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Cowbird Parasitism in Grassland and Cropland
in the Northern Great Plains


In seeded grassland, the mean frequency of parasitism was 25% among the host species whose nests we found (Table 2). The frequency in Red-winged Blackbirds (47%) was distinctly higher than for other species combined (21%). Most blackbirds nested in two fields in upland vegetation.

Table 2.   Frequency of Parasitism in Seeded Grassland, Natural Grassland, and Cropland Fields.
Species Nests Frequency (%)
Seeded Grassland
    Vesper Sparrow 10 20
    Grasshopper Sparrow 45 22
    Bobolink 12 25
    Red-winged Blackbird 74 47
    Western Meadowlark 26 19
    Other 18 17
Natural Grassland
    Clay-colored Sparrow 49 24
    Western Meadowlark (southern) 294 47
    Other 35 37
    Horned Lark (early) 24 0
    Horned Lark (late) 60 83
    Vesper Sparrow 84 26
    Lark Bunting 23 61
    Grasshopper Sparrow 13 38
    Red-winged Blackbird 19 26
    Other 10 40
Note:  In natural grassland, data on Western Meadowlark nests in the southern study area are shown and nests in the northern area are included with other nests. Horned Lark nests were classified as early (found on or before 15 May) or late (found after 15 May). Numbers of nests located may not reflect nest densities (see Methods).

In natural grassland, the mean frequency of parasitism was 34% (Table 2). Western Meadowlarks in the southern study had a higher frequency of parasitism (47%) than the mean frequency of all species in the northern study (31%). We found too few meadowlark nests to make an accurate estimate of parasitism frequency in this species in the northern study.

In cropland, the mean frequency of parasitism was 39% (Table 2), mainly because of a high frequency among Horned Larks (Eremophila alpestris) late in the breeding season. We found 24 unparasitized nests of Horned Larks before the first cowbird egg was found (15 May). Of the 60 nests found after 15 May, 83% were parasitized and received 1-6 cowbird eggs (mean = 2.3). This heavy parasitism contrasts with the moderate parasitism (26%) experienced by Vesper Sparrows (Pooecetes gramineus), the other common species in cropland. Our observed cowbird/host ratios corresponded roughly to observed patterns of variation in frequency of parasitism. The lowest ratio (1 : 84) was observed in seeded grassland, where the lowest overall frequency of parasitism occurred (mean = 25%). Higher ratios were observed in natural-grassland fields in the northern area (1 : 13) and in cropland (1 : 10), where birds experienced moderate to heavy parasitism (mean = 31% and 39%, respectively). In natural grassland, the ratio was higher (1 : 5) in the southern study than in the northern study (1 : 13). This higher ratio corresponded to a higher frequency of parasitism in the southern study (47%).

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