Other facilities across Canada decided to join in, and the idea snowballed from there.
Peter Duffey, the head of the Canadian Air Traffic Control Association, said: “The next thing we knew, our members were buying pizzas left, right and centre for the colleagues in the US.
“As it stands right now, I believe we’re up to 36 facilities that have received pizza from Canada, and that number is growing by the hour.”
Some 10,000 air traffic controllers in the US have been working without pay since late December because of the ongoing government shutdown.
Mr Duffey estimated that, as of Sunday afternoon, some 300 pizzas had been received by American controllers, many of whom took to social media to express their gratitude.
He said many union members had been looking for a way to show solidarity with their American colleagues.
— NATCA New England (@NATCANewEngland)12 January 2019
“Air traffic control is a very stressful job,” he said. “They say you have to be 100 per cent right, 100 per cent of the time.
“People just don’t need to be reporting to work with the added stress of worrying about how to pay their mortgages and grocery bills on top of it.”
Ron Singer, the national media manager for Nav Canada, which manages the country’s civil air navigation, said Canadian and American air traffic controllers interacted “on a daily basis”.
“There’s a bond there, automatically,” he said.
Mark Sheehy, an air traffic controller in New Hampshire, tweeted pictures of the pizzas and his thanks for “our brothers from the north sending love and solidarity”.
The partial shutdown was caused by an impasse between US president Donald Trump and Democrats over his $5.6billion (£4.4bn) request for a wall on the border with Mexico – after originally saying Mexico would pay for it.
The government shutdown – which is now the longest in US history – has resulted in hundreds of thousands of workers missing their pay cheques.
Additional reporting by Associated Press