Northern Prairie Wildlife Research Center
During the studies of nesting ducks, biologists also found nests of large nongame birds such as short-eared owls, American bitterns and northern harriers. In 1990, NPWRC initiated a more extensive survey of the use of CRP fields by breeding birds in North Dakota, eastern Montana, South Dakota and western Minnesota. These four states have about 9.9 million acres of land enrolled in the program.
Biologists surveyed birds on 11,500 acres in 240 fields in 1990 and on 15,273 acres in 335 fields in 1991. Seventy-three species of birds were identified in these fields. Overall densities averaged 50 pairs/100 acres, and ranged from a high of 77 pairs/100 acres in 1991 in Sheridan County, Montana, to a low of 19.5 pairs/100 acres in Fallon County, Montana, in 1990.
The most common species on CRP fields have the greatest potential of benefitting from the program. Many of these species, including bobolinks, lark buntings, grasshopper sparrows and dickcissels, are neotropical migrants - birds that summer in North America but migrate to Central or South America for the winter. These birds are the target of conservation by Partners in Flight - Aves de las Americas - a joint effort of states, private organizations and federal agencies that began because of concern over population declines of many of these species.
A comparison of densities of common species in CRP fields with densities in cropland revealed that most grassland species were more common in CRP fields than in cropland (Table 1). Conversion of cropland to perennial cover thus added suitable breeding habitat for these species and may enhance their populations. This change is especially important because during the last quarter-century several grassland species suffered major population declines in the central United States.
The declines were especially severe for lark buntings and grasshopper sparrows, which declined by more than 50 percent since the mid-1960s. These two species are, fortuitously, by far the most abundant nesting species in CRP fields.
Northern Prairie Wildlife Research Center biologists are also investigating the productivity of songbirds inhabiting CRP fields and alternative habitats in southeastern North Dakota and west central Minnesota. Birds must not only occupy a habitat but also successfully raise young to maintain their population levels. Early indications are that only a quarter of the clutches hatch, but that nest success is quite variable. Because of this variability, further study is needed to determine whether nest success in CRP fields is as good as in alternative habitats.
The study of productivity in songbirds also includes an examination of rates of brood parasitism. The northern Great Plains is the center of distribution of the brown-headed cowbird. This species does not build a nest, but lays its eggs in the nest of a host species and leaves them for the host to incubate and to feed and protect the nestlings. Brood parasitism can reduce the nest success and the number of the host's fledged young.
The overall rate of parasitism was half as high in CRP fields as in WPA fields, suggesting that some feature of CRP fields reduces parasitism by cowbirds. Thus, CRP may have the potential of reducing the harm from parasitism across broad regions of the prairies.