Swift populations have declined dramatically as modern building practices eliminate the roof and eave spaces they depend on for nesting.
The decline of swift populations represents a welfare failure played out at the colonial and individual level as birds return annually to find nesting sites destroyed by building works. Swifts cannot easily find alternative sites — their dependence on specific nest cavities built up over years means a lost site is often a permanent loss. The individual welfare impact includes breeding failure, loss of familiar social bonds within colonies, and the physiological stress of failed nesting attempts. At the population level, swift decline represents the slow collapse of colonies that have used the same buildings for generations. Conservation actions that replace lost nesting infrastructure directly address this welfare harm and can restore breeding success within a few seasons if implemented in areas with remaining swift populations.