People can follow along and tweet at them for updates on local pollution data.

The backpacks detect nitrogen dioxide and ozone gases, and so far, the readings indicate the city wavers between moderate and high pollution levels. A "high" level is one that's above the World Health Organization's recommendations for air quality. London's air has only been deemed "fresh" a few times so far.

London's air pollution track record is notoriously bad. The city broke its annual EU air pollution limit within the first week of 2016, The Guardian reports, and also broke that annual cap during the previous five years. The persistent dirty air ultimately pushed the country's Supreme Court to publish an action plan last year on how to deal with the crisis. Even with the plan, however, London and other cities expect to exceed nitrogen dioxide levels for at least another five years.

The pigeons will tweet until Wednesday, the day they’re scheduled to reach their home in a suburban area of London. After that point, 100 people will be recruited to beta test a new pollution monitor for humans to wear. Pigeons aren't the cutest birds, but hey, they can do a lot of cool things! Here's to hoping they get to keep the backpacks.