Parasites often have detrimental effects on their hosts, and only host individuals able to cope with parasitism are likely to display induced or genetic resistance. Hosts may respond to parasitism by differential investment in offspring depending on their ability to cope with parasitism, because offspring that perform better than their siblings are themselves likely to have superior induced or genetic resistance. We tested whether nestlings of the highly colonial sand martin Riparia riparia were affected by the haematophagous tick Ixodes lividus by experimentally manipulating parasite loads of nests [nests sprayed with pyrethrum to remove parasites (sprayed), or nests sprayed with water (control)] at three stages of the breeding season. Prevalence and intensity of ticks were significantly affected by treatments. Breeding success was not significantly affected by treatment, although post-fledging survival was twice as high among nestlings from sprayed nests than from controls. Mean phenotypic traits of nestlings generally did not differ significantly among treatments, while within-brood variance in keel length (a skeletal character) and body mass were higher in control treatment broods than sprayed ones. Sedimentation rate, which reflects blood protein and immunoglobulin content, was significantly higher and less variable in sprayed than control broods. These findings are consistent with the suggestion that parasitism effects on host reproductive success act through an increase in the variance of offspring quality.