I examined ecological and behavioural variables influencing the susceptibility of red-winged blackbird, Agelaius phoeniceus, nests to brood parasitism by brown-headed cowbirds, Molothrus ater, in a prairie-nesting population of redwings in southern Wisconsin. I compared the following variables between parasitized and unparasitized nests: proximity of nests to trees and other prominent perches; physical characteristics of nests such as nest height and cover; and aspects of host behaviour, including vocal behaviour and aggression directed towards female cowbirds. I used these data to discriminate among four non-exclusive hypotheses for how brood parasites locate host nests: the perch-proximity, nest-exposure, nesting-cue and host-activity hypotheses. Parasitized nests were significantly closer than unparasitized nests to trees in the 2 years of the study. There was no evidence that nest height or nest cover differed significantly between parasitized and unparasitized nests, suggesting that cowbirds do not use nest exposure as a cue for locating redwing nests. The responses of both parasitized and unparasitized redwings towards a female cowbird mount decreased with increasing mount presentation distance from the nest. There were no significant differences in responses towards the mount between parasitized and unparasitized redwings, giving no support for the nesting-cue hypothesis. Female redwings whose nests were subsequently parasitized gave significantly more nest-associated (type I) vocalizations during the egg-laying period than did unparasitized females. These findings suggest that female brown-headed cowbirds in search of potential host nests use trees to search from, and that female redwing vocalizations may serve as a proximate cue for cowbirds to locate redwing nests at appropriate stages for parasitism, giving support for both the perch-proximity and host-activity hypotheses. Copyright 1998 The Association for the Study of Animal Behaviour. Copyright 1998 The Association for the Study of Animal Behaviour.