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 that 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 DDT 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 droppings study is that ecologists are beginning to wonder how far beyond eggshell 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 whippoorwills? It's too bad other species haven't left behind repositories of droppings that could be studied in the manner that the Canadian swifts have been.
When swifts return in April, 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 today?
Send questions and comments to Dr. Scott Shalaway, 2222 Fish Ridge Road, Cameron, WV 26033 or email sshala...@aol.com.