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Habitat Establishment, Enhancement and Management for Forest and Grassland Birds in Illinois

Area Requirements of Forest and Grassland Birds


Several recent studies have shown that many forest30 and grassland25,26 bird species are rare or absent from many small, isolated habitat blocks. Individual species' responses to reductions in habitat area, however, are variable and determining particular species' minimum area requirements has proved very challenging. Minimum area estimates vary by species, regions and habitat types. Therefore it is difficult to predict or determine the forest or grassland size at which a particular species is likely to occur.

Despite these difficulties, general patterns of species' response to habitat fragmentation are emerging. Some species are not sensitive to habitat fragmentation and occur in habitat patches of all sizes, whereas others are highly sensitive and rarely occur in small habitat patches. Several other species have sensitivities that lie between these two extremes.

In an effort to aid landowners and managers who may wish to attract or conserve specific bird species, or preferably groups of area-sensitive species, we have characterized forest and grassland bird species with respect to their tolerance to habitat fragmentation (Table 2). Tolerance to habitat fragmentation is characterized as "high," "moderate," or "low." Species with a high area-sensitivity are those shown to be most influenced by habitat fragmentation. Species with a high area-sensitivity generally restrict nesting to large tracts of contiguous habitat and rarely nest in small habitat blocks. Species with a low area-sensitivity are those shown to be least influenced by habitat fragmentation and, generally, are frequently encountered in habitat blocks of all sizes.

Table 2. Forest and Grassland bird species sensitivity to habitat fragmentation. Species with a high sensitivity are those birds least tolerant of habitat fragmentation. Moderate species show an intermediate response to habitat fragmentation and low species are those species most tolerant to fragmentation. **=Endangered in Illinois, *=Threatened in Illinois.
Forest Birdsa
High Sensitivity Moderate Sensitivity Low Sensitivity
Broad-winged Hawk Yellow-billed Cuckoo Downy Woodpecker
Pileated Woodpecker Black-billed Cuckoo Red-headed Woodpecker
Least Flycatcher Hairy Woodpecker Red-bellied Woodpecker
Yellow-throated Vireo Acadian Flycatcher Great Crested Flycatcher
Black-and-White Warbler Scarlet Tanager Eastern Wood-pewee
Worm-eating Warbler Summer Tanager Blue Jay
Cerulean Warbler Red-eyed Vireo Brown-headed Cowbird
Ovenbird Northern Parula Northern Oriole
Mourning Warbler Yellow-throated Warbler Common Grackle
Hooded Warbler Louisiana Waterthrush Rufous-sided Towhee
American Redstart Kentucky Warbler Northern Cardinal
Brown Creeper White-breasted Nuthatch Rose-breasted Grosbeak
Veery* Tufted Titmouse Indigo Bunting
  Blue-gray Gnatcatcher Gray Catbird
  Wood Thrush Carolina Wren
    House Wren
    Black-capped Chickadee
    American Robin
Grassland Birdsb
High Sensitivity Moderate Sensitivity Low Sensitivity
Northern Harrier** Eastern Meadowlark Northern Bobwhite
Upland Sandpiper** Western Meadowlark Red-winged Blackbird
Greater Prairie-Chicken** Grasshopper Sparrow American Goldfinch
Bobolink Sedge Wren Vesper Sparrow
Savannah Sparrow   Field Sparrow
Henslow's Sparrow*   Song Sparrow
    Dickcissel
    Common Yellowthroat
adata compiled primarily from studies of forest fragments in Illinois58, Missouri59, and Wisconsin60 with supplemental data from Maryland61, Massachusetts62, and the Mid-Atlantic United States.63
bData compiled from studies of grassland fragments in Illinois26, and Missouri.64

Recognizing that area is just one of several factors that influence breeding bird distributions we provide, in Figure 2, generalized area relationships for forest and grassland bird species with high and moderate area-sensitivity to habitat fragmentation. The graphs in Figure 2 show the likelihood of encountering or attracting a typical moderately or highly sensitive bird species in Midwestern forests or grasslands of various sizes. These graphs can be used as guides to assess the likelihood of encountering or attracting particular bird species in restorations, enhancements, or managed forests and grasslands of various sizes. For example, grassland areas or restorations that are 10 acres in size would have roughly a 35% likelihood of attracting grassland bird species that are moderately area-sensitive to habitat fragmentation (Table 2) and a 15% likelihood of attracting grassland species that are highly area-sensitive to habitat fragmentation (Figure 2b). On the other hand, grassland areas that are 100 acres in size would have nearly a 70% chance of attracting moderately area-sensitive grassland species and about a 40% chance of attracting highly area-sensitive species. Grasslands as large as 250 acres may be required to have a 50% likelihood of attracting grassland species that are highly sensitive to habitat fragmentation. Similar determinations can be made for forest species using Figure 2a.


gif -- graph of grassland and forest size

Figure 2 - Likelihood of encountering or attracting breeding forest and grassland bird species that are moderately and highly sensitive to habitat fragmentation in Midwestern forests and grasslands of various sizes. Figures represent the probability of encountering or attracting "typical" area-sensitive forest or grassland bird species. For example in an Illinois forest of 100 acres there is roughly a 70% likelihood of encountering a Wood Thrush (a moderately sensitive forest species), a 70% likelihood of encountering a Red-eyed Vireo (also a moderately sensitive forest species), and a 40% probability of encountering an Ovenbird (a highly sensitive forest species). See Table 2 for a complete listing of forest and grassland bird species sensitivity to habitat fragmentation. Graphs were developed from data on breeding bird distribution patterns in forest fragments in Illinois58, Missouri64, and Wisconsin60, and grassland fragments in Illinois26 and Missouri.64

Land mangers should keep in mind that the guidelines provided are provisional estimates. Establishing or managing tracts of a particular size will not guarantee the presence of area-sensitive species. Additionally, establishing or managing small areas does not preclude their possible use by area-sensitive species. Note that for all the birds in Figure 2, bigger areas are better than smaller areas. No matter how big an area is, management or enhancement activities that enlarge the amount of contiguous habitat available will benefit area-sensitive species. Conversely no matter how big an area is, actions that reduce the overall size of the tract reduce the likelihood that area-sensitive species will be found or will persist on the tract.

Isolation from other similar habitat also significantly influences forest27,28 and probably grassland bird distribution and abundance patterns in fragmented landscapes. In areas with a relatively high proportion of forest habitat, forest-interior birds may occur in smaller habitat patches than they might in areas where forest habitat is relatively scarce.27,28 Grassland birds probably respond to isolation from other blocks of similar habitat comparably, although this has not yet been documented.


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