Management of Agricultural Landscapes

Management of Agricultural Landscapes
for the Conservation of Neotropical Migratory Birds

Thirty-eight Neotropical migrants are common in the agricultural landcapes of the Midwest. Most of these species depend on herbaceous or wooded habitats, which are declining as the average size of farms in this region increases about 12% per decade. We recommend minimizing cultivation; encouraging moderate grazing; delaying spring mowing of hayfields, grassed waterways, and roadsides; avoiding nighttime mowing; encouraging longer intervals between mowing; avoiding fall mowing and annual mowing of grassed waterways and roadsides; retaining fencelines along roadsides; maintaining idle land that provide nesting cover; and changing the amount and configuration of idle land to enhance species richness and nesting success.

Agriculture is the dominant land use in the Midwest. Farmland composed 66% of 12 midwestern states in 1993. In 1992, half of the rural, non-federal land in the Midwest was active cropland, a quarter was pasture or rangeland, 17% was forest or woodland, and the rest was farmsteads, shelterbelts, idle cropland, etc.

Neotropical migratory birds constitute the major part of midwestern, farmland avifaunas. Thirty-eight Neotropical migratory birds are common in at least one farmland habitat in the Midwest, and two-thirds of the common native species are Neotropical migratory birds. Thus the conversion of midwestern forest, savannah, and prairie to agriculture, mostly prior to 1920, undoubtedly changed the abundance and distribution of Neotropical migratory birds. Changes in bird abundance are not well documented because there were no widespread, systematic surveys of bird populations before the mid-1960's. Extinction and local extirpation of some species have been documented. Grassland birds, which form a large fraction of the common farmland Neotropical migratory birds, evidently did not decline much between the 1920's and 1950's, when diversified farming was common. The modern era of intensive cropping coincides with population declines of grassland birds. Other long-term changes, mostly of declining bird populations, have been noted in Illinois and a small study area in Iowa. These declines have continued in recent years. A variety of factors, acting at all stages of the annual cycle, may be contributing to these population declines.

Considering the extent of modern agriculture, the effect that agriculture has had on bird populations, and the likelihood of continuing changes, conservation biologists need to understand how future changes in agriculture are likely to affect bird populations. Will some species continue to decline even without further changes in the landscape? Which species are most vulnerable? To address these questions, it is important to understand how agriculture has changed recently, how agricultural land-use practices affect particular bird populations, and what can be done to conserve species that are likely to be negatively affected by current and future practices.

Recent Changes in Agricultural Habitats and Landscape Structure
The changes in agricultural landscapes that most concern conservation biologists are those that have occurred on a large spatial scale. We focus on changes in agricultural habitats in the last three decades, when bird-population trends in the Midwest were well documented. The major habitats in the Midwest are crops, including small grains (wheat, oats, etc.), corn, soybeans, sunflowers, and forage (hay). Other habitats are pastureland and rangeland, wetlands, woodlands, and strip cover (e.g., fencerows, roadsides, waterways, terraces). Idle cropland is not explicitly considered a habitat in this section but is covered below under land-use practices. The amounts and relative proportions of cropland, pastureland and rangeland, woodland, and other rural land vary across the Midwest.

The total area of cropland in the Midwest in the early 1980's was similar to that in the late 1940's. There was a steady decline in cropland area, associated with surplus production, from the late 1940's to the mid-1970's. The area then increased until the early 1980's, as exports expanded. The pattern of change varied geographically. The area used for crops in the Northern Plains (Kansas, Nebraska, North Dakota, South Dakota) in the early 1980's was similar to that in the late 1940's. In the Corn Belt (Illinois, Indiana, Iowa, Missouri, Ohio), there was a dip in the mid-1970's but an increase of 9.5% between the late 1940's and early 1980's. In the Lake States (Michigan, Minnesota, Wisconsin), there was an increase of 2.1% over the same period. The area of active cropland in the Midwest declined again from 1982 to 1992, mostly due to enrollment of cropland in the Conservation Reserve Program.

Cropland Habitats
Small grains. Wheat and other small grains are grown mostly in the Northern Plains. Wheat area increased in the 1970's and has remained relatively stable since then. The area planted to oats has declined by two thirds over the past three decades.

Corn and soybeans. The area planted to corn in the Midwest declined in the 1960's, increased in the 1970's, then declined in the late 1980's. Soybean area increased steadily from the 1960's until the 1980's, then remained fairly stable. Corn and soybean area in Illinois reflected this pattern and increased from 4.9 million ha in 1945 to 8.4 million ha in the 1980's. Corn and soybeans are usually grown in alternate-year rotation.

Sunflowers. Sunflowers are grown mostly in North Dakota, South Dakota, and Minnesota. Sunflowers are a relatively new crop in these states. The rapid increase in sunflower area was followed by a decline associated with increased worldwide production.

Hayland. After a decline in the late 1960's, the area of harvested hay in the Midwest has remained stable. Traditionally, livestock provided agricultural diversity for farmers who also raised crops, and hay was harvested from untilled prairie which was sometimes seeded with grasses. On tilled land, alfalfa is the primary hay crop.

Non-cropland Habitats
Pastureland and Rangeland. The general trend in the Midwest between 1982 and 1992, was toward less pastureland and rangeland; between 1967 and 1977 the trend varied geographically. Herkert reported a 53% decline in pastureland from 1964 to 1992 in Illinois, Iowa, Ohio, and Wisconsin. Pastureland is used primarily for production of forage plants for livestock and is typically managed by fertilization and reseeding, according to the instructions for reporting NRI data. Rangeland is land on which the climax or potential vegetation is grasses, forbs, and shrubs suitable for grazing; management does not usually include application of chemicals. Pastureland is more common in the eastern part of the Midwest and rangeland is more common in the western part. In Iowa, the amount of pastureland declined by 23% between 1958 and 1967. Most of this converted Iowa land was classified as forest or cropland in 1967. At a national scale, too, much of the conversion to cropland prior to the 1980's was from rangeland and pastureland.

Wetlands. Wetlands are a prominent part of the natural landscape in the Prairie Pothole Region of North Dakota, South Dakota, Minnesota, and northern Iowa. Many wetlands have been connected to ditches or underground pipes (tile lines) to facilitate rapid drainage of cropland in the spring. Loss of original wetland area in the Midwest ranged from 164,211 ha in Kansas to 2,816,802 ha in Illinois. The percentage of wetland area lost ranges from 35-48% in the Northern Plains states, 42-50% in the Great Lakes states, and 85- 90% in the Corn Belt.

Woodlands. Woodlands on midwestern farms are mostly riparian areas, woodlots, and shelterbelts. They are more abundant in the east. In Iowa, the amount of land classified as forest or woodland increased between 1958 and 1967. Most of this land had previously been classified as pasture. Woodland habitat decreased in the Midwest from 1967 to 1977 but has been relatively stable since then. Below, we discuss shelterbelts as a form of strip cover under land-use practices. We do not consider riparian areas and woodlots further in this paper. Wooded, non-strip-cover habitats may be the only nesting habitats used by some woodland species, such as cerulean warblers.

Strip cover. Strip-cover habitats (e.g., fencerows) have declined in recent decades as the average size of crop fields has increased. 30-80% of fencerows had been removed since the 1930's.

Landscape structure. The habitats discussed here (small grains, corn, soybeans, sunflowers, hay, pastureland and rangeland, wetlands, woodlands, and strip cover) are arranged in landscapes that affect Neotropical migratory birds. Interspersion of habitat types, habitat-patch size, and other aspects of landscape structure can affect species composition, abundance, pairing and reproductive success, and population dynamics. Species richness and abundance of birds in farmland are greatest in grasslands, pasture, early-successional habitats, strip cover, and shelterbelts. Recent declines in the areal extent of these habitats have led to a simplification, or reduction in diversity, of farmland landscapes in general. In Ohio, for example, the percentage of farmland that was harvested increased from 45% to 67% between 1940 and 1982. The declines in these habitats also have been accompanied by a trend toward larger crop fields. As an indication of this trend, the average size of farms in the Midwest in 1970, 1980, and 1990 was 164, 183, and 206 ha, respectively.

Effects of Land-Use Practices on Neotropical migratory birds
The agricultural habitats that appear to have declined the most are pastureland and strip cover. These declines may have contributed to population declines of grassland and shrubland birds. Population declines also may be related to agricultural land-use practices. Land-use practices create, modify, and destroy habitat for various species. These practices include cultivation, chemical application, grazing, haying, idling cropland, and maintaining strip cover. Some of these practices have changed in recent years, but it is important to realize that a practice may be contributing to population declines (e.g., by providing poor nesting habitat) even if the extent of the practice has not changed. Here we consider each practice individually.

Cultivation
Cultivation, or tillage, of soil is integral to the process of raising most crops. The amount of cultivation varies with the tillage system. Conventional tillage consists of tilling fields up to several times per year to prepare fields for planting and to control weeds after the crop has emerged. Little crop-plant residue remains on the surface of the soil during the growing season. Tillage is often the main method of weed control in organic farming. Conservation tillage, defined by the Conservation Technology Information Center (CTIC), is any tillage and planting system that maintains at least 30% of the soil surface covered by residue after planting to reduce soil erosion by water or, where wind erosion is the primary concern, maintains at least 1,000 pounds/acre of residue throughout the critical wind-erosion period. The types of conservation tillage are no-till, ridge-till, and mulch-till. Reduced tillage, which leaves 15-30% crop residue or 500-1000 pounds/acre of residue, is a form of crop-residue management but is not currently considered conservation tillage.

Cultivation affects birds in several ways. The amount of cultivation of rowcrop fields influences bird use by affecting the amount of crop residue on the surface of the soil. An immediate effect of cultivation may be to expose arthropods and other prey to foraging birds, etc. A more lasting effect, however, is a reduction in abundance of the litter-dwelling arthropods that are prey items for many birds. Conventionally tilled fields have lower arthropod abundance than no-till fields or idle areas except during pest outbreaks in the crop. Early-summer cultivation also can disrupt nesting activity, destroying nests or causing nest abandonment.

The extent of conservation tillage has increased in recent years. Using a slightly different definition of conservation tillage from that in CTIC (1994), CTIC (1983) reported that conservation tillage was used on 10% of U.S. cropland in 1982 and reduced tillage on another 14%. Conservation tillage increased from 28% to 40% of the planted area in the Midwest between 1990 and 1994 (CTIC 1994). This increase occurred throughout the Midwest. Reduced tillage was used on another 24% of the planted area in the Midwest in 1994 (CTIC 1994). This increase in conservation tillage is an important contribution to sustainable agriculture. Other contributions are organic farming, alternative crops, and strip intercropping.

Castrale compared conventionally tilled fields (corn and soybeans) with no-till fields in Indiana and found one third more bird species on the no-till fields during the summer. No-till fields had greater bird abundance, and the eight most common species were more abundant on no-till fields. Killdeer and horned larks were more abundant on conventionally tilled fields. No-till fields are also more attractive to most Neotropical migratory birds in Iowa and Illinois.

Chemical Applications
Application of herbicides can affect birds by reducing the availability of seeds. Herbicides and insecticides reduce the abundance and diversity of litter- and foliage-dwelling arthropods. Pesticides also can cause acute or sublethal effects on birds. Gard and Hooper reviewed the effect of pesticides and contaminants on Neotropical migratory birds and pointed out that the effects on populations are poorly understood. Insecticides and contact herbicides are the main methods of weed and insect control in no-till fields.

Grazing
Grazing by livestock can have direct and indirect effects. Nests can be lost from trampling if stocking rates are high. Grazing also can affect the avifauna composition by affecting vegetation structure.

Haying
Mowing hay drastically alters the structure of the vegetation, which affects species differently depending on their habitat preferences. Mowing hay also can cause nest losses as well as mortality of fledglings and adults. If mowing is frequent, many birds may not be able to complete their nesting cycles.

Idling Cropland
Little farmland in the Midwest is idled unintentionally because the land is generally very productive. In 1987, 11% of the farmland in the midwestern states was idle cropland. Most of this land was idled by federal cropland-retirement programs. Annual set-asides are used to adjust the planted area in an attempt to influence the supply of various crops. Also, there have been multiple-year retirement programs, such as Soil Bank and the CRP.

Few Neotropical migratory birds are likely to benefit from annual set-asides. On set-asides in Illinois and Iowa, the most commonly planted vegetation is oats (Avena sativa), in which few species nest. Furthermore, the value of this vegetation as nesting habitat is limited if it is planted late in the nesting season because nesting cover is not established when breeding begins. For example, Warner found that, in the 1980's, very little hay and oats persisted over the winter; therefore, very little of this vegetation was well established in the spring. In contrast, half or more of this vegetation persisted over the winter in the 1960's and early 1970's. An additional limitation in this habitat is that mowing destroys some of the nests that are initiated. Finally, the great spatial and temporal variability of this habitat means that philopatric birds may not find suitable nesting habitat in the same place from year to year.

Land enrolled in multiple-year retirement programs, on the other hand, generally has significant value for wildlife species. Recent work has documented use of CRP fields by Neoptropical migrants. Surveyed CRP fields in western Minnesota, North Dakota, South Dakota, and eastern Montana had high species richness and high abundance of some Neotropical migratory birds compared with cropland. Many of the species that were more abundant in CRP fields had declined in the central U.S. in the quarter century before the CRP era. Lark buntings and grasshopper sparrows, for example, were the most abundant species in CRP fields, and their populations declined substantially during this period. In contrast, species that were more abundant in cropland than in CRP fields had been relatively stable before the CRP era. These findings suggest that the CRP may have benefited many Neotropical migratory birds that nest in grassland, assuming that the CRP boosted reproduction.

Nest success in CRP fields is comparable or higher than that in some other habitats. Bird use of CRP fields (exotic, cool-season grasses) differed from use of reduced-tillage, rowcrop fields. The most abundant species in CRP fields were red-winged blackbirds, dickcissels, grasshopper sparrows and bobolinks. Horned larks and vesper sparrows were the only species nesting in rowcrop fields. In CRP fields, nest success was 34% for ring-necked pheasants, 14% for dickcissels, 16% for vesper sparrows, 30% for grasshopper sparrows, and 15% for red-winged blackbirds (at least 27 active nests for each of these five species). These estimates were higher, in general, than estimates from alfalfa fields and strip-cover habitats, indicating that CRP fields are better nesting habitat than many of the alternative habitats.

Nest success in CRP fields also may be comparable to that in pastureland, although some CRP fields attract fewer species and individuals than some pastureland. Nest success did not differ significantly between CRP fields (native grasses) and grazed, native-vegetation pastures in Kansas. Nest success ranged from 11-25% in eastern meadowlarks and from 6-12% in dickcissels. Avian abundance in native-grass CRP fields was lower than that in moderately grazed, native-vegetation pastures. Other CRP fields in the same area, however, had greater grass cover and greater bird abundances than those used in the pasture comparison.

It is important to recognize that the habitat structure of CRP fields changes over time. Older CRP fields in Michigan tended to have lower avian abundance and diversity. The cover on older fields was more dense and these fields had less bare ground than younger fields.

Maintaining Strip Cover
Strip cover provides habitat for most of the Neotropical migratory birds of agricultural landscapes because it is used by birds requiring either herbaceous or woody vegetation. These areas, which include grassed waterways, terraces, fencerows, roadsides, and windbreaks/shelterbelts, usually provide habitat that is more long-term than that provided by areas enrolled in cropland-retirement programs. Food (e.g., arthropods, weed seeds) is often more abundant in strip cover than in crop fields, and complex vegetation structure provides nesting sites, song perches, and cover. Many of these habitats are associated with conservation practices used to control soil erosion, such as crop rotation, terraces, contour planting, strip intercropping, grassed waterways, and windbreaks. Some kinds of strip cover are subject to disturbances such as mowing. During the past few decades, the average size of cropped fields has increased, reducing the amount of strip-cover habitat.

Grassed waterways are heavily used by birds. These waterways are natural drainage systems or channels constructed to transport water off crop fields at a non-erosive velocity. Various grass species are planted in these waterways to slow the flow of water. In Iowa, 48 bird species used grassed waterways, compared with only 14 in surrounding rowcrop fields. The most abundant bird species using waterways were red-winged blackbirds, dickcissels, barn swallows, grasshopper sparrows, brown-headed cowbirds, song sparrows and western meadowlarks. Total bird abundance in waterways averaged 2,198 birds/census count/l00 ha, compared with 682 for crop fields. Eleven bird species nested in the grassed waterways; red-winged blackbird and dickcissel nests were most common. Considering nests of all species, more than twice as many nests were found in forbs as in grass. Sweetclover (Melilotus spp.) and curly dock (Rumex crispus) were predominant nest substrates. Orientation of crop rows relative to the waterway influenced agriculture-related disturbance. Nest densities were greater when the rows paralleled the waterways. Nest success of red-winged blackbirds and dickcissels in waterways was 8.4 and 22.0%, respectively. Predation was the most common cause of nest loss (57% of all nest losses), followed by mowing (16%).

The practice of planting and maintaining grassed waterways clearly provides habitat for a variety of Neotropical migratory birds. Mowing, however, affects the quality of this habitat. Mowing alters the structure of the vegetation, which in turn may affect the bird community. Dickcissels, common yellowthroats, and red-winged blackbirds preferred to nest in waterways with tall grass cover, whereas nest densities of vesper sparrows were greater in mowed waterways. Grasshopper sparrows nested only in waterways that had been mowed the previous year; sedge wrens nested only in waterways that had not been mowed the previous year. The timing of mowing may also affect populations. Birds that have been displaced from mowed hayfields may move into grassed waterways with have suitable vegetation structure. Mowing waterways at the peak or late in the nesting season may interfere with some birds' last nesting attempt of the season.

Terraces are another kind of strip cover that results from conservation-oriented land use in cropland. Wildlife use of terraces is poorly known but has been examined by D.W. Beck (USDA Natural Resources Conservation Service, Des Moines, Iowa unpubl. data). Beck observed 13 bird species using grassed backslope terraces and found evidence of nesting by mallards, dickcissels, vesper sparrows, red-winged blackbirds, and ring-necked pheasants.

Fencerows are linear habitats that separate agricultural fields with a fenceline and associated vegetation. Best examined bird use of fencerows with only herbaceous vegetation, herbaceous vegetation with scattered woody plants, and continuous woody vegetation. The number of species was greatest in the continuous woody type and least in the herbaceous type, regardless of season.

In Michigan, 16 species nested in herbaceous and woody fencerows; nest density in fencerows was 43.5 nests/ha. Fencerow width, adjacent field type, and area of open shrubs most influenced nest density. Wider fencerows, which were more heterogeneous and had greater shrub coverage than narrow fencerows, supported greater nest densities. Fencerows bordered by old fields had more nests than those bordered by crop fields. Nest density and abundance increased with shrub abundance. Song sparrows, American robins, northern cardinals, red-winged blackbirds, gray catbirds, brown thrashers, northern flickers, and ring-necked pheasants were the most frequent nesters in fencerows. Apparent nest success (successful nests/active nests) in fencerows was 58% overall. Raccoons (Procyon lotor), red foxes (Vulpes vulpes), striped skunks (Mephitis mephitis), and long-tailed weasels (Mustela frenata) were responsible for most nest losses. Nest success was lower for larger, ground-nesting game birds than for passerines nesting on or above the ground.

Herbaceous cover along roadsides is used by many species. In Iowa, 35 bird species were observed in roadsides compared with 26 species in adjacent rowcrop fields. Grass was the dominant vegetation in roadsides and forbs were uncommon. Increased diversity of roadside vegetation can increase bird-species richness. Abundance of some bird species in roadsides was related to vegetation height and vertical density. Both of these vegetation characteristics are influenced by burning and the composition of grasses (exotic vs. native). The number of bird species observed was inversely related to grass coverage and directly related to the amount of bare ground. Red-winged blackbird nests were the most common and nest success was 26%. Predation was the main cause of nest failure, accounting for the fate of 52% of all active nests. Mowing appeared to benefit some species; vesper sparrows and meadowlarks nested on mowed roadside shoulders.

In Illinois roadsides, 92% of all passerine nests were of red-winged blackbirds; other nesting species included dickcissels, brown thrashers, eastern meadowlarks, grasshopper sparrows, vesper sparrows, song sparrows, and sedge wrens. Nest densities in roadsides were affected by the kind of habitat in the vicinity of the roadside. The number of passerine nests in roadsides was higher in years in which a smaller proportion of the study area was planted to small grains, presumably because less of the non-roadside habitat was suitable for nesting. The study plot with the most grassland area had the highest density of passerine nests in linear, grassland habitats. Similarly, roadside study plots with the highest density of pheasant nests were in proximity to other prime nesting habitats such as hay. In Iowa, the highest densities of red-winged blackbirds were found in roadsides adjacent to idle grasslands and hayfields.

Densities of passerine nests in Illinois roadsides also were affected by characteristics of the roadsides. Nest densities were greater in interstate roadsides than in secondary roadsides and increased with width of the roadside. Nest densities were 4-5 times greater in interstate roadsides dominated by brome-alfalfa (Bromus inermis-Medicago sativa) than in those dominated by fescue (Festuca spp.). There were over six times more passerine nests on managed (mowing deferred until 1 August) than on unmanaged (frequently mowed) secondary roadsides. In this Illinois study, as in the Iowa study, mowing was confined to the roadside shoulders; the sloped sections were left unmowed.

Higher nest densities in a particular landscape do not necessarily indicate that a species is doing better there; high densities could lead to lower nest success. Ring-necked pheasants in Illinois had low annual nest success in linear habitats (farmland corridors and managed roadsides) in years with low amounts of grassland (strip-cover habitats, forage crops, small grains) per hen in the spring. Also, nest success in roadsides was lower in years with high nest densities than in years with low densities. Similar findings have not been reported for passerines.

Shelterbelts are a common feature of midwestern landscapes. For many decades, federal agencies have encouraged farmers to plant shelterbelts or windbreaks to decrease wind erosion and to protect crops, livestock, and farmsteads. Establishment of shelterbelts can provide positive economic returns. Shelterbelts usually consist of one to five rows of trees and shrubs but can be much wider, especially around farmsteads. Because shelterbelts usually contain trees, they provide habitat for forest and forest-edge species.

In North Dakota, at least 64 species of birds are known to have bred in shelterbelts or tree claims (trees planted by homesteaders). The most common species were the brown thrasher, mourning dove, vesper sparrow, least flycatcher, eastern kingbird, black-billed cuckoo, yellow warbler, American goldfinch, gray catbird, clay-colored sparrow and American robin. The vesper sparrows and American goldfinches were presumably not nesting in the shelterbelts. Yahner recorded 87 bird species using farmstead shelterbelts in Minnesota and documented nesting in 17 of these. In South Dakota, the number of bird species increased with shelterbelt area both during spring migration and during the breeding season. The density and diversity of breeding birds was significantly correlated with both the age and size of shelterbelts in North Dakota.

The configuration of shelterbelts can affect the species composition of the avifauna. Shelterbelts with only a few rows of woody plants attracted more birds associated with open habitats, whereas those with many rows attracted more birds associated with forested habitats. Yahner, however, provided correlations between variables describing vegetation structure and avian community or population variables in shelterbelts and found that the perimeter and length of shelterbelts were associated more often with measures of community structure than were area and width of shelterbelts.

The avian species composition of shelterbelts also can be affected by vegetation structure and landscape context. Martin and Vohs found the highest bird species diversity in South Dakota shelterbelts with developed tree canopies and lush grass layers; dense shrub growth under the trees was not preferred. In Minnesota, vegetation structure was a major factor determining bird community structure in shelterbelts, with older belts having lower densities of shrubs, higher densities of trees, and greater bird-species richness. Distance to wooded and old-field habitats, and the amount of cropland and pastureland surrounding shelterbelts, influenced which birds were found in shelterbelts. A concentration effect occurred in isolated shelterbelts, which were more likely to have certain species, probably because of the paucity of similar habitat in the vicinity. In addition to species occurrence, it is important to examine nest success. Apparent nest success (successful nests/active nests) of mourning doves and American robins was 32% and 56%, respectively.

Conservation Stratagies
This review suggests that current agricultural practices are contributing to population declines of farmland birds, although not necessarily to the exclusion of factors acting during migration and wintering. Recent population declines could be due mainly to loss of habitat or to a combination of loss of habitat and degradation of habitat (e.g., low nest success or survival rates in attractive habitats and landscapes). Conservation measures to deal with these declines will depend on the relative importance of habitat loss and degradation, the objectives of a conservation strategy, the costs of implementing a strategy, and the feasibility of the strategy.

Owners and operators of farms can implement agricultural practices that will benefit Neotropical migratory birds on midwestern farmland. Many farmers appreciate having non-pest bird species on their farms and nearby. If farmers are forced to choose, however, between economic survival and having wildlife to observe and hunt, they will choose survival. Even in situations in which economic survival is not at stake, farmers will consider costs. It is in these situations where information and education may make the difference in favor of wildlife generally and Neotropical migratory birds specifically. Here we take the practical approach of recognizing that recommendations for farmland must be aimed at a broad audience that includes private landowners as well as professional wildlife managers. We offer three objectives that we believe will have broad support, review a general framework that likewise should have broad support, and summarize specific recommended modifications to current agricultural practices.

Objectives
A primary objective of a conservation strategy for Neotropical migratory birds in midwestern agricultural landscapes should be to conserve the remnants of prairie and savannah that still exist. Most of the prairie has been lost, increasing the value of the remnants. Although restoration of midwestern prairie and savannah ecosystems on a large scale is impractical, the remnants can be used for biological investigations, ecological education, and core areas for additional restoration. Although the persistence of grassland and open-woodland birds in the Midwest may not depend on these remnants, it is important for the conservation of biodiversity in general to save or restore representative natural ecosystems with a large proportion of their native flora and fauna. Of similar importance are the remnant woodlands, riparian areas, and wetlands required by a variety of bird species.

A second objective of a conservation strategy should be to ensure the population viability of species that are (1) listed under the Endangered Species Act (ESA), (2) listed by individual states, or (3) are of special concern to managers because their populations are declining or vulnerable (USFWS 1995). Forty-one of these might occur in midwestern farmland. Investments that could prevent listing of additional species under the ESA may be economical in the long run, given the costs of listing, preparing recovery plans, etc. Beyond this practical reason, drastic population declines of rare species will result in loss of these species from some ecosystems, which may affect ecosystem functioning and be undesirable for other reasons.

A third objective should be to identify priority species or groups of species beyond those identified under the second objective. A different conservation strategy for these Neotropical migratory birds might be called for, depending on whether the primary concern is for reversing population declines or for promoting biodiversity. Some species may require large blocks of grassland habitat, and others may benefit from increased availability of edge habitat in the form of strip cover. Ideally, management can benefit all groups.

General Framework
Identification of sources and sinks is an important step in implementing a conservation strategy for Neotropical migratory birds in farmland. This step has been taken for forested habitats in the Midwest. Many managers now understand that creating attractive habitat may not necessarily benefit target species if that new habitat is a sink for those species. The review of nest success in farmland indicates that few species are known to be reproducing at levels sufficient to balance estimated mortality. Even the nest success estimates in CRP fields and moderately grazed pastures, reviewed in the "Idling cropland" section, are not encouraging.

After sources and sinks have been identified, habitat, land-use, and landscape features that distinguish them can be examined to determine which features contribute to differences in vital rates. Such analyses will lead to recommended practices that will change the attractiveness of sources and sinks or increase individual fitness in them. Simulation modeling can be useful in evaluating potential actions. Finally, after actions have been taken, the status and trends of target species or guilds should be monitored to evaluate the results. All of this can be done in the context of a comprehensive framework for conservation and management planning. A major limitation of this process is our incomplete understanding of factors affecting population dynamics and ecosystem functions in agricultural ecosystems.

Specific Recommendations
Rodenhouse made the insightful observation that farming might develop along two tracks. Small, owner-operated farms would predominate close to urban centers where farm families could obtain supplemental income. These farms would provide habitat for many Neotropical migratory birds because expensive inputs (large machinery, fuel, inorganic fertilizer, and pesticides) would be minimized and farming would be diversified. Large, externally owned farms would exist in rural areas with the most productive soil. These would be intensively managed and provide little non-crop habitat. These tracks certainly seem reasonable, considering current economic conditions in the Midwest.

Agricultural practices on larger farms may best be influenced by modifying agricultural programs and policies. These farms can be expected to attract species nesting in cultivated fields: horned larks, vesper sparrows, and killdeer. Although these species are not common in cropland, their cropland habitat is very extensive. Populations of these species large. Monitoring of population trends of these species should continue. As long as their populations are relatively stable, these species should not receive high conservation priority. Vesper sparrow populations have declined recently in parts of the Midwest, according to data from the North American Breeding Bird Survey, and had low nest success in Iowa.

Smaller farms should be encouraged to enhance the attractiveness of their land to Neotropical migratory birds, especially grassland birds that have declined in recent decades, such as grasshopper sparrows and bobolinks. Long-term set-aside, such as the CRP, provide much of the grassland habitat in the Midwest today and should be encouraged. A wide variety of other species, including woodland species, also can benefit from farmland. The following recommendations are organized by agricultural practice, as covered above.

Cultivation. Tillage should be minimized. Rodenhouse recommended that crop residue be retained on the soil surface, which could be done by reducing the number of times a field is cultivated annually and by using subsurface tillage where appropriate. A greater variety of Neotropical migratory birds may be attracted by increasing the diversity of crops grown, especially if the structure of the vegetation is quite different. Crop diversity also will contribute to sustainable agriculture.

Grazing. Grazing is not incompatible with a diverse avifauna. Moderate grazing of pastures may enhance local habitat diversity. It has been suggested, however, that public grasslands managed for wildlife should benefit species that prefer habitat that is in relatively short supply, that is, grassland that is not frequently grazed or hayed. This recommendation could be extended to private land in the Midwest. Each grassland species has a particular kind of preferred vegetaion for nesting, facilitating management for particular groups of species.

Haying. Haying appears to be a major problem for Neotropical migratory birds because hayfields have low nest success. They are highly attractive to certain species, but the cutting interval is usually too short to allow complete nesting cycles. Rodenhouse recommended that spring mowing be delayed as long as possible, nighttime mowing be avoided, and the intervals between mowing be as long as possible. Additionally, hayfields with warm-season grasses should be encouraged because they would be cut later in the year than alfalfa.

Idling cropland. Idled cropland would seem to require relatively little management because it already attracts a variety of species. Practices such as mowing or burning, however, could modify many areas and change the composition of the avifauna. Mowing during the nesting season can lower nest success substantially. It may be possible to enhance bird reproduction and survival by changing the amount and configuration of idle land; nest success may be higher in block habitats than in linear ones with proportionately more edge habitat. In a Minnesota study, nest success was lower near woody edges of tall-grass-prairie remnants than it was farther from such edges. Finally, if certain kinds or configurations of idled cropland are found to be source habitats, they should be strongly encouraged to counteract the many apparent sinks in agricultural landscapes.

Patterson and Best examined bird use of CRP fields that differed in structure and composition of vegetation and made recommendations aimed at benefiting grassland birds. The species composition of the vegetation should be based in part on vegetational attributes that attract birds. A wider variety of planted species than has been used in the past would enhance bird use. Mowing of weeds should be done after the peak of the nesting season. Some large tracts of habitat should be maintained for the benefit of area-sensitive species. Other recommendations could be added to their list. Grazing may enhance species richness if the structural diversity of the habitat is increased. Prescribed burning, every few years, is often recommended for other grassland habitats to maintain attractive nesting cover, and presumably would benefit birds using CRP fields. Multiple-year programs, such as the CRP, appear to offer many more benefits to Neotropical migratory birds than annual set-aside programs. Grassland cover that is at least a year old is established well enough in the spring, when migratory birds arrive, to attract breeding pairs. After a few years, vegetative and sexual reproduction by plants fills in gaps from the initial seeding, increasing the height and density of vegetative cover.

Maintaining strip cover. Recommendations for strip cover depend on whether management is intended to favor grassland species or edge species. Management for the former should favor herbaceous cover. The benefits to Neotropical migratory birds of permanent herbaceous cover can be enhanced by changing common practices. Timing of mowing of grassed waterways should be considered. Because peak nesting in grassed waterways occurred in July in Iowa, Bryan and Best recommended that mowing in waterways be deferred until the end of August or early September. Fall mowing is not recommended because residual cover would be reduced in the subsequent winter and spring, and annual mowing is discouraged.

Beck found several factors that decreased wildlife use of grassed-backslope terraces, such as backslope steepness, poor grass stands, narrow width, grazing, and herbicide drift. He suggested that improved management might involve flattening backslopes, improving grass stands, improving grazing management, and planting alternative vegetation.

If herbaceous vegetation on roadsides is mowed, it should be mowed in mid-to-late August. Also recommended is prescribed burning in the spring every 3-5 years. Both mowing and burning should be conducted in blocks to ensure that some portions of roadsides are undisturbed at any time. Fencelines should be retained along roadsides because roadsides with fencelines are less susceptible to agricultural encroachment and fencelines provide singing perches for birds. Mowing roadside shoulders provides nesting habitat for some species, but mowing should be restricted to early spring and late summer to reduce nest losses.

An adoption model for roadside (and other) habitat management involves farm operator attitudes and perceptions. Warner made the following roadside management recommendations for grassland birds: (1) fescue and bluegrass (Poa) sods should be seeded to brome-alfalfa and/or native grasses and forbs, (2) mowing should be delayed until after 1 August, (3) widths of roadside tracts should be maximized where possible, and (4) the establishment and protection of woody plants should be encouraged. In areas where brown-headed cowbirds are abundant, of course, the effect of woody plants on frequency of brood parasitism will have to be considered carefully. The prescribed burning recommended by Camp and Best, which would slow the establishment of woody plants, could be used in areas where woody plants were not desired. They also noted that bird use of roadsides with well-established native vegetation has not been evaluated. Such roadsides may become more abundant in the future as the agencies charged with managing roadside vegetation seek methods that will benefit wildlife and minimize invasion of weeds.

Managing for breeding, edge species should be focused on providing diverse habitat in which nest success can potentially be high. Fencerows should be >3m wide, according to the recommendation of Shalaway, who worked in Michigan. This would allow for greater shrub/tree growth and potentially reduce the likelihood of nest predation. Farther west, where brown-headed cowbirds are more abundant, the possible enhancement of habitat for this species would have to be considered. Shalaway also suggested that selective mowing and woodcutting in fencerows can create a mosaic of vegetation types attractive to a variety of birds. He also recommended retaining or creating at least 1-2 large snags per km of fencerow. Artificial nest-boxes may attract eastern bluebirds and tree swallows, both Neotropical migratory birds.

Management recommendations for shelterbelts, given by Yahner, include: establish shelterbelts at least 0.6 ha in area and 8 rows in width, consider proper spacing within and between rows, use a diversity of plantings, discourage livestock grazing, discontinue mowing and cultivation after the shelterbelt becomes established, and retain snags as nesting and foraging sites.

General. Our final recommendation is that biologists and managers strive to improve their working relationships with private landowners and the organizations that represent them. The concept of benefiting Neotropical migratory birds in agricultural landscapes is different from much of traditional wildlife management, which has focused on managing land controlled by state and federal governments. Those seeking to benefit Neotropical migratory birds can learn from those who have sought to benefit ducks and upland game species, most of which occur on private land. Many game managers have learned the value of education, outreach, and attention to landowner concerns. Progress may require compromise and consideration of large-scale solutions, such as influencing agricultural policy. The recent efforts to renew the CRP brought together many wildlife and farming organizations that shared a common interest. Serious consideration has been given to the major effects of agricultural policies on wildlife in agricultural landscapes. With enhanced communication, sound science, and a common conservation strategy, we are confident that farmers and wildlife can continue to coexist in the Midwest.

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