Pigeons are a common sight in the UK city of London, but eagle-eyed observers may have noticed something a little different about a few of them recently.
Some of them had tiny backpacks on.
These pigeons weren't heading off to school but rather monitoring levels of dangerous compounds in the air, as a part of the first ever Pigeon Air Patrol.
(Twitter / Pigeon Air)
Ten pigeons have spent the last three days helping to monitor pollution levels in London.
And they're didn't just keep the results to themselves -- in true bird fashion, they tweeted what they learned.
Local Twitter users were able message the account @PigeonAir with their location, and the birds tweeted back with results based on guidelines by the World Health Organisation.
The results rank from fresh -- indicating low pollution -- to extreme, indicating critical levels of pollution that are harmful to humans. When air pollution levels are extreme people are advised to stay indoors for their own safety.
Some of the compounds measured included nitrogen dioxide, mainly produced by road traffic, and ozone, the main gases in urban air pollution.
Plume Labs, the team behind the flying detectors, teamed up with DigitasLBi and Twitter UK for the campaign.
DigitasLBi Creative Director Pierre Duquesnoy says people don't tend to engage with air pollution issues.
"The heart of our idea was to make air pollution more accessible and relevant to people. The Pigeon Air Patrol is a perfect example of how data combined with creative storytelling can raise awareness of a serious health and environmental issue."
The pigeons were well cared for during their time on the Air Patrol. As racing birds, they were already hardier than their wild counterparts, and live around four times as long.
(Twitter / Pigeon Air)
The sensors are tiny, just 5cm x 3.8cm x 1.25cm, and the vests they're stitched to give the birds complete freedom of movement, Plume Labs says.
On top of that, the flock was minded by its own vet.
It isn't Plume Labs' first foray into air quality monitoring. They also run an app, Plume Air Report, which collects data from an assortment of official air quality monitoring stations worldwide.
And the company is currently running a crowdfunding campaign for a product very similar to what's been taking flight this week, except this time people can be a part of the Air Patrol.
Instead of attaching the Plume sensor to the back of a bird, a portable device can be clipped onto a bag, a bike, or a dog's leash.
It's accompanied by a smartphone app which allows the user to monitor the pollution levels in real time.
The new project will begin in June, Plume Labs says, and they're looking for more than 100 people in London to help beta-test.
"Air pollution is a huge environmental health issue, killing 10,000 people every year in London alone," says Plume Labs' CEO Romain Lancombe.
"Putting air sensors on the back of pigeons goes beyond raising awareness of this problem and helps Londoners understand the impact of pollution in an accessible, tangible and immediate way."