Researchers used a special technique that measures the amount of light reflected off the birds' feathers. The darker the feathers, the more soot they contained.
Climate change will change how you eat
Warming temperatures mean higher prices for some staples, while some foods could become less nutritious
Air pollution harm to unborn babies may be global health catastrophe, warn doctors
New UK research links toxic air to low birth weight that can cause lifelong damage to health, raising fears that millions of babies worldwide are being harmed