. The Canadian field-naturalist. Natural history; Sciences naturelles. 2007 l(.i and Johnson: Cowhirds in the Northhrn Grhat Plains 249. 10 0123456789 Cowbird Densities (Vy /100 ha) l-KHiKi- 3. Relationship hctwccn rcmaie cowhird densities and the percentage of the total number of passerine nests that were parasiti/ed in Conservation Reserve Program grasslands in North Dakota, South l3akota. Minnesota, and Montana, IW()-2()()6. The data from the North Dakota (N) statewide study were included for compar- ison (1942 and 1493 were averaged). cases of parasitism in the northern Great Plains (
. The Canadian field-naturalist. Natural history; Sciences naturelles. 2007 l(.i and Johnson: Cowhirds in the Northhrn Grhat Plains 249. 10 0123456789 Cowbird Densities (Vy /100 ha) l-KHiKi- 3. Relationship hctwccn rcmaie cowhird densities and the percentage of the total number of passerine nests that were parasiti/ed in Conservation Reserve Program grasslands in North Dakota, South l3akota. Minnesota, and Montana, IW()-2()()6. The data from the North Dakota (N) statewide study were included for compar- ison (1942 and 1493 were averaged). cases of parasitism in the northern Great Plains ( Lin/ and Boiin iycS2: Davis and Sealy 2()(){); Koford^et al. 2()()()). There are many less-cited studies of cowbird parasitism (see Shaffer et al. 2003) with low-to-mod- erate rates of cowbird parasitism in this region ( Hill and Sealy 1994; Grantors et al. 2001: Davis 200^3: Woolfenden et al. 2004). For example, in Manitoba. Davis (2003) found an overall parasitism rate of ^ for six grassland species in southern Saskatchewan over a 5-year period. Although our overall rate of cowbird parasitism was 25%, we did find higher rates of parasitism in some years (, in 2001), weeks (, in the second week of June), counties ( in McPherson County, South Dakota), and individual species (, for the Red-winged Blackbird). The results from our studies can serve as a reminder that cowbird parasitism rates are not constant across space or time ( among years or within a breeding season) and that cowbirds differentially piirasitize hosts within the same habitats. Differences in parasitism lev- els among studies in the northern Great Plains may simply reflect variation among species, regions, habi- tats, or years. For example. Davis and Sealy's (2000) overall parasitism rate of was based on data col- lected at three study sites over two years: parasitism rates were low-to-moderate at two of the study sites (18 and 20%) and heavy at the
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