Video: Track pollution by soot on the hundred-year-old bird's body

US researchers analyzed soot on bird museums in many cities to monitor air pollution for 135 years.

Birds that are longer dated are dirtier while birds are cleaner.

A new study concludes that the fading of the bird collection in museums can be used to estimate the amount of soot in the air over time.

The team took samples of more than 1,000 birds collected in 135 years to learn and evaluate the effects of soot in the air. In American cities such as Chicago, Detroit and Pittsburgh.

To monitor the weight of soot, the team took pictures of birds and measured light reflected from them. They found that birds that are longer dated are dirtier while the birds are cleaner.