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I studied brood parasitism among 110 Aythya nests and filmed 125 interactions at 14 Canvasback (A, valisineria) and 5 Redhead (A. americana) nests to describe host-parasite behavior. Redheads outnumbered breeding Canvasbacks by about 5.4 to 1 on the Delta Marsh, Manitoba, and heavily parasitized Canvasback nests ((x) over bar +/- SE = 7.6 +/- 2.8 eggs/nest). High rates of parasitism resulted in delayed host embryo development, up to 23% longer incubation periods, and extended brooding behavior at nests containing unhatched or late-hatching eggs. Females followed potential hosts to locate nests, made prelaying nest visits, and selected active rather than deserted nests to parasitize. Parasitic laying was not markedly deterred by host resistance and occurred at all stages of host laying and incubation, resulting in only 34% of parasitic Redhead eggs hatching in successful Canvasback nests. Laying females aggressively displaced incubating hosts, causing eggs to be accidentally displaced from nests. Host Canvasback eggs were five times as Likely to be cracked and twice as likely to be dislodged from nests than were parasitic Redhead eggs. Hosts limited defensive actions to pushing against intruders or pecking at the heads of parasitic females, resulting in some parasitic eggs being deposited outside nest bowls. Incubating females may be constrained from using highly aggressive behavior to deter brood parasites partly because of fitness costs of fighting and breaking or displacing their own eggs. The existence of host defensive behavior suggests that brood parasitism may present significant fitness costs, even among species with nidifugous precocial young, by affecting the critical processes of synchronous hatching and nest exodus.