Air traffic control staff in Portland, Maine, with pizza sent in from Moncton, New Brunswick. "I couldn't be more proud of what my members are doing", said Peter Duffey, president of the Canadian Air Traffic Control Association (CATCA).
The SLJY says that there has been no progress in contract negotiations.
Talks on a new contract for the controllers have dragged on for more than a year.
But now, Duffey said, units have been randomly selecting other units to send pizza to, sometimes based on similarities they think they share.
A collection started by employees had blossomed into an industry-wide show of solidarity, the Canadian Air Traffic Control Association told the BBC.More news: LeBron James to miss another three games with groin injury
Around a quarter of the federal government is out of operation due to the shutdown.
From there, the generosity spread to other air traffic control centers across the USA, picking other places where they had connections.
Since Friday evening, air traffic control units from across Canada have been sending pizza pies of solidarity to their United States colleagues to try and boost morale as the government shutdown means they are working without pay.
As of midday Monday, Duffey said over 400 pizzas have been sent to 52 different air traffic control facilities in the USA - "and that number just keeps growing".
A three-week government shutdown has resulted in 800,000 essential workers across the US working without pay.
"It was a natural thing for them to do".More news: What are the options if Theresa May's Brexit deal is voted down?
Duffey said the response from the American controllers has been "very, very heartwarming".
"They are in the middle of the government shutdown and as of yesterday did not receive their pay cheques".
Air traffic controllers in Seattle were sent pizzas from their Canadian counterparts as part of a show of solidarity amid the ongoing USA government shutdown.
Controllers from both countries interact "on a daily basis" and have "a bond there, automatically" as they manage North American airspace, Ron Singer, the national media manager for Nav Canada, said.
"We've always stood with our USA counterparts", Duffey said.More news: China's foreign trade hits historic high in 2018