7
“10,000 Bricks With Built-in Bird Homes Installed in UK Homes to Give Nest Holes to Swifts”
(slrpnk.net)
“Over 30,000 swift bricks—essentially a normal building brick with a hollow inside—have been sold in the country, and more than 10,000 homes have been built with swift bricks incorporated into the design.”
This article is about a year old, so the number is probably higher now.
