It's a grape. Is it possible a previous owner planted a grape there? Or failing that, Vitus riparia, river grape, is native through various parts of Canada...

OMG yes it is! I knew there was something familiar about it... but I was not thinking about grapes. It reminded me of virginia creeper (grape family) but without the palmate leaves.

As to where it came from... my neighbour across the back lane grows hardy grapes and doesn't harvest them. I guess the birds do... ;-)

Thanks! That's a keeper!