Wildlife: Diet affects chimney swift population
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In April chimney swifts will return, but every year there seems to be fewer. One reason that chimney swift numbers are down is that we cap chimneys to keep them out. No one wants a chimney full of birds, nests and droppings.
A recent study in Canada revealed another factor that has played a role in the decline of chimney swifts. Chris Grooms, a research technician at Queen's University, discovered an abandoned five-story chimney in a campus building that contained a pile of swift droppings more than 6 feet deep.
The chimney had been capped in 1992, and swifts had used the chimney since 1944. Grooms thought that by analyzing the fecal matter researchers might be able to determine their diet, so he enlisted the help of an ecologist named John Smol.
Accessing the chimney through a small door at its base, researchers dug through the dry and now odorless droppings to get inside and take samples. They then shipped the samples out to other scientists who identified the insect remains and determined chemical residues left behind.
Most of the insect parts came from beetles and true bugs (insects such as stink bugs, leafhoppers and cicadas). Beetles were most common at the bottom of the debris while true bugs became more common toward the top.
Chemical analysis of the remains revealed that levels of DDE, created through the metabolism of the insecticide DDT, increased from the bottom of the pile of swift droppings to the top. And as the levels of DDE increased, the frequency of beetle remains decreased and true bug remains increased.
Beetles are more sensitive to DDT than true bugs, so beetles became less common as DDT use increased from the 1940s through the 1960s. This caused swifts to eat more true bugs than beetles.
This was bad news for swifts because beetles are a better food; they contain more calories than true bugs. This may explain why, from 1968 to 2005, chimney swift numbers dropped 95 percent in Canadian surveys. Over time the use of DDT forced swifts to consume more true bugs, an inferior food, and fewer beetles simply because beetles became less common. So the birds spent more time feeding on the wing and less time taking care of nestlings.
Capping chimneys would be expected to reduce the population of chimney swifts. But who would have predicted that an insecticide banned in the 1970s would help explain the decline of chimney swifts in 2012?
First Published January 20, 2013 12:00 am