Soot trapped in the feathers of songbirds over the past 100 years is causing scientists to revise their records of air pollution. From a report: US researchers measured the black carbon found on 1,300 larks, woodpeckers and sparrows over the past century. They've produced the most complete picture to date of historic air quality over industrial parts of the US. The study also boosts our understanding of historic climate change. [...] This new study takes an unusual approach to working out the scale of soot coming from this part of the US over the last 100 years. The scientists trawled through natural history collections in museums in the region and measured evidence of black carbon, trapped in the feathers and wings of songbirds as they flew through the smoky air. The researchers were able to accurately estimate the amount of soot on each bird by photographing them and measuring the amount of light reflected off them. "We went into natural history collections and saw that birds from 100 years ago that were soiled, they were covered in soot," co-author Shane DuBay, from the Field Museum and the University of Chicago, told BBC News. "We saw that birds from the present were cleaner and we knew that at some point through time the birds cleaned up -- when we did our first pass of analysis using reflectance we were like wow, we have some incredible precision." Their analysis of over 1,000 birds shows that black carbon levels peaked in the first decade of the 1900s and that the air at the turn of the century was worse than previously thought.