Northern Prairie Wildlife Research Center
NTMBs constitute the major part of midwestern, farmland avifaunas. Thirty-eight NTMBs are common in at least one farmland habitat in the Midwest, and two-thirds of the common native species are NTMBs (Table 1). Thus the conversion of midwestern forest, savannah, and prairie to agriculture, mostly prior to 1920, undoubtedly changed the abundance and distribution of NTMBs. 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 (e.g., Dinsmore 1981, 1994). Grassland birds, which form a large fraction of the common farmland NTMBs (Table 1), evidently did not decline much between the 1920's and 1950's, when diversified farming was common (Warner 1994). The modern era of intensive cropping coincides with population declines of grassland birds (Herkert 1991, 1995; Warner 1994). Other long-term changes, mostly of declining bird populations, have been noted in Illinois (Graber and Graber 1963) and a small study area in Iowa (Lowther 1984). These declines have continued in recent years (Graber and Graber 1983, Warner 1994). 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.