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 behaviour. We report a new instance of plasticity. Brood parasitism in South American 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 season. We report that for two parasitic species, relative hippocampal volume is smaller during the non-breeding than the breeding season, and that sexual dimorphism present in summer in one of the species is not found in winter. These results support the hypothesis that the avian hippocampal formation shows neuroanatomical plasticity associated with seasonal changes in spatial memory demands.