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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.

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.

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

Isolation from other similar habitat also significantly influences forest^27,28 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.