Avian brain plasticity has been demonstrated by seasonal variations in
neuroanatomy correlated with changes in singing and hoarding behaviou
r. We report a new instance of plasticity. Brood parasitism in South A
merican cowbirds involves memory for location of hosts' nests, and is
associated with an enlarged hippocampus relative to telencephalon size
. This effect holds between sexes and species during the breeding seas
on. We report that for two parasitic species, relative hippocampal vol
ume is smaller during the non-breeding than the breeding season, and t
hat sexual dimorphism present in summer in one of the species is not f
ound in winter. These results support the hypothesis that the avian hi
ppocampal formation shows neuroanatomical plasticity associated with s
easonal changes in spatial memory demands. (C) 1997 Elsevier Science B
.V.