So 20 miles per hour across 24 hours gets us a distance of 480 miles.

Going from Europe to the Americas by way of Scotland and Iceland is going to be a bit of a problem for that bird, as it can expect pretty consistent 10-20kt headwinds for the entire journey. America to Europe by that route is a comparatively easy trip.

I doubt that owls are capable of effective dynamic soaring, but that would drastically reduce the energy requirements.