SPECIES AND SEX-DIFFERENCES IN HIPPOCAMPUS SIZE IN PARASITIC AND NONPARASITIC COWBIRDS

To test the hypothesis that selection for spatial abilities which requ ire birds to locate and to return accurately to host nests has produce d an enlarged hippocampus in brood parasites, three species of cowbird were compared. In shiny cowbirds, females search for host nests witho ut the assistance of the male; in screaming cowbirds, males and female s inspect hosts' nests together; in bay-winged cowbirds, neither sex s earches because this species is not a brood parasite. As predicted, th e two parasitic species had a relatively larger hippocampus than the n on-parasitic species. There were no sex differences in relative hippoc ampus size in screaming or bay-winged cowbirds, but female shiny cowbi rds had a larger hippocampus than the male.