Lake Renwick Heron Rookery Nature Preserve is the largest rookery of the twelve located in northeastern Illinois and is the only site where these particular species—double-crested cormorants, great egrets, black-crowned night herons, great blue herons, and cattle egrets—nest together.

The rookery is co-owned and managed by the Forest Preserve District of Will County and the Illinois Department of Natural Resources (IDNR). Their holdings consist of two land parcels known as Units A and B. Unit A's 321 acres includes the 170-acre lake. The rookery encompasses two groups of primary breeding islands, which are actually gravel bars on the northeast side of the lake, measuring no more than 20 feet wide and 50 feet long. Unit B, know as Lake Renwick East and at 511 acres, provides much of the foraging habitat for the rookery birds, including a 200-acre lake with more than 40,000 feet of shoreline—ideal for wading birds.

Naturalists who perform public programs liken the nesting arrangements at Lake Renwick to a high-rise apartment, with each species of birds occupying a different "floor." At the top of the tree line nest great blue herons, the largest and most dominant species at the rookery. Just beneath the great blues are found double-crested cormorants.

The middle level is taken over by great egrets, whose large size and pure white color cause them to stand out among their neighbors. Great egrets were almost hunted to extinction because of their brilliant breeding plumage, leading to their classification as state endangered. However, their numbers increased sufficiently so that in 1994 their status was upgraded to state threatened. At the lowest level of the trees and on the ground are the black-crowned night herons, which are endangered in Illinois, and cattle egrets.

The success of Lake Renwick as a breeding colony can be seen in the number of nests recorded over the last 12 years. The most dramatic colonization over that time has been by the great blue herons. From less than 50 nests in 1983, 336 were counted in 1994. The next most populous species today is the great egret, whose number of nests climbed from 60 in 1983 to 309 in 1994.

Double-crested cormorants first colonized Lake Renwick in 1986, with four nesting pairs. This was the first record of their having ever established nests in Illinois. Since then, their numbers have risen to the point where they are now the third most populous species at Renwick. Two years ago, 266 cormorant nests were counted. The status of cormorants was recently upgraded from state endangered to threatened.

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Only the black-crowned night heron suffered a severe reduction in the number of nesting pairs. From a high of 270 nests in 1983, their numbers have dropped to only 114 in 1994, largely due to the increasing populations of great blues and great egrets and to predation.

Cattle egrets, whose natural habitat tends to be open fields, farms, and marshes, have always been low in number at Lake Renwick. Four nesting sites, producing six young, were counted two years ago. Unlike the other species at Renwick, which feed primarily on aquatic life, cattle egrets eat mostly insects.

Rookery Benefits from Its Very Nature...
The success of Lake Renwick as a rookery can be traced to the natural elements of the site itself. The two main breeding islands are situated in the middle of the lake, providing protection from land-based predators like raccoons, foxes, and coyotes, and are situated near a ready and ample supply of aquatic food. Although the islands offer a great deal of protection, a 1994 internal study estimated that raccoon and great-homed owl predation resulted in a 12 percent loss in active nests for great blue herons, great egret, and cormorants, and a minimum lost of 18 percent for black-crowned night herons. Much of the aquatic food is supplied by Lake Renwick East, where large-mouth bass, bluegill, gizzard shad, golden shiner, carp, black crappie, pumpkinseed sunfish, green sunfish, and rudd have been catalogued.

Even though the neighboring lake provides much of the birds' dietary needs, herons and great egrets will fly as far as 40 miles round trip to forage. Not only do dozens of ponds, water-filled quarries, and strip mines lie within a 20-mile radius of Renwick, the Illinois and Michigan Canal and two rivers, the Des Plaines and DuPage, also provide foraging opportunities.

...and Land Preservation Efforts
Also instrumental in the rookery's success was the dedication of Lake Renwick as a nature preserve, which ended seven years of confrontation between conservationists and developers. Lake Renwick is actually a quarry, born in 1914 when workers for the Chicago Gravel Company tapped into fresh water springs.

The new lake was utilized by the local population immediately, with a bathing facility, complete with sandy swimming beaches, bathhouse, bathing suit rentals, high diving board, and a pier, constructed that same year. Also in 1914, an ice-storage building was built on-site, and ice harvesting began until 1924 when the ice house was destroyed by fire.

Other historical features of the site include a dance hall, which later became a popular roller skating rink, and a restaurant, known as Powell's Mill, which was built in the 1930s. Effluent and garbage from Powell's Mill were discharged directly into the lake, and in 1940 the waters had become so polluted, health officials forced the closing of the swimming facilities.

As to when the islands at Lake Renwick were first colonized by birds, no one can say with certainty. A publication by the Audubon Society cites that herons first came to the islands in 1961, but a photograph from 1960 shows herons and egrets already atop their nests. Reports from Chicago Gravel Company employees place the birds on the islands in the early 1940s.

In 1982, Chicago Gravel announced that it was putting its holdings up for sale. Subdividers drew up a plan to ring the lake with townhouses, to be called Renwick Shores. A coalition of environmentalists, including the then Illinois Department of Conservation (now IDNR), Forest Preserve District of Will County, Nature Conservancy, U.S. Department of the Interior, and local Audubon chapters (most notably the Will County Audubon), joined ranks to protect the unique natural features that Renwick possessed.

On January 18, 1990, the Forest Preserve District of Will County and the Department of Conservation purchased Lake Renwick from the Chicago Gravel Company for $776,000. When it was granted nature preserve status, the future of the rookery was assured.

Today, visitors to the site view the birds from a distance of 750 feet. District staff provide viewing scopes for the public, and public programs are presented on select days and times. Last year, nearly 1, 300 people visited the rookery.

Given the increasing development in the Chicagoland area, the effort put forth by organizations and individuals to save Lake Renwick has paid off in setting aside a parcel critical to the future of several bird species. With the success of the rookery, birders will have the opportunity to observe these species in a natural setting into the 21st century.

Bruce Hodgdon is the media liaison for the Forest Preserve District of Will County. Photographs courtesy of Christopher Mayer, the forest preserve's public information naturalist.

42 * Illinois Parks & Recreation * May/June 1996