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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
Cropland
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%).