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The Conservation Reserve Program:
Habitat for Grassland Birds

Conclusions


Densities of all species were primarily influenced by County, reflecting the uneven geographical distributions of the species. Eight of the twelve species varied significantly among years and six species had significant County-Year interactions. The effects of Year could represent either actual changes in the continental population size or annual shifts in distribution. The interaction is mostly consistent with the latter explanation. Causes of distribution shifts are largely unknown, but most likely express species' responses to direct or indirect effects of precipitation and temperature (Wiens 1974).

No conservation practice was found to be uniformly better than another in terms of overall density of breeding birds (Table 1), but certain practices seemed to favor certain species. The percentage cover of some plant forms was related to the density of several bird species. Lark Bunting, Red-winged Blackbird, and Brown-headed Cowbird were all less common in fields with high coverage of grass; the reverse held for the Bobolink and Sedge Wren. It should, however, be emphasized that these relationships apply only within the range of cover values observed. For example, it would be inappropriate to conclude that Lark Bunting densities would be highest if grass were absent. Several species — Lark Bunting, Grasshopper Sparrow, Savannah Sparrow, Brown-headed Cowbird, and, marginally, the Western Meadowlark — had densities negatively related to the coverage of legumes. Only the Clay-colored Sparrow positively related to legume cover. Three species — Savannah Sparrow, Common Yellowthroat, and especially Red-winged Blackbird — were positively related to the coverage of water. The abundance of individual species varies according to the features of the available habitat; as plantings in CRP fields mature, these will change. Continued research into these successional changes is indicated.

The Conservation Reserve Program does not restore native prairie, as is evident from the vegetation present on CRP fields (Table 2). Nor does it restore avian communities; some species favor native vegetation, others prefer introduced plants (Wilson and Belcher 1989). Nonetheless, the Program does offer breeding habitat for a wide array of grassland bird species. Johnson and Schwartz (in press) noted that six species that were much more commonly found in CRP fields than in cropland (Lark Bunting, Grasshopper Sparrow, Clay-colored Sparrow, Bobolink, Dickcissel, and Baird's Sparrow) had suffered significant population declines during the past 25 years. The CRP has the potential to help reverse those declines.

Studies are now underway to determine the productivity of nongame birds in CRP habitat. There is no reason to anticipate that reproductive success is lower in CRP fields than in alternative habitats, many of which are subjected to greater disturbance from grazing, haying, and cultivation. Nesting success of upland-nesting ducks has been found to be as high in CRP fields as in grassland areas dedicated to waterfowl production (Kantrud 1993). A fundamental value of the Conservation Reserve Program is that the alternative land use for most CRP fields is annual cultivation, with attendant soil erosion and depauperate bird communities.


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