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Memory in Food-Hoarding Birds

Sara J. Shettleworth


In England in the winter a bird feeder stocked with peanuts is all the apparatus one needs to get an animal to show behavior suggesting it has remarkable powers of memory. Indeed, its memory appears to far surpass in capacity the memory that any other animal has ever shown in the laboratory. The feeder attracts many birds. Among them are great tits, blue tits and marsh tits, which are all small, lively birds related to the North American chickadees. The great tits and the blue tits congregate at the feeder, eating as fast as they can. They interrupt their meal only to chase away their competitors. A marsh tit nonetheless darts in, grabs a peanut and flies off. It is back almost immediately to grab another. It stores the peanuts nearby, each in a different site, until the feeder is empty. Then it searches out its hidden food.

In the American Southwest another bird, Clark's nutcracker, a relative of the jays and crows, shows similar behavior. In the late summer the nutcracker harvests the seed of piņon pines. It repeatedly fills its sublingual pouch, a pocket under the tongue, and then flies several miles to bury the seeds. Often the burial sites are on bare, south-facing slopes where the snow will not be deep later on. A nutcracker may bury as many as 33,000 piņon-pine seeds in caches of four or five seeds each (see Figure 3.1). Throughout the winter it returns and digs up its thousands of caches.

How do these birds find their hoards? Does that tit or the nutcracker remember where it has stored each peanut or cache of piņon seed? Until recently observers of hoarding behavior in the wild doubted that the birds rely on memory. For one thing the bird would need a capacious memory indeed to remember the sites of hundreds or thousands of individually hoarded items. The memory would also have to be long-lasting. Even a short-term hoarder such as the mash tit does not recover its stores until hours or days later after it deposits them, and a long-term hoarder such as Clark's nutcracker does not return to a hoard for months, perhaps not until spring, when nutcrackers feed their young on piņon seeds. Moreover, in principle the hoards could be recovered without the aid of memory. A hoarder could store food only in certain kinds of sites, such as south-facing slopes or holes in bark. To recover its stores it would need only to search in sites of that kind. Then too it could employ memory only for the area in which it hoarded rather than for the individual sites. Inside that area it could search by trial and error or conceivably by cues such as odor.

Psychologists studying learning and memory in animals are becoming increasingly aware, however, that certain species have adaptive specializations that make them particularly good at learning and remembering things it is important for them to know. Among the well-known instances are the ability of many birds to learn their species' songs, the ability of rats and other animals to remember spatial locations, the ability of bees to remember the location of flowers and the ability of many animals to avoid noxious food. If food-storing birds really do remember large numbers of storage sites over long periods, their memory could be another example of an adaptive specialization, one that would enable the birds to recover their stores efficiently. A bird that could remember where it had hidden its food would make fewer errors recovering it and spend less time and energy than birds searching randomly. Recent studies indicate that at least some food-storing birds do remember the sites of their hoards quite well.