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Management of Agricultural Landscapes for the Conservation of Neotropical Migratory Birds

Effects of Land-Use Practices on NTMBs


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 (Herkert 1995). 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. The effects of such practices in combination, called farmland structure, is covered elsewhere (Freemark et al. 1995, Rodenhouse et al. 1995).

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 (CTIC 1994). 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 (CTIC 1994).

Cultivation affects birds in several ways (Best 1985, Rodenhouse et al. 1993). The amount of cultivation of rowcrop fields influences bird use by affecting the amount of crop residue on the surface of the soil (Rodenhouse and Best 1983, Basore et al. 1986). 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 (Hendrix et al. 1986). Early-summer cultivation also can disrupt nesting activity, destroying nests or causing nest abandonment (e.g., Rodenhouse and Best 1983).

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 (fig. 3). 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 (Barrett et al. 1990). Other contributions are organic farming, alternative crops, and strip intercropping.


gif -- Graph of conservation tillage changes

Figure 3. Recent changes in estimated extent of conservation tillage in three midwestern regions (CTIC 1994).

Castrale (1985) 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 NTMBs in Iowa (Basore et al. 1986) and Illinois (Warburton and Klimstra 1984).
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