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Aspects of Cowbird Parasitism in Southern Oklahoma

John A. Wiens
The Wilson Bulletin
Vol. 75, No. 2 (Jun., 1963), pp. 130-139
Stable URL: http://www.jstor.org/stable/4159150
Page Count: 10
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Aspects of Cowbird Parasitism in Southern Oklahoma
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Abstract

From 15 June to 6 August 1960, and from 8 June to 4 August 1961, Brown-headed Cowbird parasitism in the breeding bird population about the University of Oklahoma Biological Station, Marshall County, Oklahoma, was studied with respect to incidence, success of cowbirds and hosts, and the laying period of the cowbird. Parasitism was noted at 31 of 84 nests (including 33 Red-winged Blackbird nests which were not parasitized). Six of these 31 nests contained two cowbird eggs: in each of the remaining 25 a single cowbird egg was laid. Nests of the Bell's Vireo, Dickcissel, Cardinal, Lark Sparrow, Orchard Oriole, Blue Grosbeak, Painted Bunting, and Field Sparrow were parasitized. Parasitized nests of these species were usually in somewhat exposed situations on the edges of bottomland thickets, bordering open mixed grasslands. Nests found in more open areas without edge growth were generally not parasitized. Parasitized nests did not differ appreciably from nonparasitized nests with respect to height from ground or substrate. Seven of the 31 parasitized nests (22.6%) and six of the 20 nonparasitized nests of the same species (30.0%) were successful. Thirty-seven cowbird eggs and 73 host eggs laid in parasitized nests produced nine (24.3%) and 14 (19.2%) young, respectively. Twenty-five young (36.2%) fledged from 69 eggs laid in nonparasitized nests of the same species. The laying dates of 28 cowbird eggs and 97 host eggs were determined. The distribution of these dates indicates that two peaks existed in egg production in the cowbird, one in mid-June, the other in early July, while host egg production gradually decreased from early June on. The pattern of cowbird egg production was thus independent of host availability. Cowbird egg-laying declined after mid-July, the last egg being laid on 29 or 30 July. Cowbird-host laying periods were generally coincident, but one cowbird egg was apparently laid in a partially destroyed nest, and another was laid in a nest containing a well-developed cowbird chick.

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