SEASONAL-CHANGES OF HIPPOCAMPUS VOLUME IN PARASITIC COWBIRDS

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.