BROWN-HEADED COWBIRDS SKEW HOST OFFSPRING SEX RATIOS

Abstract

Predators (or parasites) can have both direct and indirect effects on prey (or host) demography. Recent theory suggests that the impact of either effect may be greater if predators and parasites skew the sex ratio of survivors. Brown-headed Cowbirds (Molothrus ater) are obligate brood parasites. Many studies have shown that adult cowbirds directly cause host offspring mortalities. Cowbird young may indirectly cause additional mortalities by increasing competition in the nest. The sex ratio of surviving host offspring will be skewed if one sex is a poorer competitor and thus more likely to die. We studied whether and how cowbird parasitism affects host offspring sex ratios in Song Sparrow (Melospiza melodia) nests. The proportion of female sparrows in parasitized nests (0.28 ± 0.01) was half that in unparasitized nests (0.57 ± 0.05) at six days post-hatch and at fledging (0.27 ± 0.06 vs. 0.52 ± 0.11). Consistent with their being at a competitive disadvantage, female sparrows were smaller and lighter than male sparrows in parasitized mixed-sex (both sexes present) nests. This indirect effect of cowbirds on the sex ratio of surviving host offspring may dramatically affect host demography. We suggest that predator- or parasite-mediated sex ratio biases could be important to the demography of many prey or host species.