What would the many chimney swifts in my neighborhood do if the chimneys all disappeared?

They would adapt by using other high, protected hollow spaces for roosting and nesting, including trees.

North American chimney swifts first adapted to the loss of hollow trees by seeking out the chimneys built by European settlers. Reports of chimney nests first appeared in the 17th century.

Most chimney swifts now share nesting space in chimneys, but some still use trees. A 2003 review article in the journal Avian Conservation & Ecology identified the most likely chimney alternatives, based on historical records from 1803 to 2013. The researchers listed 13 tree species, both deciduous and coniferous, used by the birds in eastern North America.

The trees were all large, averaging 41 feet in height, and most were in remote settings in unlogged forests. They included yellow birches, maples, sycamores, cypresses, white pines and other pine trees. Some had broken tops or other damage that created chimney-like openings.