Northern Prairie Wildlife Research Center
Almanac and Sketches Here and
There (Leopold 1949)
To effectively manage grassland bird habitats in Wisconsin we should know as much as possible about habitat requirements for individual species and communities as well as the factors that influence habitat selection and the ability of populations to sustain themselves.
Each grassland bird species has a particular range of habitat conditions to which it is well suited (Wiens 1969, Sample 1989). Because grassland bird habitat requirements are diverse, management designed to benefit one or a few species will not adequately accommodate the needs of all other species. Effective management produces a diverse spectrum of habitats across the state and takes into account amount of habitat, habitat size and shape, habitat distribution, surrounding land use, woody cover and edge issues, habitat structure, disturbance patterns, and the impacts of land management practices. The following sections provide guidelines and recommendations for all of these topics.
Amount of Habitat
We estimate that a total of roughly 3 million acres of potentially suitable agricultural grassland habitat now exists in the state, excluding roadsides. This figure undoubtedly varies from year to year, as it includes an annual average of approximately 620,000 acres of lightly to moderately grazed pasture (one-third of the total pasture acreage in the state), 630,000 acres of small grains, 940,000 acres of old fields, fallow fields, unharvested cover crops, and idle cover crops (U.S. Department of Commerce 1994), 200,000 acres of late-cut hay (cut after early Julymostly grass hay), and approximately 620,000 acres of grassland in the Conservation Reserve Program (CRP). Since we do not know much about the quality of this habitat for birds (i.e., what proportion of fields are of adequate size and shape, how much of the surrounding habitat is suitable for birds), we conservatively estimate that roughly one-half of this total acreage of agricultural habitat, or 1.5 million acres, is currently suitableand potentially high qualityhabitat for grassland birds.
In addition to this agricultural habitat, a recent inquiry to land managers in Wisconsin indicates that we currently have approximately 120,000 acres of permanent, publicly owned and leased grassland habitat in blocks of 100 acres or greater in the state (D. Sample, unpublished data). The amount of grassland in blocks smaller than 100 acres is unknown but may equal or exceed another 120,000 acres, which would bring the total amount of permanent, non-agricultural grassland in the state to 240,000 acres. As with agricultural lands, the acreage that is high-quality for grassland birds may be less than this total.
By combining the information above, the total amount of potential agricultural and permanent habitat for grassland birds in Wisconsin is conservatively estimated to be approximately 1.7 million acres. We suggest, based on professional judgement, maintaining these suitable agricultural and permanent grassland habitats along with an additional 250,000 to 750,000 acres of managed grassland bird habitat in permanent cover. These additional acres should fit the recommendations for grassland habitat established in this guidebook and would include improvements to and expansions on current public properties managed as grasslands (such as DNR Wildlife Areas and State Natural Areas), some additional land acquisition and leases, and significant amounts of privately owned grasslandssuch as those with permanent conservation easements.
As a general guideline, additional habitat managed for grassland birds should be distributed throughout the state as follows: Southwestern Upland 25%, Southeastern Ridges and Lowlands 20%, Central Plains 15%, Lake Michigan Shoreland 10%, and Northern Highland/Lake Superior Lowland 30%.
Habitat Size and Shape
Most of the agricultural and grassland landscapes in Wisconsin are highly fragmented, characterized by a patchwork of fields, woodlots, hedgerows, farmsteads, and other habitats and land uses. Large open expanses of grasslands are the exception and not the rule. Because of the fragmented nature of these landscapes, habitat size becomes a management issue for grassland birds.
There are a number of grassland species that require large tracts of habitat; these are the area-sensitive species. Species requiring areas more than 250 acresor in some cases even 500 acresin size throughout their breeding ranges include greater prairie-chicken, sharp-tailed grouse, northern harrier, possibly upland sandpiper, and barn owl. Our observations indicate that yellow rail, Wilson's phalarope, short-eared owl, and Nelson's sharp-tailed sparrow are also restricted to large sites in Wisconsin. Other research in Wisconsin indicates that Brewer's blackbird is area-sensitive as well (Niemuth 1995). Additional species described as area-sensitive in other midwestern and eastern states are sedge wren, vesper, savannah, grasshopper, and Henslow's sparrow, bobolink, eastern meadowlark, and western meadowlark (Samson 1980, Johnson and Temple 1986, Bollinger and Gavin 1992, Herkert 1994, Vickery et al. 1994). However, in Wisconsin we found all these species in a wide range of field sizes. Area requirements of species may vary geographically (Herkert et al. 1996), and methods used to determine habitat size have differed among studies.
The area required for a healthy, viable grassland bird population is not well known, but it is surely larger than the field size required for a particular grassland bird species to select and occupy a site, which is the focus of most research on minimum area requirements. In one of the few population viability analyses for a grassland bird species, Temple (1992) estimated that an isolated population of sharp-tailed grouse in Wisconsin would require a patch of suitable habitat at least 10,000 acres in size. While this species occurs on much smaller sites in the state, those populations may not be viable. Therefore, minimum area requirements can not be used alone to determine adequate habitat size for a viable population.
Grassland bird habitat should be managed at three different scales: large landscapes of more than 10,000 acres, medium landscapes of 1,000 to typically around 5,000 acres but potentially up to 9,000+ acres (Sample et al. 1994, Henderson and Sample 1995), and small blocks of 40-1,000 acres.
Large-scale and medium-scale management areas foster the health of grassland bird communities and bird species that have varying degrees of area sensitivity. Large areas also typically contain greater natural variation in habitat structure, soil type, and topography than small areas. Some existing large landscapes are mostly publicly owned, but most new large landscapes will likely be a combination of private and public land. Most should be located in areas that were historically native grasslands to take advantage of the land formations, climate, and ecological features that naturally support management of open grassland habitat. However, some areas of cleared forest or former wetlands that are currently important to grassland birds, such as the Buena Vista/Leola Grasslands and the Clark/Marathon/Taylor county area, should also be considered.
Most of the land in these large areas can remain in private ownership, with compatible agricultural uses, but the core of the project should be an area of permanent grassland at least 2,000 acres in size. Securing this core grassland for new areas may involve some acquisition and permanent conservation easements. At least 35% of the land within the rest of the management area should be in permanent or long-term grass cover including pasture, long-term set-aside such as CRP land, conservation easements, prairie remnants, and other idle grasslands75% of which should be in blocks of 40 acres or more scattered through-out the area. The remainder (52% of the total project area) could stay in active crop production, ideally in small grains and hay (cut after 15 July or adjacent to idle grass cover). Removal of most linear tree and shrub cover, especially in fencelines and roadsides, should be encouraged throughout the management area, especially around the core grassland area, which will reduce the amount of potential nest parasite and predator habitat (Henderson and Sample 1995, Fitzgerald 1996).
Habitat blocks should approximate circle or square shapes to minimize the edge-to-interior ratio as much as possible. Habitat strips should be at least 220 yards wide for the same reason (Figure 3). The aim here is to reduce the negative effects of increased nest predation, parasitism, and possibly competition near edges.
Habitat DistributionWhen planning the distribution of blocks of grassland habitat in large areas or landscapes, it is important to consider the predominant land uses and development patterns of the area, as well as other ecological factors. This will maximize the benefits to grassland birds and will wisely use limited time and money available to resource managers.
There are two major ways to distribute habitat across landscapes: consolidated (clumped) and scattered. The aim of both is to maximize the amount of grassland bird habitat in large blocks. We suggest that managed grassland habitat blocks should be scattered in landscapes where the predominant surrounding land uses already are dominated by suitable grassland habitats. Buena Vista/Leola Grasslands is a good example of such a landscape, although increasing acreage devoted to irrigated row crops and cranberry bogs are changing the landscape make-up there. In this scenario, there is a good likelihood that any managed grassland parcel will be close to and complement the existing suitable habitat. Siting decisions should maximize the use of adjacent suitable habitat whenever possible.
Habitat blocks can be consolidated in two different kinds of landscapes:
In the case of intensively agricultural areas, little suitable habitat already exists in the landscape. It may be necessary to initiate several large-scale plantings or native grassland restorations (at least 500-1,500 acres) to create enough quality grassland habitat in one landscape. However, as we suggested above, many of these landscapes are inappropriate for grassland management for social or economic reasons. Some of the existing large landscapes with lots of suitable, idle grasslands will be appropriate for either scattered or consolidated habitat distribution.
Surrounding Land Use
Effective Habitat Size
Because grassland birds are influenced by land uses adjacent to blocks of managed habitat, effective habitat size is really the management block plus the amount of suitable grassland habitat adjacent to or very near the managed block. The placement and configuration of habitat blocks in relation to other habitat types becomes an important determinant of effective habitat size and of the species composition, species richness, and density of grassland birds that use the site. Examples of idealized arrangements of grassland habitats on a typical farm or Wildlife Area are presented in Figures 4 and 5. Note that managers should consider the degree of management control overand the permanence ofsuitable surrounding land uses. Some types such as CRP land may be less secure over the long term than others, such as grassland with a conservation easement.
Buffers of grassland vegetation between managed grassland habitat and either farmsteads or woody vegetation can increase effective size. In particular, row crops, although not quality grassland bird habitat, can be used to buffer managed grassland habitat from woody edges and may therefore buffer against nest predation and parasitism. The presence of some row crops may be used in other cases to contribute to the overall size of a treeless landscape.
Suitable vs. Unsuitable Adjacent Habitats
Unsuitable habitats for grassland birds include grasslands with too much woody cover (i.e., generally more than 30% cover), habitats with abundant linear woody cover, farmsteads, and suburban developments. Row crops (although providing foraging habitat for some species), most parks, and golf courses are also unsuitable habitats; while they are used by some grassland birds they typically are not productive habitats. Since most hay is cut well before 15 July, hay should be avoided in general, especially in southern Wisconsin, unless mowing can be delayed. Grass-dominated hayfields that are typically cut late in the season are an exception to this rule. Concentrations of such fields exist in the northern half of the state, including Clark, Taylor, Marathon, Douglas, Bayfield, Ashland, St. Croix, and Door counties. The presence of idle grassland cover near hayfields may provide re-nesting habitat for birds displaced from a mowed field after cutting.
Woody Cover and Edge Issues
Blocks of open grassland habitat should be structurally open and relatively free of farmsteads and major linear woody edges such as woodlots, hedgerows, and woody fencelines that fragment the habitat and create edges for nest parasites, predator habitat, and corridors for predator movement. In most cases woody cover should be kept to a maximum of 5% of grassland habitat. This is especially true when management is focusing on obligate grassland species that do not require much if any woody vegetation. In such cases it is desirable to have the edge of any managed grassland habitat greater than 110 yards from any major woody edges or developments such as farmsteads and homes that are present in the area. In general, woody cover below 3.25 ft (1 m) high can exceed the suggested 5% maximum but should not exceed 20%.
However, the amount and type of woody vegetation depends largely on the targeted bird species or community as well as on the size of the management block. Scattered shrubs, shrubby patches, and small trees (less than 30% total cover and less than 10 ft tall) may be beneficial to shrub-grassland species, such as loggerhead shrike, Bell's vireo, and field and clay-colored sparrow. Oak savanna species such as American kestrel, red-headed woodpecker, eastern kingbird, and orchard oriole benefit from scattered mature, open-grown trees. Most other grassland species readily use small shrubs and trees as perches, even if they don't require them. For species that do require woody vegetation, irregular patterns of scattered woody plants and irregular, shrubby edges to woodlots are preferred over hard, linear edges (Ratti and Reese 1988) (Figure 6).
Habitat Structure and Other Features
Differences in bird communities among habitats are largely determined by differences in habitat structure rather than by differences in plant species composition, although the two are certainly related. The following features of habitat structure are particularly important to grassland birds.9
One of the most common ways to categorize grassland species is into three groups based on general vegetation height-density preferenceshortgrass, midgrass, and tallgrass. However, while vegetation height-density is an important and easily measured structural feature of vegetation for grassland birds, for some species it may not be the most important feature affecting whether or not they occur in a given habitat or field. For example,
Accordingly, grassland bird species of management concern are ultimately placed into one of six categories in Table 5 (shortgrass, midgrass, tallgrass, species requiring woody vegetation or nest structures, species requiring open water, and species requiring large areas), which includes brief descriptions of both the general and specific habitat features required by grassland birds. All grassland bird species (including those not of management concern) are placed in to one of the six categories in Appendix F.
Grassland birds also may occupy suitable micro-sites within a larger, less suitable habitat patch, which complicates habitat description and ultimately species management. For example, a field of dense switchgrassa tallgrass habitat that normally harbors such species as sedge wren; common yellowthroat; song, swamp, and Henslow's sparrow; and red-winged blackbirdmay have small patches that are poorly established or weedy. These areas may be occupied by typical shortgrass species such as grasshopper or vesper sparrows. Most fields, particularly large ones, have variations in topography or soil type that affect vegetation structure, and these differences in structure are often reflected by subtle differences in bird community composition in a given habitat. Therefore, managing a field as a particular habitat type is no guarantee that only certain species or bird communities will occur there.
Diversity of vegetation structure and cover types should be promoted at a variety of scales: within most fields or blocks of habitat, among fields or blocks of habitats, within a landscape, within natural divisions, and within the entire state. Structural diversity can be reflected by differences in vegetation height and density, woody cover, amount of exposed soil, grass to forb ratio, litter depth and cover, and other variables. Natural factors such as soil type, hydrology, topography, and climate should be used to best advantage in producing diversity at all scales. Managers should maintain a spectrum of different field ages (for example fallow fields, old fields, and old and young plantings of grasses and forbs) by varying the length of disturbance cycles; this also encourages structural diversity among fields.
Much of the habitat managed as grassland in the state, especially in areas where soils are not dry, thin, or sandy, contains relatively tall and dense vegetation. Overall, there is a great shortage of grassland vegetation in the short and medium height-density categories.
Within a field less than 80 acres, only one cover type should typically be represented. Within that field, however, structural diversity can vary and should be achieved by planting diverse plant species, including roughly 10% forb cover. Fields or blocks of habitat greater than 80 acres may consist of either more than one cover type or a single cover type. The degree of diversity also varies depending on the target species or communities, some of which require large expanses of habitat with homogeneous structure. We end up with a mix of habitats and landscapes, many diverse, some more homogeneous, with the result that the overall character of grassland habitat in the state is diverse.
Most grassland habitats in Wisconsin are in agricultural landscapes. During the breeding season, many of these habitats are subject to farming-related disturbances that may severely lower the reproductive performance of nesting grassland birds (Rodenhouse and Best 1983, Frawley 1989).
Even among bird species that nest earlysuch as meadowlarks and horned larkor species that nest latesuch as dickcissel and American goldfinchthere is substantial overlap between the peak nesting season and agricultural field operations, such as planting, cultivating, fertilizing, and harvesting (Best 1986, Frawley 1989). The conservation of undisturbed or idle habitats during the breeding season is therefore important to the conservation of grassland birds. Also, since most species prefer at least some residual vegetation in the spring, it is important that some sites are not harvested or burned following the previous fall or late summer. Species that require significant standing residual vegetation or litter layer in the springsuch as Henslow's sparrow or eastern meadowlark, respectivelyprefer habitats that have not been disturbed since the previous spring.
grassland bird habitat requirements are diverse, management designed to
benefit one or a few species will not necessarily benefit all other species.
general rule of thumb for grassland management unitsat all scalesis
the larger the better, even for small blocks of land.
grassland birds are influenced by land uses adjacent to blocks of managed
habitat, effective habitat size is really the management block plus the
amount of suitable grassland habitat adjacent to or very near the managed
grassland cover near hayfields may provide re-nesting habitat for birds
displaced from a mowed field after cutting.
of vegetation structure and cover types sould be promoted at a variety