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And even more troubling -- no one knows why, the society says in the report released last month.
As a result of the study, the Connecticut Audubon Society is calling for research into the problem along with cuts in pesticide use and the creation of man-made nesting sites.
The recommendations and action plan are included in the Connecticut State of the Birds 2013 report, “The Seventh Habitat and the Decline of Our Aerial Insectivores.”
The 17 species – known as aerial insectivores because they eat bugs on the wing – include barn swallows, whippoorwills, common nighthawks, chimney swifts, purple martins and tree swallows.
The long-term population decline threatens the very survival of the birds, the report says. The report also contains an article about a similar decline in Connecticut’s bat population, which also eats aerial insects.
Such birds forage in the “seventh habitat” – or the air. Billions of insects, arthropods and other bugs inhabit that space in a shifting mass sometimes referred to as aerial plankton.
“Unless we reverse the trend, population collapse is something of a mathematical certainty,” said Milan Bull, senior director of science and conservation for Connecticut Audubon Society. “The implications will be even more profound if it turns out that the main cause of the collapse is related to changes in aerial plankton.”
The report lists as possible causes for the decline of the birds:
- Reduced availability of man-made nesting sites such as barns, open chimneys and gravel rooftops;
- Loss of open-country foraging habitat;
- Changes in insect availability;
- Exposure to environmental contaminants; and
- Reduced availability of calcium, a consequence of acid rain.
The entire report can be read online.
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