Seasonal changes in neuron numbers in the hippocampal formation of a food-hoarding bird: The black-capped chickadee

The volume of the hippocampal formation (HF) in black-capped chickadees (Po ecile atricapillus) varies across the seasons, in parallel with the seasona l cycle in food hoarding. In this study, we estimate cell density and total cell number in the HF across seasons in both juveniles and adults. We find that the seasonal variation in volume is due to an increase in the number of small and large cells (principally neurons) in the fall. Adults also hav e lower neuron densities than juveniles. Both juveniles and adults show an increase in cell density in the rostral part of the HF in August and a subs equent decrease toward October. This suggests that the net cell addition to the HF may already start in August. We discuss the implications of this ea rly start with respect to the possibility that the seasonal change in HF vo lume is driven by the experience of food hoarding. We also speculate on the functional significance of the addition of neurons to the HF in the fall. (C) 2000 John Wiley & Sons, Inc.