Abstract

We studied the interaction between brood-parasitic Brown-headed Cowbirds (Molothrus ater) and Red-winged Blackbirds (Agelaius phoeniceus) in eastern Washington, and report four results. First, the overall rate of cowbird parasitism in our redwing nests was 7.7%. Second, the density of breeding redwings strongly affected parasitism rates among marshes within our study area, probably because of the importance of group defense. Third, over 20% of cowbird eggs laid in redwing nests at our study site appeared in inactive nests. We show that the frequency of this "inappropriate laying" by cowbirds is a function of the number of trees available in redwing nesting areas, possibly because perch sites allow female cowbirds to gain better information on the status of prospective host nests. Finally, the percentage of redwing nests parasitized increases dramatically as the breeding season progresses. We suggest that the late-season decline in rate of chick starvation found in our redwing population may increase cowbird reproductive success, even though late redwing nests are more likely to be lost to predation than early redwing nests.

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