This story originally appeared on Grist, and it is reproduced with permission. Shielding your picnic lunch from London’s plentiful pigeon population is almost as much of a tourist tradition as taking a selfie with Big Ben. But one group of pigeons have a job quite different than stealing your sandwich: measuring the city’s air pollution. Equipped with air quality sensors and GPS trackers in small, feather-light backpacks, six racing pigeons from the Pigeon Air Control project flew around London to get on-the-ground (or in-the-air?) readings of nitrogen dioxide and other toxic compounds. Last week the birds started tweeting. And no, that’s not the chirps of a long-awaited springtime you hear—it’s the pigeons’ Twitter account, which promises to provide air quality readings for Londoners who tweet at the handle @PigeonAir. The three-day campaign from Pigeon Air Control, from March 14 to 16, was mainly a publicity stunt to draw attention to dirty...