In a study of 52 individuals belonging to 35 species or subspecies of passerine birds it was shown that the volume of the hippocampal complex relative to brain and body size is significantly larger in species that store food than in species that do not. Retrieval of stored food relies on an accurate and long-lasting spatial memory, and hippocampal damage disrupts memory for storage sites. The results suggest, therefore, that food-storing species of passerines have an enlarged hippocampal complex as a specialization associated with the use of a specialized memory capacity. Other life-history variables were examined and found not to be correlated with hippocampal volume.