How can true swifts rest if they cannot perch on twigs?

True swifts are highly specialized for an aerial lifestyle and possess tiny, weak feet that prevent them from perching on twigs or wires like many other birds. Instead of perching, they rest by clinging to vertical surfaces, such as the walls of caves, chimneys, or hollow trees, using their sharp claws.12

When not nesting, these birds are almost exclusively aerial, often drinking, bathing, and even mating while in flight. To conserve energy during periods when food is scarce and they cannot forage for flying insects, swifts are capable of entering a state of torpor, during which they reduce their body temperature to match the environment.123