Cowbird Parasitism in Grassland and Cropland
in the Northern Great Plains
Introduction
Significant population declines have been noted recently in populations of several grassland bird species of the northern Great Plains, including the Clay-colored Sparrow (Spizella pallida), Lark Bunting (Calamospiza melanocorys), Grasshopper Sparrow (Ammodramus savannarum), and Bobolink (Dolichonyx oryzivorus) (Robbins et al. 1986, Johnson and Schwartz 1993, Peterjohn and Sauer 1993). In general, population declines of migratory species have been attributed to increased mortality from habitat destruction on the wintering grounds or poor reproduction on the breeding grounds (Hagan and Johnston 1992).
Habitat loss and fragmentation on the breeding grounds of grassland birds are known to contribute to poor reproductive success (Best 1978, Gates and Gysel 1978, Graber and Graber 1983, Johnson and Temple 1986, 1990). Johnson and Temple (1986) found that birds that nested in remnants of tall-grass prairie near wooded edges produced fewer young than birds that nested far from wooded edges.
Brood parasitism by Brown-headed Cowbirds may also contribute to poor nesting success in grassland habitats. The frequency of brood parasitism among grassland species in the northern Great Plains is not well documented, but examples of heavy parasitism are known (Table 1; also Knapton 1979, Johnson and Temple 1990). Breeding populations of the Brown-headed Cowbird reach peak abundance in the northern Great Plains (Robbins et al. 1986; Peterjohn et al., Chapter 2, and Wiedenfeld, Chapter 3, this volume). Brown-headed Cowbirds bred on the Great Plains long before European settlement (Mayfield 1965), but their populations have probably been enhanced by human activities. Since settlement, most of the prairie has been converted to cropland and pasture; tree plantings and agriculture have fragmented the landscape, domestic livestock have been introduced, and fires have been suppressed. Significant increases in cowbird numbers occurred between 1966 and 1991 in the northern Great Plains (Peterjohn et al. Chapter 2, Wiedenfeld Chapter 3).
Additional research is needed to understand the effects of brood parasitism on reproductive success of grassland birds and the interactions (if any) of parasitism with habitat alteration. We examined parasitism in three habitats on the northern Great Plains by using data from three independent studies. The study fields were in a matrix of cultivated lands and mixed-grass prairie in south central North Dakota. Because the abundance of cowbirds relative to their hosts may affect the frequency of parasitism, we also examined the ratio of female cowbirds to hosts based on counts of birds in the breeding season.
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