I am an Ontario resident who spends winters in the US — could I owe US income tax as a snowbird?

Possibly. The US has its own residency test — the Substantial Presence Test — which counts the days you spend in the US over a rolling three-year period using a weighted formula. If the total meets the threshold, the US Internal Revenue Service can treat you as a US tax resident, making your worldwide income potentially subject to US tax. This is purely a US rule; how it affects you as an Ontario resident is a Canadian concern only insofar as it triggers double taxation.

The Canada-US Tax Treaty contains a tie-breaker that generally protects true Canadian residents from being treated as US tax residents even if they meet the Substantial Presence Test, provided they file IRS Form 8840 (Closer Connection Exemption) or rely on the treaty tie-breaker. The form must be filed with the IRS each year you are over the threshold — failure to file can result in the IRS treating you as a US resident.

If you are close to or above the US threshold, track your days carefully and consider consulting a cross-border tax adviser. US filing rules are distinct from Canadian ones.