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.

19 December 2016 Last updated at 16:54 GMT

Catherine Smith narrowly defeated Conservative Eric Ollerenshaw in Lancaster & Fleetwood with a majority of 1,265.
In Burnley, Julie Cooper secured 14,951 votes, unseating Liberal Democrat Gordon Birtwistle in a 6.3% swing.
Across the rest of the county, Labour held on to six seats while the Tories retained eight.
MP elect for Lancaster & Fleetwood Catherine Smith said she would represent and champion the area "to the best of my ability".
She also paid tribute to her predecessor Eric Ollerenshaw's work in the constituency.
Mr Ollerenshaw said he hoped he "hadn't let anyone down" after a "long and hard battle".
Speaking to BBC Radio Lancashire after her win in Burnley, Julie Cooper said: "It's a fantastic end to the day.
"I have worked for this for a very long time. I promise to give 100% for the people of Burnley."
The married English teacher and Burnley FC season ticket holder added: "People have been told times are getting better but that is not what it seems if you have lost your job or are suffering welfare cuts."
Mr Birtwistle said: "Labour threw the kitchen sink at this seat and we had far less resources."
He added that he now plans to "go back to being retired, like I was before I was elected".
In Blackburn, Kate Hollern retained the seat for Labour, increasing the majority that Jack Straw held by 2,904 votes for a total of 12,760.
Mr Straw had been the town's MP since 1979 before he recently retired.
However, the news wasn't so good for Mr Straw's son. Will Straw, Labour's candidate for the Rossendale & Darwen constituency failed in his bid to unseat Conservative Jake Berry.
To find out your election result, go to BBC Election 2015.
Monitoring birds on their long distance migrations may provide early warning of bird flu outbreaks, say scientists.
[NEXT_CONCEPT]
RHI at Stormont - Watch the key moments in 60 seconds.
[NEXT_CONCEPT]
Labour has made two gains in Lancashire taking one seat from the Conservatives and another from the Liberal Democrats.