I’ve come across a lot of resources that highlight the migration routes of these amazing birds, but when it comes to specific locations, I’m coming short on finding the answer. Have these birds been spotted across the Mediterranean Sea, or throughout the Caribbean, and even on the coasts of India or Japan? How far do they venture off course?
National Geographic has a relevant page for the Arctic tern
Can arctic terns be spotted everywhere?
Arctic terns can be found just about everywhere as their travels take them to every ocean and every continent.
How far do they venture off course?
Arctic terns hold the record for the longest migration of any animal in the world, annually making the journey from the Arctic Circle to the Antarctic Circle. One particularly committed tern made the trip in nearly 60,000 miles, or more than twice the circumference of the planet. Terns rack up all those miles by meandering across oceans and continents rather than flying directly north or south.
And the webpage All About Birds has a global sightings map.
Have these birds been spotted across the Mediterranean Sea, or throughout the Caribbean, and even on the coasts of India or Japan?
The map shows sightings in the eastern Caribbean, the Mediterranean coast of Spain, in southern Israel, northern Japan, Bankok, etc. But the sightings don't go as far as confirming across or throughout those regions.
A major source of birder information is
Go to "Explore", "Species Maps", and type in the name of your bird and (helpfully) the date range). The All About Birds link from Weather Vane's page links to a default 5-year range anyway.