Canadians Living in USA & Mexico

I'm asking this quesiton on behalf of my parents. Here is their situation. They pay taxes in Canada but have a residental address in Oregon. They spend time in Canada, the US & Mexico each year. Do they have to fill out any forms when returning to Canada? Do they have to fill out any forms to be able to "live" in the United States & how often do they have to come back to Canada?
Any ideas where I would find this information? I've tried the Canadian Gvt. website but can't make heads or tails of it. Thanks Muchly!

I have no idea about Mexico, but legally a Canadian can visit the US for up to 6 months per year and not need a visa, nor lose their benefits in Canada.

Generally, according to both Canada and the US, if you are in the US for 6 months, you should also be home for 6 months.

If they have a permanent residence in the US, and have income in the US, they would be required to claim income tax to both Canada Revenue and the IRS. I'd suggest speaking with a financial planner to figure out how to not get dinged twice for taxes.