A black-capped chickadee weighs about as much as a few sheets of paper. In a single autumn, it will hide tens of thousands of seeds, one at a time, in separate spots scattered across the forest. Then, all winter, it finds them again. To pull this off, the bird's hippocampus, the brain region involved in spatial memory, undergoes a surge of new neuron growth during the caching season. That still fascinates neuroscientists. For much of the 20th century, scientists believed adult brains could not make new neurons at all. Research on birds such as chickadees helped show that assumption was wrong. So the little gray bird at your feeder in January is relying on a memory system that has been actively reshaped over the previous few months. For a creature that weighs less than half an ounce, it's one of the most impressive feats of memory in the animal world.