Since 1928, thousands of chimney swifts have roosted at Fleming Hall, a university building in Kingston, Ontario. For decades, they fed on local insects, and excreted the remains down one of the building’s chimneys. Around 2 centimetres of droppings, or ‘guano’, built up every year until the chimney was finally capped in 1992. To this date, Fleming Hall contains a hardened guano tower, two metres tall and 64 years in the making, which preserves a layered record of the swifts’ meals.
Now, a team of scientists, led by Joseph Nocera, have used this archive of historical poo to explain why the swift populations have fallen by 90 per cent since their heyday.
The guano tower was discovered by Chris Grooms from the Kingston Field Naturalists, who brought it to the team’s attention. They reached it via a 2-foot-wide square door at the bottom of the chimney, and found a two-metre-tall column. “One has to be somewhat of a contortionist to get in,” says Nocera. “The guano is compacted and very dry, like a popcorn-cake. It has a slightly musty smell, and the area is very dusty. Overall, it’s not a terribly comfortable place to work!”
The team cut slices down the entire length of the column. They studied the insect remains within it, the levels of different chemical elements, and the amounts of pesticides such as DDT.
The insect shells revealed that, during the 1940s, the swifts were mostly eating beetles. As the 50s came around, they shifted towards ‘true bugs’ (a term referring to a specific group of insects, rather than ‘bugs’ in general). This coincides with the introduction of DDT, which hit beetles more than many other insect groups. Starved of their main prey, the swifts turned to bugs, which are more resilient to DDT sprays and quicker to evolve resistance.
Once the use of DDT started to decline, the beetles rebounded and regained their prime position on the swifts’ menu. The use of the pesticide reached its nadir in the 1970s, when it was banned from agricultural use under the Stockholm Convention. However, DDT is still used in countries that didn’t sign up to the convention, and more broadly to control malarial insects.
In fact, Nocera’s study shows that DDT levels have risen slightly since the 1970s, possibly because of this background use and the pesticide’s infamous ability to persist in the environment. And, at the same time, the swifts made yet dietary shift from beetles to bugs.
Nocera thinks that these changing diets were important for the swifts. Bugs make for harder meals because they have a greater range of chemical defences, and they provide fewer calories than beetles. “It could take a lot of small bugs to equal the content of catching one large beetle,” he says. “Chimney swifts spend most of the day in flight and are on tight energy budgets. Any disruption to that would result in negative consequences, such as fewer resources to successfully rear chicks. The dietary change we observed was likely a trigger of swift population declines.”
That might explain why the swifts started to disappear, but Nocera thinks that other factors helped to perpetuate the decline, including changing climate. It could also be that the communities of insects that feed the swifts have permanently changed as a result of the early DDT wave.
To test that, Nocera’s team is planning to analyse the DNA of the guano tower’s insect remains, to identify the species that the swifts were eating. “We want to test whether the most common prey items in recent years are the same as the most common prey items in previous years,” he says.
They are also going to study guano columns from other chimneys around North America, to see if the fate of the Queen’s University swifts represents continent-wide on changes. They already have samples from places in Quebec, Manitoba and Connecticut.
Reference: Nocera, Blais, Beresford, Finity, Grooms, Kimpe, Kyser, Michelutti, Reudink & Smol. 2012. Historical pesticide applications coincided with an altered diet of aerially foraging insectivorous chimney swifts. Proc Roy Soc B. http://dx.doi.org/10.1098/rspb.2012.0445