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Density and Fledging Success of Grassland Birds in Conservation Reserve Program Fields in North Dakota and West-central Minnesota

Introduction


Several species of grassland birds are significantly less numerous in North America in the mid-1990s than they were in the mid-1960s, when extensive population monitoring began with the North American Breeding Bird Survey (BBS). More grassland species have undergone population declines than have other classes of birds (Askins 1993, Knopf 1994, Peterjohn et al. 1994, Herkert 1995).

Several factors may have contributed to these widespread declines. One important factor has been loss of habitat (Askins 1993, Herkert 1995). Although most of the original prairie was lost long before the BBS started, habitat loss has continued in recent decades (Herkert 1991, Samson and Knopf 1994). Grassland birds now breed in many kinds of grassland habitat that are structurally similar to various types of prairie vegetation. Recent population declines in the northeastern United States may be due largely to loss of old-field habitats that have undergone succession to woody vegetation unsuitable for grassland birds (Askins 1993). Habitat loss in the Midwest has been due to loss of pasture and hayland, along with a general loss of strip cover as farming has become more intensive (Herkert 1991, Askins 1993, Herkert et al. 1996). Alfalfa (Medicago sativa) has generally replaced more diverse vegetation as the primary source of hay (Bollinger et al. 1990, Warner 1994, Herkert et al. 1996). Older hayfields, which provide more diverse habitat, are usually not promoted by current practices (Bollinger 1995). Few species nest in row crops and small-grain fields, although many species nest in alfalfa fields, pastures, and idle grassland (Best et al. 1995). Finally, many of the small fields common in current agricultural landscapes do not attract the full suite of breeding grassland birds, perhaps because some species are area sensitive (Herkert 1994, Vickery et al. 1994, Warner 1994).

Low quality of suitable nesting habitat may have contributed to population declines of grassland birds (Askins 1993). Prairie remnants and fields of non-native grassland, although suitable for nesting, may be of lower quality than large expanses of prairie. Small fields have a high ratio of edge to area, and many fields have wooded edges that may contribute to high frequencies of nest predation and brood parasitism by Brown-headed Cowbirds (Molothrus ater; Best 1978; Gates and Gysel 1978; Graber and Graber 1983; Johnson and Temple 1986, 1990).

If habitat loss, fragmentation, and degradation have contributed to population declines, then restoration of large amounts of grassland would be expected to slow or reverse those declines. The Conservation Reserve Program (CRP), administered by the U.S. Department of Agriculture (USDA), dramatically increased the amount of grassland in the late 1980s, particularly in the tall- and mixed-grass regions of the central United States. The addition of this habitat presents the opportunity to evaluate the benefit to birds of restoring a large amount of grassland habitat.

The potential benefit from the CRP is enormous. This program has taken millions of hectares of highly erodible cropland, almost a tenth of U.S. cropland, out of production under 10-yr contracts (Johnson and Schwartz 1993). Except in the southeastern United States, most of this land was seeded with perennial grasses and legumes, creating suitable feeding and nesting habitat for some bird species but possibly eliminating some habitat for a few species that nest in cropland and very short vegetation (Basore et al. 1986, Johnson and Igl 1995, King and Savidge 1995). Fields enrolled in the CRP cannot be grazed but can be partially hayed in weather-related emergencies. Johnson and Schwartz (1993) examined bird use of CRP fields in the northern Great Plains and found that most grassland species had higher breeding densities in CRP fields than had been reported for cropland.

Among the species that have higher breeding densities in CRP fields than in cropland are several that declined in the central United States from 1966 to 1990 according to BBS data (Johnson and Schwartz 1993, Johnson and Igl 1995). These declining species include Grasshopper Sparrow (Ammodramus savannarum), Baird's Sparrow (A. bairdii), Clay-colored Sparrow (Spizella pallida), Dickcissel (Spiza americana), Lark Bunting (Calamospiza melanocorys), and Bobolink (Dolichonyx oryzivorus). Savannah Sparrow (Passerculus sandwichensis), which also had higher densities in CRP fields, has experienced a continental population decline (Peterjohn et al. 1994). Other studies have confirmed the widespread use of CRP fields by grassland birds (Granfors 1992, Millenbah 1993, Klute 1994, Patterson and Best 1996). The potential of the CRP to slow or reverse declines of these species (e.g., Reynolds et al. 1994) depends on whether birds nesting in CRP fields have higher reproductive success than they would have had in the absence of the CRP. Ducks such as the Mallard (Anas platyrhynchos), which also declined during the 1980s, had relatively high hatching success in CRP fields compared with hatching success on Waterfowl Production Area (WPA) fields (Kantrud 1993, Reynolds et al. 1994).

To compare CRP fields with another grassland habitat frequently used by grassland birds that have declined, I studied birds in CRP fields and idle grasslands on WPA fields. Numerous WPA fields, mostly small tracts of grassland and wetlands in the U.S. prairie pothole region, are managed by the U.S. Fish and Wildlife Service (USFWS) to provide nesting and brood-rearing habitat for waterfowl. These fields attract all of the declining species listed above (Renken and Dinsmore 1987), although these species rarely nest in cropland, the habitat that CRP fields have replaced (Johnson and Schwartz 1993, Best et al. 1995). Thus, addition of CRP fields to the landscape has provided these species, which typically nest in hayfields and pastureland (Best et al. 1995), with an additional habitat they may find suitable for nesting. To determine how similar the avifaunas of CRP and WPA fields were, I estimated densities of breeding birds on the study areas. I also estimated fledging success. Variation in fledging success probably contributes greatly to variation in reproductive success, and techniques for monitoring nests are better developed than are techniques for estimating survival of juveniles.


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