That interview was on Jan. 5. Here's what happened in the poll averages shortly afterward.
It's important to remember that the polling average is dependent on polls, and polls take time to conduct. So it's impossible to link a poll average on a particular day to an event that happened that day, since there would have been no polls to assess it.
If we overlay the polls that followed the question (shown over the period of time that the polls were in the field), we can see what happened after Trump started talking about Canada. Cruz slipped a bit — and Trump started doing much better.
We have some correlation. But do we have causation?
Monmouth University asked Republicans nationally how they felt about Cruz's eligibility to be president, given his birthplace. (Legal experts broadly agree that it is not an issue.) Nearly all Republicans felt confident that Trump is a "natural-born citizen" -- the requirement for serving as president. A third of Republicans were more skeptical about the Canadian-born Cruz, who is a citizen by virtue of his mother being an American citizen when he was born.
A poll from the Democratic-leaning Public Policy Polling conducted shortly after Trump first discussed the issue found more direct evidence. "Only 32% of Iowa Republicans think someone born in another country should be
allowed to serve as President, to 47% who think such a person shouldn't be allowed to serve as President," the pollsters write. "Among that segment of the Republican electorate who don't think someone foreign born should be able to be President, Trump is crushing Cruz 40/14."
All of which suggests that the issue may have done some damage.
Cruz and Trump broadly share a base of support, so we would expect that Cruz's decline might help boost Trump. What's more, people who were undecided between the two may have been inspired to make up their mind in Trump's favor after the dispute. Cruz and Trump supporters are also likely to be more concerned about immigration issues than other Republicans, making questions about nativity a more potent line of attack.
Whatever the cause, Cruz's decline comes at about as bad a time as possible for his campaign. Donald Trump's gotten lucky repeatedly over the course of the last seven months, and it seems more than possible that he got lucky with this topic, too.