I enjoy watching birds at my feeders, and this year I have a whole family of chestnut-backed chickadees that have been here since spring. They are active morning and afternoon, but with a little “siesta time” in early afternoon, even during our short winter days. They have been busily storing sunflower seeds, zipping back and forth in all directions from the feeders. They spill enough seeds on the snowy ground that the juncos can easily nab them.
Chickadees stash their seeds in bark crevices, cracks in wood, tufts of moss and any other little cranny they can find, typically only one seed per site. My local pair of red-breasted nuthatches undoubtedly finds some of those stashes as they clamber up and down the tree trunks and branches, and they may even watch the chickadees at work.
The hoarding behavior of two of the seven species of North American chickadees has been studied in some detail (although the others also store food). One of the best-studied species is the black-capped chickadee, which ranges all across North America, chiefly in northern deciduous forests and, to some extent, in mixed deciduous-coniferous forests, where they concentrate their activities in the deciduous trees. These birds can remember their seed-storage sites for up to four weeks, and they can remember the relative quality of the stored food for about a day. They relocate stored seeds hierarchically, first by using local landmarks, such as trees, and then zeroing in on more fine-scale cues, such as twigs or conifer needles.
Researchers have found that the food storage and retrieval behavior of black-capped chickadees is closely linked to activity in a particular part of the brain, namely the hippocampus, which is involved with spatial memory. This part of the brain increases in fall, by the addition of new neurons—just when food-storage behavior really kicks in. Furthermore, there are geographic differences within this species in spatial memory performance and hippocampus size: birds in more northern populations do better in relocated stored food than birds from southerly populations, and the difference is genetic.
The spatial memory of mountain chickadees has also been the subject of intense study. This species lives in the Rocky Mountain area, at various elevations, chiefly in coniferous forests. Individuals from higher-elevation populations have better spatial memory and larger hippocampi than those from populations only 600 meters lower. Hungry mountain chickadees are better at relocating a food stash than are well-fed birds, although no difference in hippocampus size was detected; presumably high motivation stimulated greater attention. Mountain chickadees keep track of who is watching when they are hiding food: if another chickadee is watching or a potential thief of a different species, such as a red-breasted nuthatch, is observing, the bird with the food is more circumspect and tries to hide the food out of sight of the watchers. Interestingly, young mountain chickadees reared in captivity had smaller hippocampi and poorer spatial memory than same-age wild-caught birds.
I have not found any serious studies of food-caching by our chestnut-backed chickadees. But it might not be surprising to learn that northern birds of this species are better at spatial memory that more southern birds, for example from California. They might also be more circumspect about their food storage — more watchful of potential thieves.
• Mary F. Willson is a retired professor of ecology.