Bird Brains

Set out a few seeds and watch the neighborhood scrub jay pick them up hurriedly in his beak and fly off a short distance to hide them in various holes he has carefully dug in your lawn. If you watch long enough you may spot him sneaking back to his seeds, retrieving them from their hiding places, and nibbling away. What many people do not realize is that this friendly little jay has probably hidden hundreds to thousands of seeds in hundreds to thousands of locations all over yours' and your neighbors' yards, only to remember where each seed was hidden and to retrieve them days, weeks, or even months later.

Right: Perched at a feeding station, a white-crowned sparrow prepares to pull the nearby string in search of a peanut. The peanut is hidden in a small hole that is plugged by a knot in the string.

Studies have shown that food-storing birds, such as jays, crows, magpies, nutcrackers, chickadees, and nuthatches, perform this incredible feat using a particular form of memory--memory for spatial locations. They have also revealed that spatial memory relies on the hippocampus, a structure in the brain of many animals, including birds, mice, rats, monkeys, and humans. This structure is also involved when memories must last a long time.

The hippocampus is larger in many birds and mammals that store or "cache" food than in similar species that are non-storers. Furthermore, the size of the hippocampus corresponds to seasonal differences in caching behavior. In the fall, some food-storing birds spend more time caching, cache more items, and leave their caches for longer periods than they do in the spring. Correspondingly, these birds have a larger hippocampus in the fall than in the spring.

Perhaps most important, the hippocampus of food-storing birds shows considerable neural plasticity. That is, the size and number of cells in the hippocampus can change. For example, a higher rate of cell birth, or neurogenesis, occurs during the fall when caching activitiy is at its peak. It was believed for some time that all animals are born with a fixed number of brain cells that gradually decreases with age--that the brain is incapable of forming new cells. Quite to the contrary, the Clayton laboratory has shown that food-storing birds are born with too few cells to accurately remember where their caches are hidden. Studies have shown that young mountain chickadees must be allowed to cache before their hippocampi grow. In fact, as few as three episodes are necessary to trigger this brain growth.

These remarkable brain and behavior changes in the wild make for interesting and challenging study in the laboratory. Recently, we have used a peanut-shopping exercise to investigate the role of the hippocampus in food retrieval in birds. In this exercise we place a number of feeding stations in various locations around a room and hide a piece of peanut in one feeder. During the training phase, we ask a bird to find the hidden peanut. Later, we test the bird's ability to remember where the peanut is hidden by measuring how long it takes for the bird to return to the peanut-containing feeder.

When all of the feeders look the same, the birds are forced to rely on memories formed about the spatial location of the feeder with the peanut to find the food. When the feeders are decorated with colored tape, they also form memories about the visual aspects of the feeder.

A bird with a damaged hippocampus can easily find the hidden peanut if the feeders are made to look different from one another. However, if the feeders all look the same, the bird is unable to locate the hidden food right away and searches about the room randomly. This finding confirms our understanding that hippocampal damage leads to impaired memory formation for tasks that rely on spatial cues but not for those that rely on color or visual cues.

Peanut shopping is not quite the same as caching and retrieving food in the wild; however, it is a simple task that taps into the same type of memory and can be used with many birds, including those that do not ordinarily store food. Instead of the birds caching and retrieving food, we cache the food for them and watch them as they search, find, and remember where the food was hidden.

Our latest study sheds more light on this interesting area of research. We chose 17 wild-caught Alaskan black-capped chickadees to represent our food-storers and 16 wild-caught California white-crowned sparrows as our non-storers. These birds were kept on a relatively short-light schedule of 8 hours light and 16 hours dark, similar to that experienced in the wild during the fall and winter, when food-storing birds usually cache their food. We then trained them on the peanut-shopping exercise described earlier.

As we predicted, chickadees were better at remembering the location of the hidden peanut than sparrows when asked to remember the location for a short time (5 minutes), and especially better when asked to remember the location over a long period of time (90 minutes).

We then changed the light schedule. Half of the birds were kept on an even shorter light schedule, and the other half were changed to a relatively long light schedule, similar to that experience in the wild in summer. For both species, the summer birds were better peanut shoppers than the winter birds. However, as we predicted, the winter chickadees were far better shoppers than the winter sparrows.

The truly unique results occurred in studies involving male and female Alaskan chickadees. The females were better peanut shoppers than the males by virtually every measure. This observation was surprising because in studies involving a variety of species, including humans, males usually perform better than females on tasks that call for spatial memory. Only one other study in birds has shown that females are better than males at spatial-memory tasks. (See "Caching Baby Birds.")

The next phase of this research will address a variety of important questions.

• Did our laboratory-induced change of seasons stimulate the hippocampus of the captive birds to grow?
• Is the hippocampus larger in female Alaskan black-capped chickadees than in males, or do the females use their hippocampus in different ways than the males?
• Do female Alaskan black-capped chickadees show more neurogenesis in their hippocampus? If not the hippocampus, then where else might we look to solve the puzzle?

So, as you can see, bird brains hold great secrets of how the brain functions when confronted with learning new things. They also hold great promise. We now know that bird brains are exceptionally plastic (capable of change) and may hold the key to unraveling one of the many ways that the brain forms memories. They may also be a vital link in understanding how the brain repairs and/or replenishes itself and aid in our attempts to prevent deterioration associated with age, disease, or physical injury.

Bird brain? Why, thank you very much!

Diane Lee (left) has been a postdoctoral fellow in Nicola Clayton's laboratory since 1995. She completed her Ph.D. in biological psychology at UC Berkeley in 1993. Clayton (right) emigrated to the United States in 1995 from Oxford, England, where she was a university research fellow at Oxford University. That same year she joined the division's Section of Neurobiology, Physiology, and Behavior as an assistant professor. She received her Ph.D. in animal behavior in 1987 from the University of St. Andrews in Scotland.

Caching Baby Birds

Research by Nicola Clayton, Juan Reboreda, and Alex Kacelnik has shown that female cowbirds are better than male cowbirds when required to remember spatial locations. The female cowbird typically hides her eggs in the nests of other, unsuspecting birds to get the "host" birds to raise her offspring. Quite a nasty, but impressive, trick! Known as brood parasitism, the females that engage in this activity must remember not only where the potential host nests are located, but when the hosts' eggs are about to hatch so that she can time laying her eggs accordingly. Then she must remember where and when to go to retrieve her babies. You can think of it as caching baby birds. Similar to caching food, brood parasitism occurs seasonally and is accompanied by an enlarged hippocampus.