Common swifts are aerial specialists that nest exclusively in cavities in buildings. Modern construction and renovation eliminates nest sites, causing population collapse. The UK swift population has declined by 60% since 1995.
Swifts returning to find nest sites blocked experience repeated failed nesting attempts. Pairs that have bred successfully at a site for years will persistently return to check blocked entrances. Loss of nest site access means total reproductive failure with no alternative available. The welfare impact compounds over the lifetime of a pair as a good nest site represents decades of breeding success.