@AndrewA87 , sorry for my confusion.
In general , when you receive govt. pension ( like Social Security or similar) , it is taxed ONLY by the "source" country. This is despite the "saving clause" which allows each country to follow/ administer its own tax rules.
Please read Article XVIII para 5 of the US Canada tax treaty ----> CANADAWEB.PDF
I will come back on this -- need to refresh my memory about the US -Canada tax treaty ( the most recent version --2008/09 ).
pk
