From publication:

pg 61

Pigeon smog reporters: A flock of
racing pigeons in London was outfitted
with tiny air-pollution detection
backpacks for a three-day experiment
in monitoring urban health. People
received smog level reports on Twitter
at @pigeonair, accessing the data
almost 40,000 times. The makers of
the pigeon backpacks are now offering
bicyclists and other people similar
mini air-detection devices.