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Fluorescent Bird Droppings Reveal Bluebird Flight Paths

Brian Handwerk
for National Geographic News
June 30, 2005
 
Tracking eastern bluebirds is easy when they've dined on artificially
fluorescent fruit.

Researchers in South Carolina discovered this recently, when they tried a new method for tracking birds on the wing. All the scientists had to do, they say, was to follow the glowing droppings to discover how the birds use "corridors" to travel between habitats.

"By carefully measuring each flight they took—typically less than 65 feet (20 meters)—and putting those separate flights together in a computer simulation, we could project where a bird who ate a certain fruit would end up in 45 minutes when [the fruit] passed through its digestive system," said Douglas Levey, a zoologist at the University of Florida in Gainesville.

Biologists, of course, are not shy about getting their hands dirty. Animal droppings yield a wealth of natural information. And scientists have artificially enhanced excrement in a vareity of ways to make it easier to trace.

Scientists sometimes track larger animals, for example, by putting magnets in the animals' food and using magnetic detectors to find their droppings.

Researchers have also used fluorescent powders to track animal prints and even to make the animals themselves more visible by coating them with the glowing dust.

But at the Savannah River Site National Environmental Research Park, a U.S. Department of Energy facility near Aiken, South Carolina, Levey's team combined these techniques.

They coated the fruits of the wax myrtle (Myrica cerifera) bush with a diluted form of sticky Wiltproof—an agricultural adhesive to which fluorescent powder binds.

"The birds can't see it," Levey explained. "We can't see it—but under ultraviolet light it will glow."

The team sprayed tens to hundreds of thousands of fruits. Eastern bluebirds (Sialia sialis) in the area soon ate most of them.

The birds then obligingly dropped their feces into well-placed plastic flowerpots near popular perches throughout a large, wooded test area. In the pots, fluorescent undigested pulp and seeds were visible under a flourescent scope.

The team describes its study in the current issue of the journal Science.

Habitat Corridors

The study was designed to track the use of habitat corridors—an increasingly popular concept in wildlife management.

Many conservationists believe that habitat areas, and thus animal populations, must be linked by travel corridors that allow easy movement.

"There's a lot of controversy over whether corridors work," Levey said. "It seems intuitive that they should work. But people see all the time that animals move though places that they shouldn't be. So it's unclear how much [animals] truly depend on corridors."

John Blake, a forester and ecologist with the U.S. Forest Service at Savannah River, South Carolina, collaborated on the bluebird project, viewing it as a valuable tool to study corridor use.

"We felt that there was a lack of scientific data to support the implementation of [habitat corridors] except in a few obvious cases," he said. "From our standpoint, it provides a stronger scientific basis."

The project area covered hundreds of acres. Fruit-rich wax myrtle bushes were situated in a central patch, while several areas of mature pine forest were cleared to create shrubby meadows, each about the size of a football field.

The fields were separated by hundreds of meters of pine forest. Some were connected with corridors, while others were not.

The birds did indeed use the corridors. The proof: Researchers found much more fluorescent droppings in the connected habitat areas than in those that were isolated.

But when and how the birds used corridors was unclear at first.

"We knew corridors were working, but we never saw a bluebird fly down the center of a corridor," Levey said. "We never saw the birds use them."

In fact, the birds benefited from the corridors by using their edges, traveling along the forest side of each clearing.

"You know [a model] is useful when it reveals something that is obvious only in hindsight," Levey noted. "These results agree with intuition, and we suspect that other animals make use of corridor edges as well."

The results may ultimately benefit many species beyond bluebirds.

"We're looking to establish some fundamentals that can be applied to other sensitive plants or animals to help make better decisions about land management," Blake, the Foreset Service ecologist, said.

Levey hopes next to explore an even more subtle migration pattern—the effect of corridors on plants.

"People tend to accept pretty quickly that animals use corridors, but they don't think of plants moving," he said.

Corridors may benefit plants by aiding the animals that move their seeds and pollen.

"But they also benefit plant enemies like herbivores and seed predators," Levey noted. "So what is the net impact of corridors on plants? What is the balance?"

The answer remains unknown. But for now, some valuable bird droppings have provided a unique picture of wildlife movement.

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