Sexual dimorphism and species differences in HVC volumes of cowbirds

Cowbirds exhibit extensive Variation in their social, territorial, and repr oductive behaviors. Nissl-stained brain sections of specimens from a previo us study (J. C. Reboreda, N. S. Clayton, & A. Kacelnik, 1996) were used to study the gross anatomy of a song control nucleus in 3 South American cowbi rds (bay-winged, Molothrus badius; shiny, M. bonariensis; and screaming, M. rufoaxillaris). Cowbird high vocal center (HVC) volumes were consistently higher in males than in females in all 3 species. The largest HVC size of f emales found in bay-winged cowbirds is consistent with observations that fe males of this species, but not of the other 2 species, occasionally sing. T he extent of the sexual dimorphism of relative HVC size was highest for the sexually dichromatic and promiscuous shiny cowbirds and smaller for the mo nochromatic and monogamous bay-winged and screaming cowbirds, suggesting th at selection pressures associated with morphological traits and social syst ems are reflected in brain architecture.