This is a chimney swift video.
By Dawn M. Smith:
Multiple Threats Contribute to the Decline of the Chimney Swift
Populations of this small insectivorous bird are falling with pesticide residuals, changing insect populations and habitat loss each likely playing a part.
The chimney swift (Chaetura pelagica), built for roosting upright on the walls of caves and tree hollows relies on flies, beetles, termites, wasps and spiders for nourishment. This avian insectivore takes all of its food while flying.
In North America, Chimney Swift populations have been on the decline for a number of years. The reasons for this varies from region to region, and it is likely that in some places multiple factors are at work.
Providing Nest Sites Helps Chimney Swifts Recover in Some Areas
Chimney Swifts suffer from loss of nesting sites. Natural nesting sites include tree hollows and caves. With reduced forest habitat there are fewer tree hollows. In suburban areas, most homeowners won’t leave dead trees in place for wildlife. Restricted cave access for human safety reasons adds to habitat loss. But Chimney Swifts have proven adaptable, nesting in chimneys instead. Now, however, more people put wire mesh caps on their chimneys, and these nest sites are also disappearing.
British swift migration: here.