A giant pile of solidified bird droppings at the bottom of a century-old chimney may hold clues behind a mysterious decline in the populations of some common Ontario bird species.

The mini-mountain of bird droppings -- or 'guano' -- was uncovered by some nifty detective work, and is located on the campus of Queen's University in Kingston.

Chris Grooms, vice-president of the Kingston Field Naturalists, read that 50 years ago thousands of Chimney Swifts would roost in the structure at Fleming Hall. That was before the university capped the chimney in the early 1990s in an attempt to drive out the pesky birds.

"I started to look a little deeper -- literally -- into the chimney," Grooms said.

Until recently there has been a dearth of data on the long-term eating habits of Chimney Swifts. The small, dark-feathered birds are found across eastern North America but their population in Ontario has been falling for several decades.

Scientists have never been sure exactly why.

After Grooms received permission to access the chimney's bottom through a trap-door built in the 1920s, he began painstakingly working through the bird-droppings.

Lying on his back, he hollowed out a four-foot by four-foot space he could stand in. He then picked through the two-meter pile of guano, centimeter by centimeter, collecting samples for lab analysis; a process he says that took about "six or seven hours over two days."

Making his task even less appealing were thousands of insect pellets excreted by the birds due to their lack of nutritional value.

Groom's work eventually resulted in a dietary track record for Chimney Swifts that dated back 50 years. Canadian scientists examining the results released a study this week theorizing that the widespread use of the insecticide DDT in the 1950s may have severely affected the diets of the birds.

The working hypothesis is as follows: the poison killed beetles that were a staple food for the Chimney Swifts, forcing the birds to find less nutritious food sources.

Grooms works in the Paleoecological Environmental Assessment and Research Laboratory at Queen's, the same one as Dr. John Smol, who specializes in the analysis of lake sediments. Smol encouraged Grooms when the latter first approached the professor about studying the massive pile of preserved bird droppings.

"It could be either two meters of poop or potentially something really important," Dr. Smol recalls telling Grooms.

The scientists analyzed the dropping samples by measuring trace amounts of radioactive material present in each one. When the Soviet Union and the U.S. were conducting nuclear tests last century, the fallout scattered across the Earth. The level of radioactivity in each layer of droppings tells scientists its age. The Canadians found that Chimney Swifts began shifting their diet away from beetles in the 1960s when DDT use was widespread.

DDT, once hailed as a miracle pesticide, has long been known to cause thinning in the eggshells of birds like Brown Pelicans and Peregrine Falcons. The insecticide has been banned in the U.S. since 1972.

Smol says the ramifications of the study go far beyond answering questions behind the population decline of chimney swifts in Ontario. Thousands of chimneys or similar structures may contain similar evidence, he said. And the dating technology could be applied to track the dietary records of bird populations everywhere.

"You can potentially use this paleo-poopology all over the world," he said.