All the digging during the massive project has displaced countless rats from their long-established burrows, said a pest control expert who appeared on CBC Radio's Metro Morning on Monday.
As a result, the rats are looking for new places to live.
"What we're seeing is a lot of spillover," Steve Graff of Abell Pest Control told Metro Morning host Matt Galloway on Monday.
"Rats set up their own territories … with the construction, the rats are being pushed out of that space," he said. "They're pushing out farther away, trying to take over the next available space. And they're fighting with the rats that have established their homes in those areas.
"So we're seeing a lot of movement, and it's affecting business downtown."
Slated for completion in 2016, the ongoing renovation of the century-old Union Station is a massive public works project.
The new station will feature a glass atrium that spans the tracks. An extensive excavation below track level is needed to make way for a new pedestrian shopping concourse.
All this is happening while the station — which serves 65 million Via Rail, GO Transit and TTC passengers annually — remains in use.
At the height of the excavation, more than 45 truckloads of material was removed nightly. All that digging has sent countless rats in search of new burrows.
Galloway, who often spots rats scurrying through Toronto streets during his early-morning bike ride in to work, asked Graff about Toronto's rats. Here's an excerpt of their conversation.
So how does this rat relocation play out?
"They need to find that [new] burrow … or they could perish with the cold. They're kind of at a desperate spot, if they've been displaced. They're going to be looking for areas to get inside of buildings and out of the cold. And that's where exclusion comes into place for businesses and home owners.
"If they're the alpha male rat, who is used to having the priority in that colony and the best food, those rats are going to be very aggressive, they're going to take anything they want when they get pushed out. The bigger they are, the stronger they are."
How far can rats move?
"Rats are highly mobile. They'll travel over a kilometre just to find food from their burrow. So it's very easy for them to keep pushing farther out." He said a rat with a nest near Union Station can pretty much travel anywhere in Toronto's downtown core.
So how do pest control experts keep rats out of homes and businesses?
Graff said his company first does a lot of inspection work. "We're looking for those openings and pointing them out to them. We seal off entrance points and set up traps to help keep the numbers down."
We've heard rats are very intelligent. Is this true?
"They are very smart and they do learn," said Graff. "If they get hit with a snap trap but don't actually get caught, they learn from that experience and they'll learn to avoid the snap trap. They are afraid of new things. So they're pretty wary. If you put something new into their environment, they'll avoid it and try and stay away from that object until they know it's something that's not going to hurt them."
New York City is reputed to have about two million rats. Anything like that in Toronto?
Graff said no one has made an effort to get an accurate count on Toronto's rat population. He said it probably doesn't compare to New York, where he's seen rat activity in person.
"Any type of green space in New York City, you see rat burrows there. Here in Toronto, you don’t see that type of activity."
Why are people so unsettled by rats?
Graff said the fact rats scurry around in the dark and can carry disease is enough to make anyone squeamish.
"Any big city has rats," he said. "They're not something people want to cuddle up with. One rat is too many for a lot of people."