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The Cowbird and Certain Host Species in Michigan
Andrew J. Berger
The Wilson Bulletin
Vol. 63, No. 1 (Mar., 1951), pp. 26-34
Published by: Wilson Ornithological Society
Stable URL: http://www.jstor.org/stable/4157909
Page Count: 9
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A study of Cowbird parasitism in Washtenaw County, Michigan, from 1946 through 1949 involved twenty host species and two additional species believed to be hosts. The Cowbird laying season extended from April 21 to July 26. Of 112 parasitized nests observed, 53 contained one Cowbird egg, 36 contained two, 15 contained three, six contained four, and two contained five. Of 196 Cowbird eggs observed, 60 (30.6%) hatched. Forty-three of these 60 young Cowbirds (21.9% of the 196 eggs) fledged. In each of four nests, a Cowbird egg was laid after incubation had begun: in two cases while eggs were still unhatched; in two cases after the young had hatched. Cowbird eggs were laid in seven deserted nests. The interval between nest-desertion and deposition of the Cowbird egg in five Song Sparrow nests varied from one to 26 days and averaged 10.2 days. The relative time of hatching of host and Cowbird eggs is important in the fledging success of the respective species. Host young were fledged from each of six nests of five species in which the host eggs hatched first. In only two of nine nests in which Cowbird eggs hatched first were host young also fledged. Both host and Cowbird young were fledged from two Song Sparrow nests in which a Cowbird egg and at least one host egg hatched the same day. Host eggs hatched first in two Song Sparrow nests which fledged three host and one Cowbird and four host and one Cowbird respectively. Cowbird eggs hatched first in two Song Sparrow nests which fledged three and four Cowbirds respectively but no sparrows. Parasitized nests of field-nesting species which I observed were in portions of fields bordered by a woodlot or thicket, whereas non-parasitized nests were not near such vegetation.
The Wilson Bulletin © 1951 Wilson Ornithological Society