document: There is growing evidence that wild birds move the disease around the world as they fly thousands of miles to their winter homes.
  Researchers studied the genetic code of flu viruses in birds from 16 countries infected during the 2014 outbreak.
  They say bird flu was carried by migrating birds from Asia to Europe and North America via the Arctic.
  Bird flu is an infectious disease of poultry and wild birds.
  The strain studied was H5N8, which first appeared in South Korea in early 2014.
  The virus later spread to Japan, North America and Europe, causing outbreaks at poultry farms between autumn 2014 and spring 2015.
  "Bird flu is a major threat to the health and well-being of farmed chickens worldwide," said lead researcher Dr Samantha Lycett of the University of Edinburgh.
  "Our findings show that with good surveillance, rapid data sharing and collaboration, we can track how infections spread across continents."
  The study suggests that the virus spread along two main migration routes - or flyways - for wild birds:
  According to international scientists, contact with infected wild birds or materials contaminated with their droppings was the most likely route of transmission.
  Commenting on the research, Dr Derek Gatherer of the University of Lancaster said H5N8 is the latest in a long line of bird flu outbreaks to cause concern.
  "Flyways are the routes that migrating ducks, geese and other wildfowl follow every year from their breeding grounds to their winter retreats," he said
  "Just as a human airline passenger can spread human flu from continent to continent, each migratory bird that carries bird flu is a potential spread risk to other points along its flyway."
  The study, published in the journal Science, was conducted by the Global Consortium for H5N8 and Related Influenza Viruses.
  Follow Helen on Twitter @hbriggs.
summary: Monitoring birds on their long distance migrations may provide early warning of bird flu outbreaks, say scientists.