Middle Fork Savanna is an image, a 550-acre miniature of northeastern Illinois as it looked in 1800. The image is blurred and distorted by alien plants and animals and by railroad embankments and drainage tiles, but we can still catch a glimpse of what Illinois was before the Potawatomi were thrown out.
The Middle Fork is in Lake County just west of Lake Forest. It occupies a strip of land split into eastern and western halves by a busy rail line that carries Metra, Amtrak, and assorted freight trains. The Tri-State Tollway is immediately west of the savanna and Waukegan Road is a short distance east. Illinois Route 60 forms the southern boundary, and a little hamlet called Rondout sits at the northern end. Rondout has body shops and landscaping outfits, places essential to the maintenance of Lake Forest but too gritty to locate there.
The middle fork of the North Branch of the Chicago River flows through the savanna. It has been channelized, and its local name is the Skokie Ditch. Turning the savanna into a protected preserve has been a joint project of the Lake County Forest Preserve District, Lake Forest Open Lands, and the Nature Conservancy, which for fairly obvious reasons chose to call the place Middle Fork rather than Skokie Ditch Savanna.
The preserve is a very diverse place, with four distinct habitat types within its boundaries. Lovely old oak groves are mixed with marshes. Along the eastern side is an extensive grassland that was a cow pasture until recently. And on the western side of the tracks is a combination of grasses, wildflowers, and low shrubs–the sort of place early settlers called a “brushy prairie.”
I’ve spent several mornings over the past two months wandering through the Middle Fork trying to put together an inclusive list of the nesting birds that live there. I had originally planned to spend nearly every morning in June there, but the weather got in the way.
The weather also produced a mosquito crop that is easily the equal of Upper Peninsula levels. Since bird surveys require that one stand still for long stretches of time, I have taken to wearing a head net to keep the bugs away. The net–which, as it happens, I bought in a hardware store in da U.P.–makes me feel like I’m wearing the Tarnhelm, a wondrous helmet crafted by Mime for Alberich and eventually used by Siegfried, that had the power to make you invisible to your enemies. I am not literally invisible, but the bugs are kept far enough from me that they can’t even annoy me by buzzing in my ears. Repellents keep the bugs from biting, but they don’t keep them away. I also have tried to stay away from chemicals ever since the morning when a heavy application of deet–the active ingredient in most repellents–dissolved the plastic earpieces on my glasses.
But I digress. What I have found at Middle Fork is a highly diverse bird population. Each of the habitat types has its own distinctive community. One of those–the group of birds associated with the brushy prairie–raises some interesting questions about the way we have been managing our prairie remnants and restorations.
All but one of the oak groves at Middle Fork has a dense understory of the nasty alien shrub called European buckthorn. Buckthorn was originally introduced to this area as an ornamental. It escaped from cultivation, carried by birds that ate its fruits and defecated the seeds. It is now a major pest in all our forest preserves.
Savanna groves were once open places. The understory was grasses and such low shrubs as gray dogwood and brambles. A dense canopy of buckthorn–which typically grows 10 to 12 feet high–shades out all these native plants. The resulting environment does seem to benefit some forest birds. The buckthorn-infested groves at Middle Fork have wood thrushes and ovenbirds, species that depend on tall shrubs for nesting or for singing perches. I also have one record each for our two local species of cuckoos–the black-billed and the yellow-billed–and both are from groves with buckthorn. Our only singing male veery–a threatened species in Illinois–is also in a buckthorn grove.
The one grove that has been cleared of buckthorn and burned has a quite different character. Yellow warblers live in that grove, along with yellowthroats and indigo buntings. These are usually thought of as edge species, but they seem to be doing well in the interior of an oak grove with a ground layer of low shrubs mixed with grasses.
Up in the canopies of both the buckthorn groves and the nonbuckthorn grove, I find blue-gray gnatcatchers, wood pewees, great crested flycatchers, white-breasted nuthatches, and northern orioles. I have some evidence of nesting by a pair of Cooper’s hawks, an endangered species that probably used to live in the presettlement savannas. But the grove where the hawks were is also the place where I keep seeing a great horned owl. Great horned owls do not like competition, and that grove was probably not big enough for both species.
Up until this year I had been working on a nesting survey at Somme Woods in Northbrook. Somme is a much smaller preserve–150 acres compared to 550–but it contains habitats similar to those at Middle Fork. Many nesting species are common to both places. But Somme has been the object of major restoration work over the past 15 years. Part of that work has involved girdling trees. You girdle trees by removing the bark in a strip a few inches wide all the way around the trunk. Girdling kills the trees, but it leaves the trunks standing.
Numerous birds use holes in standing dead trees as nesting sites. Woodpeckers dig them or natural processes of decay create them, and a variety of species move in. Somme’s 150 acres has many more of such hole nesters flickers, downy woodpeckers, black-capped chickadees, and house wrens than the 550 acres at the Middle Fork.
The old pasture at the Middle Fork has nice numbers of some prairie species. Bobolinks and eastern meadowlarks are there, and a few savanna sparrows. The marshes have green-backed herons, yellowthroats, swamp sparrows, and redwings.
Perhaps the most intriguing habitat at the Middle Fork is the brushy prairie. The minds of those who manage prairie remnants or direct restoration efforts have been dominated by a vision of the prairie as a place with no wood. It’s all grasses and wildflowers that die back to the roots in fall. Fires sweep over it regularly and prevent woody plants from ever taking hold. A prairie manager looking at the brushy areas at Middle Fork would almost certainly be thinking about using a combination of lopping sheers and frequent fires to clear out those gray dogwood clumps. Give us a few years, he would say, and well have this looking like a proper prairie.
Buckthorn has inspired much of this kind of thinking. It is a nasty pest that has to be cleared out, and a sort of guilt-by-association process has made native shrubs–dogwoods, hawthorns, sumac, and others–into villains too.
But Middle Fork has me suspecting that proper prairies can have a substantial number of woody plants. Gray dogwood, by far the most common woody species there, grows in dense clumps that shade out any grasses that might provide fuel for a fire. When fire strikes, a few sterns on the windward side of the clump get burned, but everything in the lee of that first rank survives. And the roots of the burned stems are unharmed, so they can send up new shoots. Hawthorns, cherries, and even small oaks take advantage of that sheltered environment and grow right out of the middle of the dogwood clumps.
Bobolinks, meadowlarks, savanna sparrows, grasshopper sparrows, and Henslow’s sparrows live in prairies where there is little or no wood. The brushy prairies support a completely different community. Put a patch of brush in the middle of a prairie and you will probably see yellow warblers, song sparrows, field sparrows, blue-winged warblers, willow flycatchers, catbirds, and yellowthroats moving in. Cedar waxwings will build their nests in the small trees in the middle of the dogwood clumps. Indigo buntings like the dogwoods as nesting sites, but they need some tall trees as singing perches. At Middle Fork they nest in the brushy prairie near the tracks. The males sing from the power lines that parallel the railroad. In southern Cook County this kind of habitat would also be the place to look for Bell’s vireo and the yellow-breasted chat.
Restoration and natural-areas management are applied ecology, and what makes them endlessly interesting is that there is always more to learn. It may be that a brushy prairie is not a degenerate form of the grassy prairie, but an authentic community in its own right.