If you have been out watching the migration this spring, I bet you have tried once or twice to spot what the warblers are hunting. It’s a game probably all birders attempt at least a few times each year—and one that we almost always lose.
A Magnolia Warbler or an American Redstart flits above you, tapping its bill on the underside of one leaf over here, then flies to a second leaf to repeat the action over there, and then—wings beating frantically, tail spread for lift—it picks at a third leaf, right over your head, just 10 feet up. What are the targets? What is it chasing? Focus your binoculars on the leaf instead of the bird, and odds are good that you won’t see the prey.
Take up this challenge regularly and you learn that you and your binoculars are no match for warblers and their naked eyes. In fact, if you really want to see warbler prey, you generally need to pull down a limb, break off a branch, exchange your bins for a magnifying glass, and thumb through the leaves carefully for several minutes. With patience and persistence you might finally spy a tiny caterpillar or two.
Meanwhile, the Magnolia Warbler captures and consumes a couple dozen in the branches above you.
“Warblers never receive credit for the good that they do,” Edward Howe Forbush argued in his chapter “Food for Warblers” in Frank Chapman’s Warblers of North America, “ because the insects that they eat are mainly of small size [and consumed] before they have the chance to work noticeable injury.” Forbush was writing in 1907 when songbirds were best supported by pointing to their economic value as defenders against forest tent caterpillars, apple tree tent caterpillars, gypsy moth caterpillars, and other recognized scourges of the time.
“The habits and haunts of the warblers are so varied that, collectively, they exert a repressive influence on nearly all orders of insects,” Forbush wrote. “They form a sort of aerial police whose chief function is to put down uprisings of injurious insects.” Caterpillars are their primary targets, he noted. “Practically all the warblers feed very largely at times on measuring worms [geometrid moth larvae] and other hairless caterpillars.” And they capture them at high speed and in such mind-boggling numbers that they can only be tallied over brief stretches. Forbush’s sharp-eyed assistant, F. H. Mosher, recorded a Chestnut-sided Warbler in one tree eating 22 caterpillars in 14 minutes “and other insects that could not be seen plainly.” A Nashville Warbler ate 42 caterpillars in 30 minutes, as well as “many other insects that could not be plainly seen.” A Black-and-white Warbler ate at least 28 caterpillars in 10 minutes “and probably took many more.”
In the century since Forbush, Mosher, and Chapman, North America has undergone both a cultural shift that they might have sensed coming and an ecological shift that they almost certainly could not have foreseen.
The cultural shift means songbirds no longer must be defended for their economic values. Today, backyard birds are treasured for their intrinsic beauty, and warblers in particular are celebrated for bringing the wild world into suburbia. Anyone with a few trees and shrubs in the backyard can hope to see a Magnolia, Black-and-white, Nashville, or other warbler stop by to feed during migration, and if you have more than a few trees, you might even have a pair or two nesting on your property.
Whether those hopes become reality, however, may depend on how successfully you have battled the enormous ecological shift of the past century, one that has shaped backyards and other suburban landscapes from Nova Scotia to California. It is a shift whose importance went largely unrecognized until the last 20 or 30 years. Indeed, its impact on birdlife still seems to remain unrecognized by many homeowner birders today.
What has happened? We have replaced native trees, shrubs, and other native plants with lawns and “pest-free” alien plants. A “pest-free” yard—that is, one without caterpillars—is a desert to warblers. They can’t stop during migration in yards where there is nothing to eat, and they certainly can’t nest during the summer in a “pest-free” yard, when their need for caterpillars to feed their young multiplies several times over as the nestlings grow.
Forbush’s understanding of the importance of caterpillars in warblers’ diets—and Mosher’s counts of their feeding rates—have been confirmed in study after study in the years since their work. Open any good source on warblers’ diets and you will see that Lepidoptera larvae are nearly always the most important food source, sometimes virtually the only food source, especially during the nesting season. And the number of caterpillars each pair requires is mind-boggling. In a 1996 study, published in The Wilson Bulletin, for example, “Factors Affecting Food Provisioning of Nestling Black-throated Blue Warblers,” C. O’Neill-Goodbred and Richard T. Holmes used video cameras to record the foraging activities of pairs of color-banded adults at 18 different nests. By the eighth day of nesting, each adult averaged 11 trips to the nest per hour— that is, 22 trips per pair (more than one every three minutes). After every delivery each adult must fly off again, to find more insect prey to carry back on the next return. In that study Lepidoptera larvae comprised 60 to 87 percent of the food delivered.
“Nesting songbirds can’t work any harder,” notes Douglas Tallamy, an entomologist and ecologist at the University of Delaware. “They are already working all day, every day.” Tallamy is a man on a mission to encourage homeowners to rethink the plants on their properties. “If you landscape only with non-natives,” he writes, “you undermine the food web.”
Non-natives are far more common in our yards and neighborhoods than many birders realize. Walk out your door with a field guide to alien plants in hand and you are likely to find one or more of the most widespread of them: Bradford pear, autumn olive, oriental bittersweet, multiflora rose, Norway maple, Japanese honeysuckle, and dozens of others. In fact, if you are new to their identification, you are likely to be shocked to discover how many caterpillar-proof, non-native plants your yard holds. Once you learn their field marks, you will also see them just about everywhere you go—on roadsides, in parks and schoolyards, and in most other semi-open areas. Most were introduced as ornamental plants over the past century and they have since spread far and wide, out-competing native plants.
Non-native plants are “pest-free,” because the caterpillars that are adapted to feed on them have been left behind in the plants’ homelands, usually Europe or Asia. Our native caterpillars do not eat them because they can’t detoxify those chemical defenses. Our caterpillars can eat only the leaves of native plants whose chemistry they have adjusted to through evolutionary adaptation. “This requires a long shared history between insects and host plants,” Tallamy points out. “Native insects only have such histories with native plants.”
In one study he and a colleague found that “areas overrun with alien plants produce 22 times fewer caterpillars” than more natural areas (D. Tallamy and K. J. Shropshire, “Ranking Lepidopteran use of native vs. introduced plants,” published in Conservation Biology in 2009).
“Every time we plant an introduced plant, we are reducing the local insect population and thus depriving birds and other wildlife of the food they need to survive and reproduce,” he writes.
Tallamy’s book, Bringing Nature Home: How You Can Sustain Wildlife with Native Plants (Timber Press, 2009), is a wonderfully rich and practical guide to helping warblers and other wildlife prosper on our managed landscapes. Some numbers he cites are depressing, of course—declining songbird populations, ever-expanding highways, accelerating suburban sprawl across the country. But his focus is on how anyone with property and a little determination and an informed understanding of the web of native plant, native insect, and native bird can make a difference.
To encapsulate the strategy, Tallamy cites a term he learned from Michael Rosenzweig’s book Win-Win Ecology: How the Earth’s Species Can Survive in the Midst of Human Enterprise: “reconciliation ecology.” “It’s a mouthful, but it’s the future; and we will play a central role in its success simply by reevaluating our use of native plants in the landscapes in which we live.”