Soot trapped in the feathers of songbirds over the past 100 years is causing scientists to revise their records of air pollution. US researchers measured the black carbon found on 1,300 larks, woodpeckers and sparrows over the past century

See the source article by following the link below:
Soot trapped in the feathers of songbirds over the past 100 years is causing scientists to revise their records of air pollution. US researchers measured the black carbon found on 1,300 larks, woodpeckers and sparrows over the past century