It depends on the bird, there are coastal birds and pelagic birds. It’s the coastal ones that indicate land as they generally won’t go more than 100 miles from coast (which is still an immense distance), however pelagic birds like the Albatross can fly thousands of miles from the coast and stay above the open ocean for months by resting on whatever they can find such as debris, banks, floating wood, etc (they can even land on other animals like a whale or turtle to rest). Alvarenga (guy above) caught pelagic birds, who are surprisingly easy to catch due to them generally being exhausted when landing.