In many naturalistic studies of the hippocampus wild animals are held in captivity. To see if captivity itself affects hippocampal structure, adult black-capped chickadees (Poecile atricapilla) were caught in the fall, injected with bromodeoxyuridine to mark neurogenesis and alternately released back to the wild or held in captivity for 4–6 weeks. Wild birds were recaptured and perfused simultaneously with their captive counterparts. The hippocampus of the captive birds was 23% smaller than the wild birds, with no hemispheric differences in volume within groups. There was no statistically significant difference in the size of the telencephalon between groups, or in the number and density of surviving new cells. Proximate causes of the hippocampal volume change could include stress, lack of exercise, diminished social interaction or limited caching opportunity; a hippocampal-dependent activity. The results suggest the avian hippocampus - a structure essential for rapid, complex relational and spatial learning - is both plastic and sensitive, much as is the case in mammals, including humans.

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