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Brood Parasite and Host Coevolution: Interactions between Shiny Cowbirds and Two Species of Meadowlarks

Michael Gochfeld
The American Naturalist
Vol. 113, No. 6 (Jun., 1979), pp. 855-870
Stable URL: http://www.jstor.org/stable/2460308
Page Count: 16
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Brood Parasite and Host Coevolution: Interactions between Shiny Cowbirds and Two Species of Meadowlarks
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Abstract

I studied host-parasite interactions of the shiny cowbird and two species of meadowlarks on the Argentine pampas. The greater red-breasted meadowlark (Sturnella loyca) suffered a high rate of parasitism (23 of 24 nests) while S. defilippii appeared to escape parasitism (0 of 11 nests). Cowbirds visiting the study area associated preferentially with S. loyca, and approached decoys more often when S. loyca song was broadcast. The S. loyca is dependent on elevated perches and usually nests near them, while S. defilippii does not require such perches and does not nest near them. The perches near S. loyca nests apparently facilitated nest-finding by cowbirds. Female S. defilippii were more surreptitious than S. loyca females during nest-building, and although S. loyca females interrupted nest building when cowbirds arrived, the cowbirds were usually more patient and succeeded in finding nearly all of the nests that I found. The shiny cowbird can benefit by focusing attention on S. loyca since that species is numerous, conspicuous, nests near elevated perches, and is less careful in nest building (compared with its more numerous and equally conspicuous congener). The close association of S. loyca male and female assure that a cowbird approaching a singing male S. loyca will be near the female. These factors render S. loyca a host with high primary suitability. Moreover, it is clearly not a rejector nor deserter species; hence it has good secondary suitability. It is not certain if it ever raises cowbirds to independence, but in this study I found no evidence that any cowbirds fledged, and I found many fledged meadowlark broods without cowbirds. I conclude that it has low ultimate suitability for the shiny cowbird. Although S. defilippii is numerous, its nests are not near perches and the male and female are not found close together; hence it has low primary suitability. I have no evidence that it is either a rejector or deserter, but experiments should be done to clarify these points. There is no evidence on the ultimate suitability of S. defilippii as a host, except the lack of reports of S. defilippii feeding fledgling cowbirds. Since there are few S. loyca-reared cowbirds (if any) a system of foster specificity does not account for the responses of cowbirds to S. loyca. Rather it seems likely that individual cowbirds learn of S. loyca's primary suitability and focus attention on it for that reason. Although ultimately selection should act against birds that lay in S. loyca nests, the high primary suitability actually allows S. loyca to "compete" for cowbird eggs against other hosts. A system based on host-recognition through imprinting is perhaps ideal since the fact that one has matured is prima facie evidence that one has been raised by an ultimately suitable host, and such a system offers protection against the kind of errors involved in choosing a host with high primary but low ultimate suitability. The use of song as a cue in distinguishing hosts allows cowbirds to benefit from the fact that hosts sing more just prior to the onset of incubation, the most propitious time for nest discovery. The parasite's time investment in nest-finding must be adequately repayed, and focusing attention on a host with low primary suitability means that cowbirds will not find enough nests in one season. Only if one could assure gaining a host with high secondary and/or ultimate suitability would excess nest-finding time be adequately repayed.

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