Summer 2004

	City Hall's green roof is an island of life, 12 stories above the street. Photos in this montage by Dennis Light.

Birds on a
Cool Green Roof

Can the roof of Chicago's City Hall take urban nature to a higher level?

By Katherine Millett

When my nephew first visited the top of the Sears Tower, it wasn't the altitude or the lake view that impressed him. It was the grasshoppers jumping on the glass outside the windows. He liked the spiders, too, spinning their webs in corners of the steel window frames. Deaf to tales of human engineering, he wanted to know how these creatures climbed 1,353 feet to the observation deck.

Below us spread downtown Chicago. For a bird or even a grasshopper, it seemed a desolate place to make a living. Around the chess pieces of Loop buildings flowed an unremitting stream of cars and people on pavement; only Grant Park and the wooded banks of the Chicago River suggested food and shelter. At roof level, sun beat down on expanses of asphalt, and fans whirred inside boxes of ventilating equipment.

Then we trained our binoculars northeast. Between a black tower and a gray one, we saw a patch of green — the roof of City Hall.

In 2001, at a cost of $1.5 million, the City transformed 20,300 square feet of the 38,800-square-foot roof at LaSalle and Washington Streets into a contoured garden landscape. Much publicized, the green roof was installed to combat the "urban heat-island effect" that traps heat and air pollution in congested downtown areas, to cool the building and reduce its energy costs, to soak up water during storms and release it slowly afterwards, and to grace the cityscape. (When summer temperatures reach 90 degrees Fahrenheit, well-watered sections of City Hall's green roof hold around 90 degrees, while the black-tar roof of the adjacent Cook County building heats to nearly 170 degrees.)

Soon after, almost unnoticed amid calculations of roof temperatures and water runoff, a small team of naturalists asked whether the roof could also serve as a home for wildlife. Making regular visits there to monitor birds, plants, and insects, they became the first, and are still the only, people in all of North America collecting data about animals on a green roof, according to Steven Peck, executive director of the Toronto-based organization Green Roofs for Healthy Cities.

The variation of depth, structure, and plants creates niches that support a much wider range of birds and bugs than a standard green roof. Photo by M.S. Thompson.

To structure the monitoring effort, North Park Village Nature Center director Drew Hart and longtime birder Jerry Garden wrote a protocol. Garden began counting birds at the project's inception, and he continues to monitor for a half hour each week during spring and fall migration periods. He records species that land on the roof as well as those that fly closer than 20 feet above it. Naturalist Sean Shaffer uses traps to check for arthropods, such as spiders and soil insects, and Stephanie Averill and Michael Thompson monitor bees weekly.

This spring, I accompanied Garden, Hart, and Shaffer to the roof. After riding a service elevator to the 12th floor and zigzagging through pipe-lined hallways, we walked into a surprising landscape. I had seen the roof from above, a view enjoyed by hundreds of people whose high-rise offices face the building, but I was startled by its contours and contrasts: prairie up close, glass and steel behind; curvy hills opposite right-angled windows; birds chirping in a crabapple tree while engines rumbled on LaSalle Street.

Wildlife habitat was never a primary objective of the City Hall roof's design. Yet experiments often have unintended consequences, and providing urban habitat for animals is turning out to be one of them. City Hall's is not a typical green roof with hundreds of identical sedums set in three inches of soil. Such easy-care installations have numerous advantages, but according to Swiss scientist Stephan Brenneisen, who has studied green roofs as habitat in Europe, they don't support much wildlife. The best way to encourage biodiversity on a roof, he says, is to vary the depth and structure of its substrate. Some animals need cover and seeds, while others need rocks and gravel.

The garden on City Hall begins with layers of waterproofing materials, designed to protect the original roof, and drainage ways to manage stormwater. Combinations of polystyrene contours, gravel, soil, and fabric lie atop these, varying from 5 to 18 inches deep to create a hilly profile.

Over several weeks in the fall of 2000 and the following spring, workers set 20,000 individual plants into the soil. The planting design, developed by Conservation Design Forum, Inc. (CDF) of Elmhurst, called for 150 different species. Some, like sedums, were known to do well on green roofs. Others represent experiments in growing native prairie species (many are typical inhabitants of dry gravel hill prairies) and long-blooming flowers within the constraints of a roof environment.

We followed a path of round paving stones through clumps of big bluestem grass, plumes of bottlebrush, and green nodes of sedum clustered under last fall's brittle flowers. A prickly pear cactus wrinkled on a sandy hillside. Towering above the dark purple heads of a wild onion, a rattlesnake master plant rattled its seeds in the breeze. New shoots were pushing aggressively against old stalks, and tangles of papery grasses dangled on bare branches of shrubbery. The landscape had the messy candor of early spring.

While Hart and Garden combed through seed heads and flung the contents in strategic patches of soil, Shaffer crawled under the crabapple tree to check a pitfall trap. He stirred through a few cherry-sized apples and came up with a pillbug, springtails, and some coiled millipedes. On his fingertip, a nematode raised its translucent body and wriggled like a tiny, charmed cobra.

Only up to 30 percent of the soil mixture is organic soil, but the animals don't seem to mind. Engineered to be light, the "growth medium" also contains expanded clay pellets, wood chips, and vermiculite.

I followed Hart to an unplanted section of the roof's north end, where he stooped next to a beehive. Two box towers were built there in the spring of 2003, each stocked with 3,000 exotic honeybees. We watched dozens of bees land at the bottom of a hive and fly inside carrying loads of yellow pollen on their back legs.

The roof's beekeepers, Averill and Thompson, say the honeybees routinely fly to Grant Park and back, laden with pollen to produce excellent honey. Yet, other bees use the roof as well. Perhaps the most exciting development on the roof to date is its discovery by native bees. Unlike honeybees, many species native to Illinois are solitary soil dwellers. They nest in the ground and tunnel to deposit eggs individually.

"They must be nesting up there," says Thompson, "because there are so many of them. The porous quality of the soil is perfect." He has observed them feeding on native flowers such as white asters, goldenrod, and coreopsis.

As we talked, Garden hopped up onto one of the metal walls that enclose the structured landscape. "There we go," he said, a smile showing beneath his binoculars. "Field sparrows, a junco, and a song sparrow. And isn't that a peregrine?" He arched his back and twisted to follow a falcon overhead, watching as it disappeared beyond the tall spire of the First United Methodist Center.

Jerry Garden checks on a birdhouse installed among the roof's tall prairie grasses. He has spotted numerous migrant birds resting in the welcoming habitat, including the common yellowthroat warbler (right). Photo of Garden by Katherine Millett. Photo of yellowthroat by Rob Curtis, The Early Birder.

During the first year, Garden saw a few sparrows each week and occasionally a flock of juncos. His list of visitors included a wren, a chickadee, a kinglet, a Cape May warbler, and an Empidonax flycatcher. The year 2003 brought 12 percent more birds and a greater variety: at least one of each species from the previous year, plus six types of sparrow, woodpeckers, thrushes, a robin, a thrasher, a starling, a Philadelphia vireo, and both a Cape May and a common yellowthroat warbler. But his biggest thrill came last October when Garden saw a rare olive-sided flycatcher determinedly push through the steel "Grand Canyon" of LaSalle Street and land on the roof. "It must have been compelled by its sense of direction to abandon a friendly route and take off into this inhospitable environment. The more green roofs we have," he says, "the more birds will get to know them."

To encourage birds to nest on the roof, Garden built eight birdhouses that the City installed on poles among the plants and on a few walls. "The house wren would make a good indicator species for the roof if it nests," he says. "Then we would know we have a habitat that draws and holds."

As the experiment enters its third year, it is beginning to yield results. The monitors know that a variety of plants and soil animals can survive year-round and that birds are taking advantage of the roof. They've also observed that, as an isolated environment connected to other green patches only tenuously by its animal pollinators, the roof depends on human custodians to stimulate its genetic health. To add to the challenge, the soil here is unnaturally shallow and warm, as the roof receives heat from the building beneath it all winter. This will be hard on large prairie grasses long-term, says Gerould Wilhelm of CDF. These plants usually root deeply and require cold-weather dormancy for their seeds to germinate. Drought-resistant ornamentals like sedums and Eurasian carpet weeds are most likely to succeed.

Argiope orb-web spiders can live on green roofs only if they have tall prairie grasses on which to build webs. Photo by Dennis Manning.

"One can plant anything on a roof that one can plant on the ground," Wilhelm says, "but one may need to replace certain individual plants every three or four years. The objectives are not the same for a green roof as for a restoration. On a roof, one might have four or five species within a quarter square-meter area, whereas an aboriginal prairie might have about 25 species in the same space."

Beneath the plants, a significant variety of arthropods churn the soil, and, says Petra Sierwald of The Field Museum, spiders "definitely like green roofs. The more complex your habitat structure, the more species of spiders you'll get." Some, like the orb-web spiders outside the observation deck on the Sears Tower, favor vertical surfaces. They balloon upward on warm currents of air, riding strings of silk they spin for the purpose. And while the thousands of grasshoppers that besieged the green roof during its first year were likely carried up on the new plants, these and other insects also can be inadvertently transported long distances by winds, sometimes reaching mile-high altitudes.

The presence of these animals, especially those that require quality habitat, bodes well for the roof's future diversity. "It's a very good sign," says Wilhelm, "when conservative species volunteer in an engineered environment."

	The green roof offers a refreshing view to workers in surrounding office buildings. Photo by M.S. Thompson.

One of the primary goals for City Hall's green roof has been to inspire more green building. Indeed, the idea is catching on. Within Chicago's city limits there are now 70 green roofs, either complete or in the works, and 51 of them are private. Many were at least partially funded by $100 million the City recovered from Commonwealth Edison after a series of power outages during 1999, but many more are in the works with other sources of funding. When all these roofs are finished, says Kevin Laberge of the City's Department of Environment, they will cover one million square feet.

No one yet knows exactly what the cumulative effect of significant areas of green roofs here will be, but the flat roofs of our dense urban centers provide ample opportunity to find out. "In most cities, the roof area is between 15 to 30 percent of the total land area," says Steven Peck. "So if we can design green roofs that generate stormwater quality and quantity improvements, reduce the urban heat-island effect, clean the air, provide green space, and support butterflies, migratory birds, and invertebrates, why not? Why not give back to nature the footprint of the buildings that rest upon her?"

After my tour of the City Hall roof, I took the elevator down and walked out to Washington Street. Arranged in a stone planter near the door was a row of purple crocuses. A bee emerged from a blossom, its legs yellow with pollen, and flew up to the roof.

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