The screaming cowbird Molothrus rufoaxillaris is described as being sexually monochromatic, although it has a rufous axillary patch that shows inter-individual size variation. We investigated potential sex differences in patch size in 82 molecularly-sexed adults. All males exhibited large patches and 23 females had small patches; however, seven females lacked a patch and nine had large patches. Patch size was associated with sex and independent of body condition, location or date of sampling. We discuss several hypotheses that may explain the incidence of this male character in some females.