How chimney swifts’ diet affects population size


In April, chimney swifts will return, but every year there seems to be fewer. One reason chimney swift numbers are down is we cap chimneys to keep them out. No one wants a chimney full of birds, nests and droppings. And no one wants live birds flying around the living room.

A recent study in Canada recently 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 poop more than six feet deep.

The chimney had been capped in 1992, and swifts had used the chimney since 1944. Grooms thought that by analyzing the swift poop, 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, a metabolite of 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 through the 1940s, 50s and 60s. 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 they had to spend more time feeding on the wing and less time taking care of nestlings. And unfortunately, there is no evidence the percentage of beetles in the diet increased after the banning of DDT in the early 1970s.


DDT is best known for its role in the decline of bald eagles, ospreys, brown pelicans and peregrine falcons. Birds exposed to DDT during the 1940s, 50s and 60s became unable to lay eggs with normal shells, a phenomenon that came to be known as “egg-shell thinning.” During incubation, eggs would crack and embryos died.

As a result, successful reproduction almost came to a halt and populations of affected species declined quickly. Most of these affected species became classified as “endangered.” Since DTT was outlawed, these species have made remarkable recoveries, and each that I mentioned has been removed from the “endangered” list.


One of the results of the Canadian chimney swift poop study is that ecologists are beginning to wonder how far beyond egg-shell thinning the effects of DDT reached. If it altered the diet of chimney swifts, did it also change the diet of other insectivorous species such as swallows, flycatchers, warblers, vireos and maybe even nighthawks and whip-poor-wills. It’s too bad other species haven’t left behind repositories of droppings that could be studied in the manner the Canadian swifts have been.

When swifts return in April, you can recognize them by their constant twitters, cigar-shaped body and long narrow wings. They fly almost continuously, always in search of flying insects.

It would be fascinating to find some chimneys piled high with poop to see if U.S swifts show the same dietary shifts as Canadian birds. I suspect they would.

Capping chimneys would be expected to reduce the population of chimney swifts, but who would have predicted that an insecticide banned in the 1970s could help explain the decline of chimney swifts in 2012?

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