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Citation. Sorenson, M. D. 1993. Parasitic egg laying in canvasbacks: frequency, success, and individual behavior. The Auk 110(1):57-69. [1685 CC]
Abstract. Time-lapse photography and frequent nest checks conducted at Canvasback (Aythya valisineria) nests revealed a high frequency of intraspecific parasitic egg laying. At least 36% of completed Canvasback clutches were parasitized by other Canvasbacks, and at least 9.7% of all Canvasback eggs were laid parasitically during the three-year study. The maximum hatching success of nonparasitic Canvasback eggs in successful nests was 79%, while the maximum success of known parasitic eggs was only 29%. Individual patterns of parasitic and typical nesting behavior were variable. In 15 cases, however, a marked female laid one or a few parasitic eggs before initiating her own nest. These parasitic eggs and subsequent nest initiations followed the seasonal peak of Canvasback nest initiations, suggesting that these females may have abandoned or had destroyed an initial nest prior to laying parasitically. I suggest that some females lay parasitic eggs after an initial nesting attempt is terminated early in the laying stage and before a second nest is initiated because the time-consuming task of nest building prevents them from having a second nest immediately ready to receive eggs. Parasitic egg laying in Canvasbacks also may function as a low-cost alternative to typical nesting when environmental conditions are unfavorable. Several younger females were known only to lay parasitic eggs in 1988, when drought conditions reduced the probability of successful nesting. Parasitic egg laying is a regular feature of the biology of Canvasbacks, but is a relatively unsuccessful reproductive tactic employed only in "best-of-a-bad-job" situations.