<p>We used data from a long-term study of the demography and behaviour in host-brood parasite system to examine whether brood reduction among host nestlings was affected by the presence of a brood parasite nestling. Red-winged blackbirds (<i>Agelaius phoeniceus</i>) were the host species: brown-headed cowbirds (<i>Molothrus ater</i>) the brood parasite. In unparasitized broods, the incidence of brood reduction ranged from near zero in the smallest one-chick broods; to above 80% in the largest five-chick broods. Surprisingly, the presence of a cowbird nestling had almost no effect on the incidence of brood reduction among host nestlings. Previous work in this system found extensive modifications to host clutch size and brood composition occur in the prenatal environment largely due to egg removal / damage by cowbirds. Thus, parasitized host broods are smaller, on average, than unparasitized broods ameliorating some post-natal competition. Previous work in this system has shown that facultative paternal care was higher in larger broods and cowbird parasitized broods. That extra provisioning appears to have offset the cost of an additional mouth to feed, averting host brood reduction. The growth of the larger male host nestlings was significantly higher in parasitized broods; the growth of smaller female nestlings was less affected by the presence of a cowbird in the broods. As parent female blackbirds in this system do not increase their provisioning to parasitized broods, it appears that the postnatal costs of brood parasitism were borne chiefly by the male parent. Thus, after hatching, male host nestlings and parent female blackbirds appeared to benefit from brood parasitism. Our work suggests a potential post-natal sexual conflict over brood parasitism. Host mothers may favour retaining a brood parasite if it means extra help from dad.</p>