PARASITIC EGG-LAYING IN CANVASBACKS - FREQUENCY, SUCCESS, AND INDIVIDUAL BEHAVIOR

Time-lapse photography and frequent nest checks conducted at Canvasbac k (Aythya valisineria) nests revealed a high frequency of intraspecifi c parasitic egg laying. At least 36% of completed Canvasback clutches were parasitized by other Canvasbacks, and at least 9.7% of all Canvas back eggs were laid parasitically during the three-year study. The max imum hatching success of nonparasitic Canvasback eggs in successful ne sts was 79%, while the maximum success of known parasitic eggs was onl y 29%. Individual patterns of parasitic and typical nesting behavior w ere variable. In 15 cases, however, a marked female laid one or a few parasitic eggs before initiating her own nest. These parasitic eggs an d 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 sugges t that some females lay parasitic eggs after an initial nesting attemp t is terminated early in the laying stage and before a second nest is initiated because the time-consuming task of nest building prevents th em from having a second nest immediately ready to receive eggs. Parasi tic egg laying in Canvasbacks also may function as a low-cost alternat ive 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 Canvasba cks, but is a relatively unsuccessful reproductive tactic employed onl y in ''best-of-a-bad-job'' situations.