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.