Salivary glands enlarge during the breeding season, and the secretion sets hard enough to hold a twig nest against a sheer surface. The twigs themselves are gathered in flight: the bird breaks a small dead branch with its feet or bill while passing, rather than landing to collect it.
Before European settlement chimney swifts nested in large hollow trees and cave walls. As old-growth forest was cleared and brick chimneys multiplied, the population shifted into human structures so thoroughly that natural nest sites are now rare. That dependence has become a liability as chimneys are capped, lined or demolished.
North American surveys show a long, steep decline, and the species is assessed as Vulnerable. The relative weight of nest-site loss against declines in flying insect prey is not resolved, and both may act together. Roosting flocks entering a chimney at dusk can number in the thousands.