysis, including a time-line chart of the nine most common fruits and when they were eaten, graphs detailing the fall and winter diets of White-throated Sparrows and Black-capped Chickadees, and other results. Among the passages that caught my eye, “Twelve of the thirteen commonest migratory species and eleven of nineteen resident species seen between 25 September and 14 November were observed eating fruit, and many of these species were wholly frugivorous during this period.”

“Poison ivy berries were eaten by ten bird species. This fruit grows in conspicuous clusters, is slow to fall, and may have been the most fully used of any fruit in the forest.

“[The disappearance of dogwood] was so abrupt that only one single dropped berry could be found in the study plot on 6 November.

“The depletion of dogwood berries and grapes in late October coincided with the departure of many of the migratory birds. Perhaps a causal relationship exists here.”

E.W. Stiles investigates that causal relationship in “Patterns of fruit presentation and seed dispersal in bird-disseminated woody plants in the eastern deciduous forest,” in the November 1980 issue of The American Naturalist. His methods are more complicated—charting fruiting times for 125 woody plants from 341 forests; measuring nutritional values and fruit and seed sizes for all listed plants; and investigating the diets and migration timing of nine fruit-eating bird species—but his results and analysis mesh neatly with Baird’s.

Stiles groups the plants into four categories by their fruits: summer small-seeded (mulberry and serviceberry); summer large-seeded (cherries and other Prunus); fall high-quality (dogwood, spice bush, sassafras, buckthorn, and others); and fall low-quality (bayberry, sumac, greenbrier, and others). Fruits of the first two groups are consumed quickly by resident birds. They are so highly nutritious that they are attacked by microbes (and so lose their value to the plants) if the birds don’t find them first. Fruits of the fourth group trade low nutrient value for staying power (which they gain because they lack attractiveness to microbes). They are eaten by hungry birds that have no choice, generally in late fall or winter, after all the other fruits are gone. According to Stiles’s calculations, this fourth group comprises the most common fruiting plants in eastern deciduous forests.

It is the fruits of the third group, however, that fuel migration for American Robins, Gray Catbirds, Wood Thrushes, Hermit Thrushes, Swainson’s Thrushes, Gray-cheeked Thrushes, Veerys, Eastern Bluebirds, Cedar Waxwings, and a number of other species.

The plants depend on the birds, too. In a classic example of coevolution they have timed their ripening in exactly the opposite direction a Cub Scout would expect. The northernmost individuals ripen first, the southernmost last, and the trend southward follows the moving line of migration closely. Also, the farther south you go, the more fall-fruiting high-quality plants you come upon. This pattern also seems to reflect these plants’ dependency on migratory frugivores. Fall migration builds as the birds head southward (as more and more individuals join in), and so group-three plants find more and more willing transporters of their seeds the farther south they grow. In Canada and our northern states, on the other hand, migratory frugivores are more thinly scattered, and so are the plants that produce high-quality fruit in fall.

One minor point of difference between these two researchers concerns poison ivy. Stiles places it in his fourth group (fall low-quality), whereas Baird found its fruits mostly gone from the woods he studied by mid-November, as migration came to a close. Poison ivy leaves turn scarlet red in late summer, whereas most other common plants remain green, and I have always assumed this was a strategy to draw migrant birds to its dry, unappealing fruit. But I can’t say I have ever paid attention to which birds feed on it or when—or how long the fruits remain on the vines. That’s a little mystery I am planning to solve this fall.

How well do you know the relationships between fruit-eaters and woody plants in your area? Which birds consume the most fruit in your backyard or favorite birding spot? Which fruiting plant draws the widest variety of species? When do the first fruit-eating migrants arrive in your yard, and which plants do they feed on? What fruits linger into winter in the highest numbers? When are those low-quality fruits finally eaten, and by which species?

If you make yourself a more alert and informed observer while you pursue answers to these questions, pat yourself on the back. You might even consider applying for a merit badge in ecology.

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