Spring 2001

Some ornithologists believe certain tree species — like elms and oaks — are beneficial to spring migrants because they support so many caterpillars. Until now there's been little data to confirm or refute that hunch.

Magnolia warbler. Photo by Art Morris/Birds as Art.

A bay-breasted warbler winters in Venezuela and breeds in Canada, but its survival during migration may depend on which tree species are planted by the Chicago Park District. Cape May warblers expertly glean caterpillars from the undersides of white oak leaves, but it takes forest preserve district land managers to assure oaks will reproduce for later generations of warblers.

In spring, neotropical migrants, including warblers, tanagers, flycatchers, and thrushes stop along the Lake Michigan shoreline, inland parks, forest preserves, back yards, and the Des Plaines and Fox River valleys on their way to their northern breeding grounds. These birds begin flying through Chicago Wilderness in April into as late as June. All are searching for food and shelter, especially after flying over the corn and soybean fields of central Illinois, where they find little sustenance. In essence, for three or four months of spring and fall, Chicago Wilderness serves as a vital way station for birds that need to arrive well-fed at their breeding grounds.

Ornithologists have long known that breeding and wintering habitat for migratory songbirds is declining. Agriculture-dominated landscapes in the northern United States, for example, don’t provide enough contiguous forest for successful breeding of some of these birds, and their wintering habitat in Central and South America is also declining. Many local, national, and international conservation groups are focusing on the wintering and breeding habitat of migratory songbirds.

The third link

But here in Chicago Wilderness, committed citizens are working to ensure these birds have what they need in between. During migration, the balance of energy consumed and spent becomes extremely important. Birds, especially females, need the energy to rebuild their reproductive glands. But most importantly, they need the energy to continue their journey. Each place they stop must provide the shelter and food they need to fly to their breeding grounds.

"We’re only just beginning to understand how the Chicago Wilderness region is important for migratory birds," says Rickie White, regional science coordinator for the National Audubon Society. Indeed, using photographs of radar images, Ronald Larkin, wildlife ecologist with the Illinois Natural History Survey (INHS), has observed the massive migration of birds over Lake Michigan and along river valleys. "The habitats near the Lake Michigan shoreline are critical stopovers for migratory birds," says Larkin. "The images show us that if you live near the lake shore, even your backyard is crucial habitat for warblers and other migrants."

"One of the reasons lakefront parks are so important is that when it starts to get light and migratory birds are over the lake, they head for the nearest land," says Rob Diehl, a Ph.D. student working with Larkin on the Doppler radar study. Lake Michigan funnels migratory birds coming from the Caribbean heading northwest and birds coming from South America and Mexico heading northeast.

The Chicago River and the Fox and Des Plaines River valleys are equally critical for migratory birds. In fact, the river valleys have always been historically important migratory flyways for birds, says Christopher Whelan, an INHS avian ecologist stationed at Midewin National Tallgrass Prairie.

Understanding the needs of migratory birds is complicated. But scientific studies are providing some answers for land managers on how to best manage the land for migratory birds — such as which trees provide the best foraging habitat for birds. One answer that seems clear is that all trees are not created equal.

Diversity

Under the guidance of esteemed ornithologist Dr. Scott Robinson, University of Illinois doctoral student Aaron Gabbe spent several springs in southern Illinois studying the needs of migratory songbirds, specifically warblers. For his study, Gabbe identified 42 tree species within one watershed in the Cache River in southern Illinois. "During migration," he says, "I followed foraging birds up to two minutes and identified what tree species they were in." He recorded his observations from sunrise to late afternoon.

Gabbe gathered sufficient data on the American redstart as well as Nashville, magnolia, blackpoll, Tennessee, and chestnut-sided warblers to make some preliminary conclusions. All six of these warblers are foliage insectivores — they glean insects from leaves — and all of them in April 1997 fed mainly on cypress, tupelo, and sugarberry in southern Illinois. It was a cold spring and the tupelo was flowering. Arthropods and insects were attracted to the flowers, and the birds were attracted to the insects. The next April, however, a warm spring hastened the tupelo’s emergence from dormancy. The tree had already finished blooming when the warblers arrived. That year the warblers foraged predominantly in kingnut hickory, red maple, Shumard’s oak and, again, sugarberry.

Once they have sufficiently stoked themselves with fuel, migrants continue north into new territory. Some of these warblers have never seen the cypress swamps of southern Illinois nor the oak savannas and woodlands of northern Illinois. Yet, stop after stop they encounter new ecosystems, following the inner urge to continue north. And stop after stop they must search for food and avoid predators in a new environment. Their energy levels get taxed, and they need to find food expeditiously.

The mighty oak

Some of the most important tree species for migratory birds in Chicago Wilderness are the oaks. The high value of oaks to migrant birds has been known at least since 1983 when Jean and Richard Graber wrote a paper for the journal Condor about their research on what and where migratory birds eat in several Illinois regions. In one site, migrant warblers fed almost entirely on butterfly and moth caterpillars, mostly taken from oaks. The Grabers estimated warblers ate 1.2 to 1.7 times their own weight in caterpillars per day.

In another study, Robert Marquis of the University of Missouri, St. Louis, and Christopher Whelan of the INHS quantified the extent of bird predation on leaf-eating arthropods on white oaks. In their Missouri study site, Marquis and Whelan found birds consumed significant quantities of these arthropods, of which about 95 percent were butterfly and moth larvae, similar to the results of Graber and Graber in Illinois. "Migratory and breeding birds keep the oaks healthy by reducing insects, which helps the oaks grow faster," adds Whelan.

County forest preserve districts in the Chicago Wilderness region have long known that oak/hickory forests are gradually declining, as species such as maples, ash, and buckthorn replace the original ones. Lack of fire is the most significant cause for the decline of our oak/hickory forests. Land managers now conduct controlled burns at some preserves, including parts of Ryerson Woods in the Lake County Forest Preserves, where sugar maple trees were overtaking native oaks. People are also planting oaks throughout the Chicago Wilderness region.

Habitat management

The Chicago Park District, county forest preserves, and many other Chicago Wilderness member organizations take bird habitat into account as they manage lands they own. Within the past few years, lakefront projects for migratory birds have begun at Montrose Harbor, south along the lake front at the Lincoln Park Bird Sanctuary, the South Shore Cultural Center, the Metropolitan Water Reclamation Plant, Olive Park near the Jardine Water Treatment, and McCormick Place.

"We recognize the restricted size of some of our lakefront parks funnels some of the migrants coming from the neotropics into a very small area," says Barry Burton, deputy director of the department of natural resources for the Chicago Park District. "So we’re planting trees and shrubs that migrants seem to need — oak, hackberry, and elm, among others," he says.

The Chicago Park District recently adopted guidelines for bird habitat management in lakefront parks, following suggestions from a coalition of birding groups called the Bird Conservation Network. "The Park District will designate areas considered to be significant habitat for birds, buffers around these areas, and secondary bird habitat," says Terry Schilling, an active volunteer in rehabilitating the Lincoln Park Bird Sanctuary along the lakefront.

The Chicago Park District has approved a plan in collaboration with the Lakeview Citizens Council to increase habitat quality and diversity within the sanctuary. Bird Sanctuary Task Force members, led by Charlotte Newfeld, have been removing buckthorn and garlic mustard, and thinning invasive ash and maplesaplings to open the understory for sun-loving plants to grow.

A $25,000 grant will be used to purchase, among other things, grasses, sedges, and forbs. "We’ve been planting buttonbush along the edges of ponds, and native honeysuckles, viburnums, and other shrubs to increase the diversity," says Schilling. "We’ll be removing sod and planting a matrix of savanna grasses and sedges in the new area where the Park District plans to expand the Sanctuary. Later we’ll add a mix of appropriate wildflowers."

Burton praises another volunteer, Doug Anderson, for his help in making Wooded Isle in Jackson Park more migratory bird friendly. "The Chicago Park District has spent $400,000 removing buckthorn and other invasive plants and replacing those with native plants such as viburnum," says Burton. "We’re making excellent progress."

An important component of managing lands for migratory birds is engaging the surrounding human community. Several stewardship projects at lakefront parks designed to teach others have begun. One of them is at McCormick Place, where habitat is being enhanced for migratory birds. "We want to use this project to introduce citizens to the idea of land management for migratory birds and to enjoy the thrill of watching colorful birds stop in Chicago to refuel before continuing on to their breeding grounds," says Judy Pollock, founding president of the Bird Conservation Network.

At Montrose, where the famous Magic Hedge attracts thousands of birds and birders each spring, work is underway to create and protect habitat for migratory shorebirds, sparrows, warblers, and other species. A ring of trees, surrounded by a meadow of different-sized grasses, will offer food and shelter to migrants. The Chicago Park District has already begun planting shrubs, and allowing the vegetation to grow, instead of mowing it. Warblers such as Connecticut, with its bold white eye ring and ventriloquial song, find the medium-height grasses and shrubs at the Magic Hedge a good place to feed and rest during migration. And making the migratory birds feel at home, for the time they are here, is just what land managers are hoping to do, with help from the many volunteer birders, tree experts, and scientists.

Sheryl DeVore is Assistant Editor of Chicago Wilderness.