Swifts are extraordinary fliers and are adapted to spend their entire day on the wing. By contrast, swifts are awkward on the ground. It is during these grounded periods at night when they are most vulnerable to predators. Because of this, roost structures that are protected from predators are an important resource for migratory swifts.

Historically, swifts both nested and roosted in large, hollow trees. Audubon recounts an observation of an estimated 9,000 individuals roosting in a hollow sycamore tree that was eight feet in diameter at the base. Trees of this size are rare today. As virgin forests gave way to urban development, swifts have adapted to nest and roost in man-made structures. Chimneys and similar structures have replaced hollow trees as the most common structures used by swifts in eastern North America.

Migratory swifts appear to be quite selective in the use of communal roosts. Within cities that contain thousands of chimneys, typically only a few dozen will be used for migratory roosts. One factor that appears to be important is the inside dimensions of the chimney. Several thousand swifts require a considerable surface area to roost communally. Another factor that appears to be important is the inside lining of the chimney. Because swifts roost vertically by clinging onto the inside surface, they require "holds" within the chimney. Chimneys that contain metal or tile flues with slick surfaces are rarely used by roosting swifts.

What factors contribute to activity schedules for migratory roost sites are poorly understood. Some roost sites seem to be used consistently for decades suggesting that they meet some criteria or are known to individual migrants. Other sites are used intermittently through the years. Still other sites are used only during specific times of the migration period suggesting that use may be population-specific. A great deal of information is needed to better understand the factors that result in different activity regimes.