Northern Prairie Wildlife Research Center
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 (Rodenhouse et al. 1993). Pesticides also can cause acute or sublethal effects on birds. Gard and Hooper (1995) reviewed the effect of pesticides and contaminants on NTMBs 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 (Castrale 1985, Wooley et al. 1985).
Grazing by livestock can have direct and indirect effects. Nests can be lost from trampling if stocking rates are high (Jensen et al. 1990). Grazing also can affect the avifauna composition by affecting vegetation structure (Skinner et al. 1984). Grazing effects are discussed further by Herkert et al. (1996).
Mowing hay drastically alters the structure of the vegetation, which affects species differently depending on their habitat preferences (Frawley and Best 1991). Mowing hay also can cause nest losses as well as mortality of fledglings and adults (Frawley 1989, Rodenhouse et al. 1993). If mowing is frequent, many birds may not be able to complete their nesting cycles. Haying is discussed further by Rodenhouse et al. (1993, 1995) and Herkert et al. (1996).
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 (USDA 1993). 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 (Harmon and Nelson 1973, Berner 1988).
Few NTMBs are likely to benefit from annual set-asides. On set-asides in Illinois (R. Warner, Univ. Illinois, Urbana, pers. commun.) and Iowa (L. B. Best, pers. observ.), the most commonly planted vegetation is oats (Avena sativa), in which few species nest (Best et al. 1995). 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 (pers. commun.) 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 (Harmon and Nelson 1973). 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 NTMBs compared with cropland (Johnson and Schwartz 1993a). 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 (Johnson and Schwartz 1993b). 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 NTMBs 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 (Patterson and Best 1996). 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 (Mayfield estimate) 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 (Patterson and Best 1996).
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 (Granfors 1992, Klute 1994). Nest success (Mayfield estimate) 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 (Klute 1994). Other CRP fields in the same area, however, had greater grass cover and greater bird abundances than those used in the pasture comparison (Hull 1993).
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 (Millenbah 1993). The cover on older fields was more dense and these fields had less bare ground than younger fields.