Food caching by black-capped chickadees
A major challenge in studying the hippocampal circuit is its sheer complexity. The hippocampal formation consists of at least 9 distinct regions. Subsets of these are connected to more than 20 cortical areas and an equally large number of subcortical structures. The avian brain offers a solution to this challenge. Like all vertebrates, birds have a hippocampal formation derived from the same embryological precursor as its mammalian counterpart (the medial pallium of the telencephalon). Like in mammals, this structure connects via its inputs and outputs to the cerebral cortex. However, the avian cortex (pallium) is nucleated, and only about four of these cortical nuclei are connected to the hippocampal formation. A small number of well-defined, well-separated and thus easily targetable inputs/outputs makes the bird hippocampal formation a dream circuit for neural recordings and manipulations.
Birds have a fascinating innate behavior to match. Food caching is a strategy used by many animals to cope with instabilities in their food supply. A small number of bird families, including corvidae (crows and jays) and paridae (chickadees and tits) employ a unique strategy of scattering hidden caches throughout an environment and using episodic-like memory to find these caches later in time. The capacity of cache memory is bewildering: in extreme Arctic conditions, chickadees have been observed making more than 7,000 distinct caches per day. They precisely remember these locations more than a month later. Importantly, the behavior is hippocampus-dependent: birds with bilateral hippocampal lesions cache and attempt to retrieve food items, but fail to search at correct locations. The hippocampus changes in size seasonally and is about 3 times larger in food-caching birds than in non-caching species of a similar size.
We study a local species that adapts particularly well to laboratory conditions -- the black-capped chickadee (Poecile atricapillus). Chickadees readily cache and retrieve food items in the lab. Combined with the unique neuroanatomical advantages of birds, this behavior is an exciting new target for the application of 21st-century circuit dissection tools. The hope is that a comparative approach of studying both avian and mammalian hippocampal systems will produce deeper insights into the ancient, fundamental principles by which this circuit operates.