sealPurdue News
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October 1996

Study: Chickadees are scrupulous savers, watchful wardens

WEST LAFAYETTE, Ind. -- Like the most diligent accountant, the Carolina chickadee appears to be a stickler for keeping track of its assets and will go to great lengths to ensure that the books are balanced in taxing times.

A recent study by Purdue University biologist Jeffrey Lucas showed that the Carolina chickadee is capable of sizing up its environment, keeping a watchful eye on factors such as availability of food or theft of stored goods, and carefully replacing any seeds that are lost or stolen.

"Our data suggest that, in addition to internal cues that prompt birds to prepare for seasonal change, the chickadee makes an independent measurement of the environment and incorporates that into its behavior," he says.

Lucas says birds seem to make conscious decisions about food storage. He is interested in how birds know when to store food vs. when to eat it, or when to "cash in" on stored food.

He predicted that if seeds were stolen from the birds' storage sites, they would reduce the amount they stored and would compensate by eating more to keep their fat reserves high.

He found, instead, that the chickadees were more concerned about counting savings than calories. Not only did the birds store additional seeds to make up for stolen goods, but they replaced the stolen seeds in a ratio of almost 1-to-1.

He presented his findings in July at the annual conference of the Animal Behavior Society.

The study could help scientists understand how animals adapt to their environment and could help researchers focus conservation efforts for animals that live in resource-poor environments.

Lucas' studies focus on the Carolina chickadee, one of several types of chickadees found in North America. The Carolina chickadee is a five-inch long bird with a gray body, a white face and a black cap. It is found primarily in the southeastern United States, with a range from Texas to northern Indiana.

He says chickadees are a key species for understanding how animals regulate energy stores, because they have enormously high metabolic rates, which can put their lives at risk when conditions are harsh.

"For all animals, and particularly for small warm-blooded animals, energy regulation is one of the most critical aspects of their life," he says. "One of the major drawbacks of being small is that storing energy through the accumulation of fat doesn't work as well as it does for a large animal."

For example, a bear can store enough fat to survive all winter, but a chickadee, which weighs about as much as two nickels, can store enough fat to survive only a single day, he says. The reason is a combination of its high metabolic rate and very little room to store extra energy on its body.

"So small animals that live in harsh winter environments have evolved alternative mechanisms of energy storage," Lucas says.

For chickadees, that alternative is to cache, or store, food in a fashion called scatter-hoarding, storing a single seed or a couple of seeds in many locations over a large area.

"Chickadees are exquisite storekeepers," Lucas says. "Chickadees that live in northern Canada, for example, will store tens of thousands of seeds over a number of acres, and then remember where those seeds are. The Carolina chickadee, which lives in a much milder environment, may store hundreds of seeds over several acres."

Two years ago, Lucas was the first to show that circannual rhythms, or internal cues, prompted chickadees to store at higher rates during the fall and winter seasons.

"We knew there were seasonal cycles tied to how much food the chickadee stored, but we didn't know if there was a tie between stored food and the amount of fat the bird had," he says.

To find the answer, Lucas developed an experiment to manipulate the value of one of those energy stores, in this case the cache, by stealing from it. His prediction was that if the value of that cache was reduced by stealing, the chickadees would eat more and store fewer seeds.

"It's like having a roommate who goes to the refrigerator and takes your food. Under such circumstances, most of us would start storing stuff in other places, or eating larger meals to prevent us from constantly feeding the roommate," he says.

In a specially designed laboratory at Purdue, Lucas studied six groups of Carolina chickadees for two months each under a constant environment. To account for seasonal changes, the experiments spanned a period of 12 months. He wanted to determine what influences how the birds utilize food they have found.

The aviary included electronic "trees" that could be set to "steal" a percentage of seeds stored by the birds by opening trap doors in the storage areas. Electronic scales on the tree weighed birds to record changes in weight.

For half the birds in each group, the stealing began on the birds' first day in the aviary and lasted for 12 days. For the other half, seeds were stolen from them after they had settled into the new environment for 12 days. Throughout the experiments, the 12 days with stealing were followed by 12 with no stealing to see how they would respond to a "safe" environment.

As seeds began to disappear, the birds spent up to 55 percent of their time finding, storing and checking on food -- about twice the amount of time normally devoted to such activities.

"If we steal from them first, the birds become wary of the environment and cache like crazy though the fall and winter and into late spring. If we steal from them later, the birds are more trustful, compensating during the winter and fall, but not quite as much as the first group, and not into the spring," Lucas says.

Seasonal changes also played a role in the birds' behavior, with the birds compensating more in the winter than in the summer, even though the laboratory environment remained constant throughout the year.

"Generally, we wouldn't expect them to store as much food in the summer when food is plentiful, but what they experience in the laboratory in July is exactly the same as what they experience in the December in the lab," Lucas says. "This indicates that the circannual rhythm acts independently of this environmental measurement that the bird is taking."

The data also suggest that the cache is a distinct source of energy for the bird and it is regulated independently of the level of fat reserves, Lucas says.

The study builds upon Lucas' previous findings with chickadees and titmice, which show there is a hierarchy of mechanisms that affect birds' behavior.

"If they're hungry, for example, they never sing or bathe," he says. "Instead, they spend all their time looking for food."

Lucas' study was funded by the National Science Foundation.

Source: Jeffrey R. Lucas, (765) 494-8112; e-mail, jlucas@bilbo.bio.purdue.edu; Web, http://www.bio.purdue.edu/Bioweb/People/Faculty/Research/Research.Faculty/lucas.NCLK
Writer: Susan Gaidos, (765) 494-2081; e-mail, susan_gaidos@purdue.edu
Purdue News Service: (765) 494-2096; e-mail, purduenews@purdue.edu

NOTE TO JOURNALISTS: A color photograph of Jeffrey Lucas banding a bird is available. Ask for the photo called Lucas/Chickadees or download here.

Photo Caption:
Purdue University researcher Jeff Lucas bands a tufted titmouse before releasing it. Lucas is currently trapping chickadees for his studies of how animals manage their energy. (Purdue News Service Photo by Dave Umberger)
Color photo, electronic transmission, and Web and ftp download available. Photo ID: Lucas/Chickadees


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