Exposure to ectoparasites increases within-brood variability in size and body mass in the sand martin

Parasites often have detrimental effects on their hosts, and only host indi viduals able to cope with parasitism are likely to display induced or genet ic resistance. Hosts may respond to parasitism by differential investment i n offspring depending on their ability to cope with parasitism, because off spring that perform better than their siblings are themselves likely to hav e superior induced or genetic resistance. We tested whether nestlings of th e highly colonial sand martin Riparia riparia were affected by the haematop hagous tick Ixodes lividus by experimentally manipulating parasite loads of nests [nests sprayed with pyrethrum to remove parasites (sprayed), or nest s sprayed with water (control)] at three stages of the breeding season. Pre valence and intensity of ticks were significantly affected by treatments, B reeding 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 diffe r significantly among treatments, while within-brood variance in keel lengt h (a skeletal character) and body mass were higher in control treatment bro ods than sprayed ones. Sedimentation rate, which reflects blood protein and immunoglobulin content, was significantly higher and less variable in spra yed than control broods. These findings are consistent with the suggestion that parasitism effects on host reproductive success act through an increas e in the variance of offspring quality.