Non-stop Flight Champions
Bar-tailed godwit: 11,500+km non-stop. This is the current non-stop champion, and has been recorded with repeated telemetry series. Note that these birds are continuous flappers, so they are not soaring for this distance. They also cannot land/launch from open water, so there’s no way for them to stop in route (confirmed with tracking).
Burning the Engine
One of the primary factors involved in long distance migration is something that Colin Pennycuick has termed burning the engine. Before migration, birds eat a lot of extra food and put on a huge amount of fat. As they fly, migrating birds burn the fat, which makes them lighter. This means they require less power to continue flying, so they do something quite interesting: they burn some of the muscle. That is, they burn the engine. This provides more fuel while simultaneously keeping the birds at a high efficiency ratio of power available to cost of transport (it is more efficient to run a small engine hard than a large engine at partial capacity). This continues for the whole trip: some fat is burned, then some muscle, and repeat. The net effect is a 50% loss of body mass for godwits and others, along with an extraordinary long-distance journey.