Our Natural Heritage: Birds and Their Habitats
by John Schwegman

ARTICLE FIRST APPEARED: 2000 Winter

How many kinds of nesting birds you have in your woods or field depends a lot on the size of your habitat area. Some birds need more space than others.

Many Illinois birds are sensitive to the size of a habitat block and do not nest where the habitat is too small. These are called area sensitive species. Both forest and grassland birds can be classified as low, moderate and high in sensitivity to habitat size.

Among forest birds, the cardinal belongs to the low sensitivity group along with blue jay, catbird, grackle and robin. The moderate forest group includes such species as the wood thrush, hairy woodpecker, red-eyed vireo and white-breasted nuthatch.

Some of the most sensitive birds to forest size are least flycatcher, ovenbird, hooded warbler, and broad-winged hawk. If you have a 10 acre woods you can expect to have chickadees, cardinals and catbirds, but the probability of having a nesting scarlet tanager or wood thrush is only about 20%. The chances of such a small woods harboring nesting pileated woodpeckers and oven birds is near zero.

If you enlarge the woods to 100 acres, however, you have a 70% chance of attracting breeding wood thrushes and tanagers and your odds on ovenbirds goes up to 30%.

At 1,000 acres, a woods can expect to have 75% of the most area sensitive forest birds normally nesting in its region and 95% of the moderately sensitive species.

Many birds in the low sensitivity group are common around our homes and familiar to everyone. The robin and grackle can make do with single yard trees but if you want to see a yellow-throated vireo, cerulean warbler or pileated woodpecker at nesting time, better head for the big woods.

Birds that nest in grasslands exhibit a similar sensitivity to area or size. Examples of low sensitivity grassland birds are the bobwhite, red-winged blackbird, goldfinch and dickcissel. Many of these can nest successfully in grassy borders of fields or even on roadsides. Grasshopper sparrows can even nest in weedy vacant lots.

Moderately sensitive birds include the meadowlarks, grasshopper sparrow, and sedge wren. On average, a 10 acre grassland can be expected to support breeding populations of 20% of the species in this group while a 100 acre tract could house 60% of them. Grassland birds highly sensitive to habitat area include northern harrier, upland sandpiper, prairie chicken, bobolink and Henslow's sparrow among others. The probability of a 1,000 acre grassland containing a species of this group is just 60%.

Factors other than size are also important to grassland nesters. Scott Simpson, of Illinois' Prairie Chicken Sanctuaries, reports that until all fence rows were cleared of trees and tall shrubs near his grassland nestling habitat, it was rare to see nesting harriers and short-eared owls. Use of the sanctuaries for nesting by these area sensitive birds has increased greatly since removal of all trees and brush from the edges of grasslands. These sensitive nesters apparently like to think they are in the middle of a vast open prairie.

Landowners wishing to help area-sensitive birds should keep a few points in mind. If you have a woods broken up by a narrow ridge-top field or if you have two nearby but separated woodlots, consider reforesting to fill in the narrow field or to connect the two woods into one larger forest. Prairie restorationists/managers and pasture managers can help area-sensitive birds by eliminating trees around their grasslands and by making grass stands as large as possible.

John Schwegman lives with nature in Metropolis, IL. He can be reached via botany@midwest.net. For more information about bird habitats, contact the IL Dept of Natural Resources at (217) 785-8686.

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