A two-metre tower of bird excrement at an Ontario university has become an unlikely archive that may reveal the reasons for the declining population of the North American chimney swift, according to new research by Canadian scientists.

The study published in the journal Proceedings of the Royal Society B document the discovery and cataloguing of the droppings in an abandoned chimney on the campus of Queen's University in Kingston, Ont.

"What we have is a history book that we didn't think we had before," said Dr. John Smol, one of the study's authors. Smol went on to explain that, to his knowledge, this is the first time anyone has ever measured stratified towers of bird droppings.

With this data, the researchers hope to show a link between the use of the insecticide DDT and the lower numbers of insect-eating birds, in particular, chimney swifts.

The chimney at Queen's University's Fleming Hall was home to a population of 4,000 chimney swifts up until it was covered over with mesh in 1993. (Chris Grooms/Queen's University)

Chimney swifts roost in smoke stacks of all kinds. The Queen's University chimney was home to a population of 4,000 up until it was covered over with mesh in 1993. The two-metre stack of guano represents 50 years of bird droppings.

"It occurred to me that this is just like a sediment core in a lake," explained Chris Grooms, the technician who discovered the deposit and reported it to Smol. Eventually, he burrowed into the base of the chimney and through the layers of excrement.

Smol runs a lab at Queen's that specializes in analyzing sediment cores. That is the process of analyzing years of deposits on the bottoms of lakes to see how the environment has changed over time.

Smol was intrigued enough by the discovery to apply his lake-analyzing methods to the chimney sample.

"My first reaction was, we may have two metres of bird droppings," laughed Smol. "But another possibility was that it actually was a time capsule. That we could actually use that information to trace what the birds were eating going back several decades."

His initial concerns were shared by Joe Nocera, another of the study's authors and a scientist with Ontario's Ministry of Natural Resources.

"It sounds ridiculous, and maybe to me at first it sounded ridiculous," Nocera said. "But then once you get over the initial effect of what it is and you think about what it can possibly tell you, and then you realize that this is actually very scientifically revealing."

The research shows that DDT did the job it was supposed to do. It killed bugs.

That was a problem for chimney swifts, though, because the beetles that DDT killed were the most nutritious part of the birds' diet. All that remained were lower-calorie insects.

Nocera speculates that could be the reason the chimney swift population is in decline, even though DDT is no longer in use. The birds spend more energy getting lower quality food, so they have less energy for reproduction. But that hypothesis will require more research.

For now, though, says Smol, the tower of bird droppings represents a way to make up for missing data sets.

"We found a way that we can reconstruct data sets that no one gathered," he said.