Comparative studies of food-storing, memory, and the hippocampal formation in parids

Birds which scatter-hoard large numbers of food items such as marsh tits, Parus palustris, use memory to retrieve their caches and have an enlarged hippocampal formation relative to the rest of the telencephalon compared with species that store little or no food. Preliminary observations suggested that captive blue tits, P. caeruleus, may store small quantities of food albeit in limited amounts. This experiment compared food-storing intensity, memory for cache sites, and relative hippocampal formation in marsh tits and blue tits. Comparisons were made both within species, by comparing wild-caught adults and hand-raised juvenile blue tits that store and those that do not, and between closely related species, by comparing food-storing adult wild-caught blue tits and juvenile hand-raised blue tits with adult wild-caught marsh tits. Food-storing blue tits stored fewer seeds than did marsh tits, and they had a less accurate memory for cache sites and a smaller absolute and relative hippocampal formation than did marsh tits. For further analysis, the hippocampal volume was divided into a rostral (front) portion and a caudal (rear) portion, separated by the first appearance of the anterior commissure. Marsh tits had both larger rostral and caudal portions than did blue tits, but the species difference in hippocampal volume was greater for the rostral than for the caudal portion. In blue tits, wild-caught adults had significantly larger absolute and relative hippocampal volumes than did hand-raised juveniles, but there was no difference in the proportion of rostral to caudal portions, irrespective of whether they had stored and retrieved food. Although food-storing blue tits did not differ from non-storing blue tits in total absolute or relative hippocampal volume, they had larger rostral portions of the hippocampal formation and small caudal portions. Possible reasons for this are discussed.